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FERAL HOG (SUS SCROFA) DISTURBANCE IN SEEPAGE SLOPE WETLANDS
By
MEGAN E. BROWN
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2014
© 2014 Megan E. Brown
To my Dad
ACKNOWLEDGMENTS
First, I would like to thank my parents, Larry Brown and Patricia Maddox, who early on
fostered my interest in the natural world and have always encouraged me to pursue my
education. I owe much of my accomplishments to their love and support.
I would like to express my sincere gratitude towards those who helped me throughout my
doctoral experience. Mark Brown gave me the opportunity to be a part of the Adaptive
Management of Water, Wetlands, and Watershed IGERT group. My advisor, Debbie Miller, has
been great mentor and friend over the past six years. My growth as a scientist has benefited from
her experience and guidance. The other members of my committee, Carrie Reinhardt Adams,
Doria Gordon, Kevin Heirs, and Wiley Kitchens have been instrumental in providing wisdom,
inspiration, and encouragement along the way. A big thank you to Tyler Barry, Sean Claypool,
Gina Duke, Hanna Hunsberger, and Allix North for spending many long, hot, humid days in the
field, trudging through snake-infested woods and wetlands, lugging a meter square that never
stayed together, and collecting hundreds of plant specimens....all in the name of data collection. I
also give much thanks to Brett Williams with Jackson Guard at Eglin AFB and John Allen with
the USDA APHIS Wildlife Services for their support of this project. I started this doctoral
journey with seven other amazing women and I am thankful for having had the opportunity to
create such wonderful friendships.
Finally, I am grateful for the tireless support of my husband, Joshua Hornbarger, who
uprooted his life to move with me to Gator Country. I also am eternally grateful for my daughter,
Odelia (3), who provides me with more joy than I ever knew possible. I hope to instill in her an
admiration of the natural world so that she too can know and experience its beauty.
This project was funded in part by a NSF IGERT fellowship, a USDA NIFA pre-doctoral
fellowship, a McKnight Dissertation Fellowship, a Society of Wetland Scientists Student Grant,
4
the Doris Lowe and Earl and Verna Lowe Scholarship (UF), and the William C. and Bertha M.
Cornett Fellowship (UF).
5
TABLE OF CONTENTS
page
ACKNOWLEDGMENTS ...............................................................................................................4
LIST OF TABLES ...........................................................................................................................8
LIST OF FIGURES .........................................................................................................................9
ABSTRACT ...................................................................................................................................12
CHAPTER
1
INTRODUCTION ..................................................................................................................14
Background .............................................................................................................................14
Research Goals .......................................................................................................................15
2
INDIRECT EFFECTS OF FERAL HOG DISTURBANCE ON FIRE SPREAD .................16
Background .............................................................................................................................16
Methods ..................................................................................................................................18
Study Area .......................................................................................................................18
Data Collection ................................................................................................................20
Data Analyses ..................................................................................................................20
Results.....................................................................................................................................21
Discussion ...............................................................................................................................22
Summary .................................................................................................................................23
3
LONG-TERM MONITORING OF FERAL HOG DISTURBANCE IN SEEPAGE
SLOPES ..................................................................................................................................26
Background .............................................................................................................................26
Methods ..................................................................................................................................27
Study Area .......................................................................................................................27
Data Collection ................................................................................................................29
Data Analyses ..................................................................................................................31
Results.....................................................................................................................................32
Discussion ...............................................................................................................................35
Summary .................................................................................................................................41
4
EFFECTS OF INTENSITY OF FERAL HOG DISUTRBANCE ON VEGETATION
DYNAMICS ...........................................................................................................................49
Background .............................................................................................................................49
Methods ..................................................................................................................................51
Study Area .......................................................................................................................51
6
Data Collection ................................................................................................................52
Data Analyses ..................................................................................................................54
Results.....................................................................................................................................55
Foliar Cover .....................................................................................................................55
Richness ...........................................................................................................................57
Camera Detection of Hog Activity ..................................................................................57
Vegetation Composition ..................................................................................................57
Discussion ...............................................................................................................................58
Summary .................................................................................................................................63
5
EFFECTS OF EXPERIMENTAL SOIL DISTURBANCE ON VEGETATION
DYNAMICS ...........................................................................................................................72
Background .............................................................................................................................72
Methods ..................................................................................................................................74
Study Area .......................................................................................................................74
Data Collection ................................................................................................................75
Data Analyses ..................................................................................................................76
Results.....................................................................................................................................77
Foliar Cover .....................................................................................................................77
Camera Detected Hog Frequency ....................................................................................80
Richness ...........................................................................................................................80
Vegetation Composition ..................................................................................................81
Discussion ...............................................................................................................................82
Summary .................................................................................................................................85
6
CONCLUSIONS ....................................................................................................................98
LIST OF REFERENCES ...............................................................................................................99
BIOGRAPHICAL SKETCH .......................................................................................................113
7
LIST OF TABLES
Table
page
3-1
Repeated measures ANOVA results for the percentage of hog disturbed plots over
time by zone on Eglin AFB from 2002 to 2012.................................................................43
3-2
Repeated measures ANOVA results for the percentage of hog disturbed plots over
time by hunting access on Eglin AFB from 2002 to 2012. ................................................43
3-3
The number of hogs removed from Eglin AFB from October 1 of the previous year
to September 30 of that calendar year. The percentage of hog disturbed plots and
rainfall totals are also indicated. ........................................................................................43
3-4
Repeated measures ANOVA results for the cover of total vegetation, functional
groups, and A. stricta by zone and access on Eglin AFB in 2002, 2010, and 2012. .........44
4-1
ANOVA results for the foliar cover of total vegetation, functional groups, and A.
stricta and species richness by disturbance intensity and exclosure on Eglin AFB. .........64
5-1
Repeated measures ANOVA results for the foliar cover of total vegetation,
functional groups, A. stricta, Dichanthelium spp., Sarracenia spp., and species
richness by disturbance frequency on Eglin AFB..............................................................86
5-2
Repeated measures ANOVA results for the foliar cover of total vegetation,
functional groups, A. stricta, Dichanthelium spp., and Sarracenia spp., and species
richness by disturbance frequency on Eglin AFB with controls........................................87
8
LIST OF FIGURES
Figure
page
2-1
The location of seepage slope sites on Eglin AFB. ...........................................................25
2-2
Topographic relationship between seepage slope, upland pine, and baygall
communities including position of zones on seepage slopes. ............................................25
3-1
3-2
3-3
Percent hog-disturbed plots from 2002 to 2012. Analysis included both UF and Eglin
AFB data. ...........................................................................................................................46
3-4
Total foliar cover by year. ..................................................................................................46
3-5
Foliar cover of functional groups and A. stricta by zone (i.e., position on slope). ............47
3-6
Foliar cover of woody species through time. .....................................................................47
3-7
Foliar cover of Aristida stricta through time. ....................................................................48
4-1
4-2
4-3
Exclosure and open plot pairs. ...........................................................................................66
4-4
The location of seepage slope sites with motion detecting wildlife cameras. ...................66
4-5
Total foliar cover by intensity by year inside and outside exclosures on Eglin AFB. .......67
4-6
Total foliar cover by exclosure by year on Eglin AFB. .....................................................67
4-7
Foliar cover of woody species by intensity by year inside and outside exclosures on
Eglin AFB. .........................................................................................................................68
4-8
Foliar cover of graminoid species by intensity by year inside and outside exclosures
on Eglin AFB. ....................................................................................................................68
4-9
Foliar cover of graminoid species by exclosure by year on Eglin AFB. ...........................69
4-10
Average number of hog occurrences per month in seepage slopes with cameras on
Eglin AFB from June 2010 through December 2013. .......................................................69
9
4-11
Two-dimensional nonmetric multidimensional scaling ordination showing cover by
species in high disturbance intensity and control plots on Eglin AFB. .............................70
4-12
Three-dimensional nonmetric multidimensional ordination showing cover by species
in high disturbance and control plots on Eglin AFB in A) 2010 and B) 2012. .................71
5-1
5-2
5-3
Total foliar cover by disturbance treatment over time on Eglin AFB. ..............................89
5-4
Foliar cover of forbs by disturbance treatment over time on Eglin AFB. .........................90
5-5
Foliar cover of grasses by disturbance treatment over time on Eglin AFB. ......................91
5-6
Foliar cover of woody species by disturbance treatment over time on Eglin AFB. ..........92
5-7
Foliar cover of A. stricta by disturbance treatment over time on Eglin AFB. ...................93
5-8
Foliar cover of Dichanthelium spp. by disturbance treatment over time on Eglin
AFB. ...................................................................................................................................94
5-9
Foliar cover of Sarracenia spp. by disturbance treatment over time on Eglin AFB. ........95
5-10
Species richness by disturbance treatment over time on Eglin AFB. ................................96
5-11
Three-dimensional nonmetric multidimensional ordination showing cover by species
by disturbance treatment on Eglin AFB in June 2010 and Sept 2013.. .............................97
10
LIST OF ABBREVIATIONS
ANOVA
Analysis of Variance
EGLIN AFB
Eglin Air Force Base
MRPP
Multi-response permutation procedure
NMS
Nonmetric multidimensional scaling
UF
University of Florida
11
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
FERAL HOG (SUS SCROFA) DISTURBANCE IN SEEPAGE SLOPE WETLANDS
By
Megan E. Brown
May 2014
Chair: Debbie Miller
Major: Interdisciplinary Ecology
Florida is home to one of North America's most unique and diverse natural ecosystems,
the seepage slope wetland. Unusual hydrology and frequent fires have resulted in a habitat that
supports a variety of insectivorous and other endemic, helophytic herbaceous plants. Feral hog
(Sus scrofa) foraging has resulted in widespread soil disturbances in seepage slope wetlands in
the Florida Panhandle. It is proposed that feral hog rooting is a serious threat to this community
because rooting sets back succession, causes changes in species composition and plant
population structure, reduces unique species, and inhibits fire spread. These potential changes in
the seepage slope plant community are particularly important because these wetlands provide
increasingly rare habitat for several threatened and endangered plant species.
We conducted plant surveys to investigate both the long and short-term vegetation
dynamics in response to hog disturbance. We demonstrate that hog disturbance has not declined
since the USDA APHIS Wildlife Services began trapping hogs on Eglin AFB in 2003 and
theorize rainfall and other potential drivers of hog disturbance. The short-term exclosure studies
provide evidence that the intensity and frequency of disturbance alter vegetation composition
even when foliar cover is partially or fully regained. These results also indicate that Aristida
stricta cover is reduced by soil disturbance, while woody cover is maintained. A positive
12
feedback is likely to result with reduced grass cover and fire spread, which further contributes to
increased woody cover. In addition, we used GPS collected data to compare areas damaged by
hogs and areas burned during prescribed fires to quantify the indirect effects of hog rooting on
ecosystem function. We found that hog rooting reduced vegetation cover and decreased fire
spread. Through this work, we gain insight to the mechanisms that affect the rate and trajectory
of vegetation development in this plant community, with significance for both theoretical and
management purposes.
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CHAPTER 1
INTRODUCTION
Background
Seepage slope wetlands and bogs of the southeastern United States are of critical
conservation concern due to their high levels of species richness and endemism (Johnson 2001).
Unusual hydrology, oligotrophic conditions, and frequent fires have resulted in a habitat that
supports a variety of carnivorous and other heliophytic, herbaceous plants. The southeastern
coastal plain contains the highest number of carnivorous plant species in North America
(Hermann 1995). Only one percent of seepage slopes that originally existed along the Gulf
Coastal Plain still remain (FNAI 1990). These wetland features have been degraded by
encroachment of woody species in the absence of fire, conversion into ponds, or drainage. As a
result, remnant seepage slopes communities provide increasingly critical but diminishing habitat
for threatened and endangered plant and amphibian species (FNAI 1990; Hermann 1995).
The Spanish explorer, Hernando de Soto, introduced domestic hogs (Sus scrofa) in the
16th Century, (Mayer & Brisbin 1991), and they are now established in 35 states and Canadian
provinces (Giuliano 2009). Feral hogs pose a serious threat to seepage slopes (FNAI 2010)
because they overturn soil when foraging (Howe & Bratton 1976). This behavior creates
hummocks and tussocks that alter edaphic properties (Singer et al. 1984; Jensen 1985) and
hydrology (Arrington et al. 1999), accelerates decomposition, alters soil nutrient concentrations
(Singer et al. 1984; Wirthner et al. 2012; Krull et al. 2013), and facilitates soil erosion (Bratton
1975; Howe & Bratton 1976).
In addition to these changes in edaphic conditions, the mechanical disturbance of hog
foraging deracinates vegetation (Engeman et al. 2007), reduces vegetation cover (Bratton 1974,
1975; Engeman et al. 2007; Siemann et al. 2009; Wirthner et al. 2012), and impacts regeneration
14
(Lipscomb 1989; Sweitzer & Van Vuren 2002; Siemann et al. 2009). Hog rooting can result in
reduced species diversity (Bratton 1975; Kotanen 1995; Tierney & Cushman 2006; Siemann et
al. 2009) and altered species composition (Bratton 1974; Aplet et al. 1991; Siemann et al. 2009;
Cuevas et al. 2010). These changes in species composition are due to both direct species
removal, as well as indirect effects on the ecosystem processes including the disruption of the
fine fuel component that facilitates fire spread. Vegetation may (Arrington et al. 1999; Tierney &
Cushman 2006) or may not be able to recover from rooting disturbances (Bratton 1975). Hog
rooting and foraging could lead to the extirpation of species (Bratton 1974; Recher & Clark
1974; Challies 1975; Spatz & Mueller-Dombois 1975). Deleterious impact from extensive,
repetitive rooting of native plants is documented as a critical threat to the conservation of
pitcherplant wetland communities in Florida (Johnson 2001).
Research Goals
It is important to understand how disturbances affect natural communities for both
theoretical and management purposes. This research explored how feral hog disturbance affects
the rate and trajectory of vegetation development in seepage slopes of the Florida Panhandle in
four interrelated studies. Chapter 2 quantifies the effect of this novel disturbance on the historic
fire regime. Chapter 3 investigates vegetation dynamics in seepage slopes over a 10-year period.
Chapter 4 evaluates vegetation development in exclosures erected in areas of varying disturbance
intensity. Chapter 5 assesses the role of frequency of disturbance and time since last disturbance
in determining vegetation composition and species richness. This research will provide better
understanding of: 1) long-term patterns of vegetation dynamics; and 2) mechanisms driving
shifts in plant composition in response to feral hog disturbance. The results of this work can be
used with continued monitoring data to direct management actions to prevent degradation of
seepage slopes on Eglin AFB.
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CHAPTER 2
INDIRECT EFFECTS OF FERAL HOG DISTURBANCE ON FIRE SPREAD
Background
Ecosystems adapted to disturbance by fire are distributed all over the globe (Drewa et al.
2002; Bond & Keeley 2005). These systems include northern latitude boreal forests, lower
latitude shrublands, grasslands, and savannas and eucalyptus woodlands in Australia. Different
fire regimes have selected for and resulted in different plant attributes and adaptations.
Worldwide these fire-maintained systems have different biota, but recent decades of fire
suppression in these systems all demonstrate: 1) risks associated with increasing fuel
accumulation (Bond & Keeley 2005); 2) a loss of fire-dependent species (Frost et al. 1986;
Hermann 1995; Frost 2000; Lett & Knapp 2003; Glitzenstein et al. 2003; Bond & Keeley 2005;
FNAI 2010); and 3) management challenges in establishing habitat needs (e.g., determining fire
return intervals, season of burn, and use of fire surrogates) (Slapcinsky et al. 2010). These threats
and challenges are especially significant in Florida where 70% of the terrestrial ecosystems are
fire-dependent or adapted (FNAI 1990).
Seepage slopes, shrub bogs, and baygalls are wetlands that occur downslope from Pinus
palustris Mill. (Longleaf pine)-upland communities. These communities have a successional
relationship dependent on fire frequency. Frequent fire upslope from shrub bogs and baygalls
maintain the herbaceous component of the seepage slope (Eleuterius & Jones 1969; FNAI 2010).
Fire return intervals are estimated to have been one to three years historically in the surrounding
uplands (Frost 1995; Frost 2000; Frost 2006), while somewhat longer in wet seepage slopes
(FNAI 2010). Further downslope, shrub bogs are estimated to have longer burn return intervals
of 20 to 50 years (Wharton et al. 1976). In the absence of fire, shrub bogs replace the herbaceous
wet prairie species and eventually, these shrub bogs succeed to baygalls dominated by hardwood
16
species (Means & Moler 1979). While fire is a disturbance in the classical sense, it is essential to
the stability of the seepage slope community (Frost et al. 1986).
Beginning in the 1920's, suppression of wildfires became public policy in the
southeastern United States. Realizing the importance of fire in the maintenance of natural
communities, land managers now use prescribed fire to mimic natural, lightning-ignited fire.
Most prescribed fire is applied in the dormant season rather than the summer growing season
(Knapp et al. 2009).
Alteration of natural fire regimes can lead to degradation of the remaining seepage slopes
(FNAI 2010). Historically, lightning ignited fires during the spring or the beginning of the
growing season (Olson 1992). This seasonal timing of spring drought and lightning-producing
thunderstorms generated upland fires that would carry down into seepage slopes (Olson & Platt
1995). It is well documented that fire suppression and, in some cases, dormant season prescribed
fires have allowed the encroachment of woody species (Frost et al. 1986; Frost 1995;
Glitzenstein et al. 1995; Olson & Platt 1995; Drewa et al. 2002; Slocum et al. 2003) and loss of
herbaceous species in seepage slopes (Eleuterius & Jones 1969; Folkerts 1982; Frost et al. 1986;
Hermann 1995).
Fire is influenced by multiple factors including fuel conditions, weather and wind
conditions (Cheney et al. 1993; Turner et al. 1994; Brooks et al. 2004), and topography (Weber
1990; Turner et al. 1994; Brooks et al. 2004). This chapter focuses primarily on the relationship
between fuel continuity and fire spread. The horizontal continuity of fuels can determine the
extent of fire spread (Brooks et al. 2004; Nader et al. 2007). For example, wider gap distance
between fuel beds can result in decreased flame contact and thus decreased spread (Weber 1990;
Finney et al. 2010). Modeling has shown that fuel gaps are more likely to inhibit fire spread
17
when fuels are heterogeneous (Kerby et al. 2007), like fuels of bunch-grass systems (Finney et
al. 2010) such as the P. palustris (Longleaf pine) / Aristida stricta Michx. (Wiregrass) ecosystem
of the Southeastern United States.
Herbivores can alter a fire regime by ingesting vegetation that is critical for fire spread
(Savage & Swetnam 1990; McNaughton 1992; Knapp et al. 1999; Waldram et al. 2008; Hierro et
al. 2011). Feral hog foraging that results in uprooted vegetation (Bratton 1974; Engeman 2007)
and increased cover of bare ground (Bratton 1975; Kotanen 1995; Engeman 2007; Doupé et al.
2010) effectively reduces the load and continuity of fine fuels. This reduction of fine fuels can
potentially: 1) reduce the extent of fire spread (Harrington & Hodgkinson 1986; McNaughton
1992; Knapp et al. 1999; Kerby et al. 2007; Waldram et al. 2008); and 2) alter the frequency
(Savage & Swetnam 1990; Van Auken 2000; Hierro et al. 2011) and intensity of future fires
(Hobbs et al. 1991; Van Langevelde et al. 2003; Briggs et al. 2005; Kerby et al. 2007; Hierro et
al. 2011). This chapter investigates the indirect effects of hog rooting on ecosystem function. We
sought to determine whether soil disturbances associated with feral hog foraging reduce fire
spread in seepage slopes on Eglin AFB.
Methods
Study Area
Sites for this study were located on Eglin AFB in Walton County in the western Florida
Panhandle (30°29′22″N 086°32′32″W) (Figure 2-1). This area has a mean annual precipitation of
158cm (DoD-Air Force 1995). Precipitation tends to peak in the summer months and is lowest in
May and October. Lightning producing thunderstorms are common in the spring and summer
months (NOAA 2002). Suppression of wildfires and limited use of prescribed fire resulted in
infrequent fire in seepage slopes on Eglin AFB until about 2002. Subsequently, Eglin land
18
managers increased the burning frequency to annual and biennial prescribed fires in both the
growing and dormant seasons (pers. comm. 2014 Eglin AFB land manager).
The soils on the eastern portion of EAFB are loamy sands. Creeks have created rolling
erosional hills with relief of up to about 30 meters (NRCS 1989). The mesic, upland pine
community (FNAI 1990) found on these loamy sands is speckled with wet prairies on hillsides
called seepage slopes (Figure 2-2). These seepage slopes result from groundwater seeping
downslope above an impermeable layer of clay that intersects the hillside. A unique variety of
herbaceous vegetation can be found in seepage slopes including carnivorous plants like
Sarracenia spp. (Pitcherplants), Drosera spp. (Sundews), Pinguicula spp. (Butterworts), and
Utricularia spp. (Bladderworts). Upslope, in the drier portions, characteristic species include
Aristida stricta, Ctenium aromaticum (Walter) A. Wood (Toothachegrass), and Rhexia spp.
(Meadowbeauties), while downslope the longer hydroperiod accommodates Eriocaulon spp.
(Pipeworts), Lophiola aurea Ker Gawl. (Golden Crest), Pleea tenuifolia Michx. (Rush
Featherling), Rhynchospora spp. (Beaksedges), and Xyris spp. (Yelloweyed Grasses). These
wetlands are dependent on ground water and dry out in times of drought. It is during dry spells
that seepage slopes are able to carry fire. Frequent fire is necessary to prevent shrubs, like
Cliftonia monophylla (Lam.) Sarg (Black Titi), from invading from the shrub bogs and baygalls
along the seepage creek and shading out helophytic herbaceous species. Other scattered shrubs
that are common in seepage slopes include Gaylussacia mosieri Small (Woolly huckleberry) and
Hypericum spp. (St. John's-wort) (FNAI 2010).
The baygalls located at the base of the slope are saturated, peat-filled seepage depressions
that support a dense forest of evergreen hardwoods. This forest is dominated by Magnolia
virginiana L. (Sweetbay), Persea palustris (Raf.) Sarg. (Swamp bay), and Gordonia lasianthus
19
(L.) J. Ellis (Loblolly bay). Sphagnum spp. (Sphagnum moss) can form mats on the ground
(FNAI 1990). A mature canopy of these fire-intolerant hardwoods indicates the lack of
destructive fire for many years (Clewell 1986). These areas may burn catastrophically in times of
drought (FNAI 2010).
Data Collection
Research questions 1 and 2. Did fire carry through a greater percentage of seepage
slope area with or without hog disturbance? Did hog disturbed areas burn? Prescribed burns were
used to examine the effect of hog disturbance and fuel continuity on the extent of fire spread. We
analyzed fire spread at 21 seepage slopes, some burned more than once (n=37), on Eglin AFB in
2010, 2012, and 2013. Areas where hog foraging resulted in discontinuous fuels were captured
as polygons using a handheld Trimble GPS. These sites, ranging in size from 2,549m2 to
16,316m2, were then burned by fire crews for management purposes, as instructed by Jackson
Guard. After each burn was extinguished, any areas of seepage slope vegetation not consumed in
the fire were captured as polygons using the Trimble GPS.
Research question 3. Was total foliar cover prior to burning greater in plots that carried
fire compared to those that did not? Burn histories for 38 control exclosure plots (no recent hog
disturbance) used in the intensity experiment and 61 experimental disturbance treatment plots
used in the frequency experiment (Chapters 4 & 5) were also examined to determine if prior to
burning unburned plots had less foliar cover than burned plots. Total foliar cover prior to the
burn was recorded at sites burned between 2010 and 2013. Plots were categorized as burned or
unburned after prescribed fires.
Data Analyses
slope area with or without hog disturbance? Did hog disturbed areas burn? ArcGIS 9.2 software
20
(ESRI 2009) was used to determine if there was a difference in: 1) the percentage of hog
disturbed seepage slope that burned and the percentage of undisturbed seepage slope that burned;
and 2) the percentage of hog-disturbed area that burned and did not burn. Areas of hog
disturbance, areas left unburned after the fire, and total seepage slope areas were calculated. The
analysis tool "Union" was used to compute the geometric intersection of the three input layers.
From this output layer, the percentage of the hog disturbed seepage slope that did and did not
burn and the percentage of undisturbed seepage slope that did and did not burn was calculated.
Student’s t-tests (PROC TTEST; SAS 9.3; SAS Institute 2011) was used to reveal any
differences in these percentages.
fire compared to those that did not? Total cover from plots used in the intensity and frequency
experiments at sites that burned was also analyzed using a Student’s t-test (PROC TTEST; SAS
9.3; SAS Institute 2011) to discern any difference in total foliar cover prior to burning in burned
and unburned plots.
Results
slope area with or without hog disturbance? Did hog disturbed areas burn? Soil disturbance
caused by hog rooting is associated with reduced fire spread. Prescribed fires burned
significantly more area in undisturbed portions (75.80%) than hog-disturbed portions (21.04%)
of the slopes (t<0.0001). Also, significantly more hog disturbed area was unburned than burned
after prescribed fire (t<0.0001). This study found that 77.32% of hog disturbed portions of
seepage slopes remained unburned after prescribed burns.
fire compared to those that did not? Pre-burn total foliar cover was significantly lower in plots
21
that did not burn than plots that burned (t<0.0001). Prior to the burn, average total foliar cover of
burned plots was 71.31%, while unburned plots averaged 39.15% cover. This indicates that there
is a threshold between approximately 40% and 70% total foliar cover below which fire spread is
reduced.
Discussion
The literature lacks research concerning indirect effects of introduced herbivores on
ecosystem processes (Stritar et al. 2010), whereas most studies quantify the effects of native
herbivores on fire regime in African ecosystems (Van Langevelde et al. 2003; Waldram et al.
2008; Holdo et al. 2009). As we hypothesized, vegetation in areas of hog disturbance remains
unburned and is associated with reduced fire spread in seepage slopes. Non-indigenous hog
foraging exposes bare ground (Bratton 1975; Kotanen 1995; Engeman 2007; Doupé et al. 2010)
and reduces fine fuel loads, ultimately reducing fire spread. This research indicates that this
novel disturbance affects ecosystem processes in this wet prairie community.
We categorized hog disturbance with less than 40% total foliar cover as high intensity
hog disturbance in Chapter 3. After three years of recovery, areas of high intensity hog
disturbance failed to have average total foliar cover of 70% or greater. This study found that
there is a threshold of between 40% and 70% total cover, below which fire is likely not to spread.
It can be interpreted from these data that areas of high intensity hog disturbance are less likely to
carry fire. Future research may be necessary to investigate the extent of high intensity hog
disturbance in seepage slopes to quantify the degree of this fuel reduction problem.
Reduced fire spread has significant implications for the physiognomy of the seepage
slope. In the absence of fire, the longleaf pine ecosystem experiences increased hardwood
density and decreased herbaceous cover and diversity (Frost et al. 1986; Hermann, 1995; Frost
2000; Lett & Knapp 2003; Glitzenstein et al. 2003; Bond & Keeley 2005; FNAI 2010). As a
22
result, most indicator species groups for bogs and wet savannas (i.e., forbs, sedges, insectivorous
plants) are extremely sensitive to reductions in fire frequency (Glitzenstein et al. 2003). Woody
plants may set up positive feedback that facilitate further woody invasion (Schlesinger et al.
1990; Callaway & Davis 1998), making restoring herbaceous dominance in shrub-encroached
areas unlikely even with the reestablishment of the historic fire regime (Archer 1989; Lett &
Knapp 2003; Briggs et al. 2005).
Losses of this community due to changes in physiognomy in the past have not been well
documented. Like other wet prairies of the southeastern United States that occur on seasonally
saturated soils, seepage slopes can be spatially narrow. Much of the acreage of this wet
community is limited to narrow areas that are less than 50 meters wide between the pyrogenic
upland savanna and deep wetlands. Wet prairies, including seepage slopes, also share a
considerable number of species with the adjacent communities. For these reasons, many authors
have regarded wet prairies as ecotones rather than distinct communities. Thus, losses in this
community due to woody encroachment and other anthropogenic activities have been overlooked
by conservationists (Clewell et al. 2009).
We acknowledge that fire behavior is dynamic by nature, influenced by fuel conditions,
weather or climate, and wind conditions (Cheney et al. 1993; Turner et al. 1994; Brooks et al.
2004) and topography (Weber 1990; Turner et al. 1994; Brooks et al. 2004). While we did not
control for environmental conditions, we found that fuel continuity is a major predictor of fire
spread in seepage slopes.
Summary
Feral hog foraging creates a complex network of novel ecosystem linkages and feedbacks
that have both direct and indirect effects on the biological and physical components of the
ecosystem. Yet, research on indirect effects of this exotic species on community structure and
23
ecosystem function is limited (Barrios & Ballari 2012). Our research indicates that feral hogs
alter fuel continuity in the landscape, indirectly altering the fire regime, and potentially resulting
in changes in community structure. Implications of this finding necessitate management plans to
remove hogs to lower densities or eradicate when possible in an attempt to reduce impacts and
preserve community structure. Further investigations, involving both direct and indirect effects
of feral hog disturbance, should involve a whole-ecosystem approach (Barrios & Ballari 2012).
24
Figure 2-1. The location of seepage slope sites on Eglin AFB.
Figure 2-2. Topographic relationship between seepage slope, upland pine, and baygall
communities including position of zones on seepage slopes.
25
CHAPTER 3
LONG-TERM MONITORING OF FERAL HOG DISTURBANCE IN SEEPAGE SLOPES
Background
Management options to address feral hog damage are limited, with trapping/hunting used
as the primary means to address the threat to natural ecosystems. The effectiveness of hog
removal at reducing hog densities and density-dependent damage is mixed. A study in 2007
assessed the effect of three years of hog trapping on hog disturbance in seepage slopes on Eglin
AFB in northwest Florida. The findings suggest that removal of hogs from the reservation has
dramatically reduced the extent of hog disturbance in seepage slopes (Engeman et al. 2007). In
contrast, recent research shows compensatory reproduction and immigration to increase in
response to hog removal utilizing traps and ground shooting (Hanson et al. 2009; Sparklin et al.
2009). Thus these common management techniques may see limited success in reducing hog
densities due to a density-dependent increase in recruitment (likely immigration) that can exceed
removal rates (Hanson et al. 2009). Hogs also have a high reproductive capacity (Dzieciolowski
et al. 1992; Choquenot et al. 1996; Taylor et al. 1998). Models indicate that pre-removal
densities can be reached in a short period of time if removal techniques do not remove a high
proportion of the population (i.e., greater than 70% instantaneous kill) (Dzieciolowski et al.
1992). Another concern is that trapping success is reduced when hogs become "trap shy" after
previous experience with traps (Saunders et al. 1993; Richardson et al. 1997). Populations of
hogs can rebound after the initial reduction. This failure to reduce hog numbers can then
correspond to failure to reduce hog disturbance (Hone 1995; Sweitzer 1998; Hone 2002;
Sweitzer & Van Vuren 2002).
Monitoring data can be used by managers to determine: 1) if a system is departing from
the desired state; 2) the success of management actions; and 3) the effects of disturbances (Legg
26
& Nagy 2006). At Eglin AFB, seepage slope monitoring was initiated in 2003 to evaluate the
extent of hog disturbance and the success of hog trapping at reducing that extent (Engeman et al.
2007). As with most monitoring programs, there were no true control sites, as half the sites were
trapped and half the sites were sport-hunted. This design can preclude conclusive results. The
observed change in hog disturbance after hog trapping began in seepage slopes on Eglin AFB
was attributed to the trapping (Engeman et al. 2007). Yet change is inherent in all ecosystems, so
the observed change may be unrelated to the management action (Legg & Nagy 2006) (i.e., hog
trapping). Without repeated and contemporaneous sampling before and after the initiation of the
management action at control and treatment sites, causality can only be inferred when a plausible
mechanism is identified and reasonable alternative mechanisms are investigated and rejected
(Schroeter et al. 1993). Also, monitoring data involving hog population dynamics should be
analyzed over a long-term time scale because hog populations can experience great interannual
fluctuation in relation to food availability (Peine & Farmer 1990; Brisbin & Sturek 2009).
The objectives of Chapter 3 are to investigate the long-term fluctuations in extent of hog
disturbance and corresponding floristic changes in seepage slopes on Eglin AFB. We analyze
environmental factors and hog removal data that may affect variability in interannual patterns of
hog disturbance. Finally, we explore the spatial patterns of hog damage within the complex
hydrological gradient within seepage slope wetlands and assess implications of damage across
that gradient.
Methods
Study Area
This research was conducted on Eglin AFB in the western Florida Panhandle
(30°29′22″N 086°32′32″W) where the mean annual temperature is 18.3oC and mean annual
precipitation is 158cm (DoD-Air Force 1995). Precipitation is greatest in the summer months
27
(NOAA 2002). Eglin AFB is a 187,548 ha reservation that represents the largest remnant of
Pinus palustris Mill. (longleaf pine) ecosystems (Compton et al. 2006; Mitchell et al. 2009). A
variety of birds and mammals, including feral hogs, can be hunted for sport on 105,218 ha of
Eglin AFB during hunting season, from mid-October until mid-February (United States Air
Force 2002). Sites were located on the eastern portion of Eglin AFB where soils are of the
Citronelle Formation, the loamy sand remains of a Pliocene-Pleistocene age delta plain. The
gently rolling erosional hills have relief of up to about 30 meters and areas of steeper slope along
creeks (NRCS 1989). The clay component in these sandy soils creates more mesic conditions
than the similar, nearby xeric P. palustris sandhills. These mesic longleaf pine woodlands are
classified as Upland Pine Forest (FNAI 1990).
Embedded in the upland pine community are seepage slopes. Seepage slopes are wet
prairies located on hillsides (Folkerts 1982). Soil saturation is sustained by the downslope
seepage of groundwater above a relatively impermeable layer of clay, organic matter, or rock.
The impermeable layer prohibits groundwater penetration and results in seeps along the hillsides
of dendritic systems of streams. The characteristic dense and diverse herbaceous vegetation
includes Rhynchospora spp. (Beaksedges), Aristida stricta Michx. (Wiregrass), Ctenium
aromaticum (Walter) A.W. Wood (Toothachegrass), Dichanthelium spp. (Witchgrasses), Xyris
spp. (Yelloweyed Grasses), Rhexia spp. (Meadowbeauties), Eriocaulon spp. (Pipeworts), and
Lycopodiella spp. (Club Mosses) (FNAI 2010). Carnivorous plants, including Sarracenia spp.
(Pitcherplants), Drosera spp. (Sundews), Pinguicula spp. (Butterworts), and Utricularia spp.
(Bladderworts), have evolved the ability to trap insects in order to acquire nutrients in these
anaerobic and acidic conditions that limit nutrient availability (Schnell 1976). Frequent fire
within the seepage slope maintains shrubs at a reduced height. In the absence of frequent fire,
28
Cliftonia monophylla (Lam.) Sarg. (Black Titi) and other shrubs invade upslope from the shrub
bogs or baygalls found along the seepage creek (Figure 3-1) and outcompete the heliophytic,
herbaceous vegetation. These wetlands dry out in times of drought due to their dependence on
groundwater (FNAI 2010).
Data Collection
Research questions 1 and 2. Has the extent of hog damage and its effect on foliar cover
changed from 2002-2012? Are hogs preferentially disturbing a particular zone of the slope? In
2002, 222 permanent plots were established at 24 randomly chosen, independent seepage slope
sites on Eglin AFB by UF (Figure 3-2). Half of these 24 sites were located in areas open to sport
hunting and half of the sites were located in areas closed to sport hunting. Each seepage slope
site was stratified by position on slope into three zones (i.e., lower, middle, upper) based on the
approximate distance from the seepage creek. Zonation represented a moisture gradient and was
delineated to account for differences in soil moisture and direct sunlight. Generally the lower
zone near the creek was equivalent to the toe-slope, the mid zone was equivalent to the footslope and the upper zone, furthest upslope, was equivalent to the mid-slope (Figure 3-1). The
lower zone was characterized by saturated, organic mud and seeping water. The middle zone,
although not present at every site due to size and topographic differences, had saturated organic
sands, but was drier than the lower zone. The upper zone consisted of sandy soils with low soil
moisture and organic content. The upper boundary of the slope was delineated by dry, sandy soil
and the dominance of upland species including a canopy of pine trees. Depending on the size of
the slope, up to five 1m2 permanent plots were randomly placed in each zone within each slope.
In the summer of 2002, 2010, and 2012, each permanent plot was characterized as
disturbed or undisturbed, depending on whether hog disturbance was present. A modified
Daubenmire scale was used to estimate cover of total vegetation, forbs, grasses, and woody
29
vegetation. The modified Daubenmire scale (0= 0%, 0.5-5 = 3%, 5-10 = 8%, 10-15 = 13%, 1525 = 21%, 25-50 = 38%, 50-75 = 68%, 75-95 = 88%, 95-100 = 98%) reflects the precision of
ocular estimation of foliar cover that can be expected with multiple observers and allows for data
comparison (Daubenmire 1959). Although there can be between-observer error, this study used
pairs of observers and observers trained by the same person in attempt to reduce observational
error to the greatest extent possible (Vittoz & Guisan 2007). Foliar cover of the keystone species
A. stricta was also estimated because it dominates the undisturbed upland pine system and
carries fire through dryer portions of the seepage slope (Noss 1989; Outcalt et al. 1999).
Land managers at Eglin AFB monitored additional seepage slopes, intermingled with
those established by UF. Both groups monitored ten of the same slopes. For the internal Eglin
monitoring project, twenty randomly selected, 1m2 plots were permanently marked with rebar at
each of 28 randomly chosen seepage slope sites on Eglin AFB in 2003. The sites were located
half in areas open to hog hunting and half in areas closed to hog hunting. Cover of hog
disturbance was estimated during surveys in 2003, 2004, 2005, 2007, and 2011. By 2010, seven
sites were excluded because access had become restricted. Data from the remaining 21 sites was
used for this study, bringing the total number of sites for the study to 34. Plots at the 21 Eglin
sites were also surveyed in 2010 by UF researchers.
Research question 3. Does hog removal or do environmental variables account for
changes in hog disturbance? The USDA APHIS Wildlife Services began removing hogs on Eglin
AFB in 2003. Hog removal primarily involved trapping with pen traps for euthanization, with
some control hunting (not sport hunting). Hog removal was conducted in areas closed to sport
hunting from 2003 to 2005 (Engeman et al. 2007). In more recent years, hog removal has been
extended to include areas open to sport hunting. The rainfall on Range C52N, approximately 4
30
kilometers north of the seepage slope sites, and number of hogs removed by trapping each year
were recorded by Eglin AFB The occurrence of prescribed burns in seepage slopes evaluated by
UF and Eglin AFB were also documented by Eglin AFB.
Data Analyses
Research questions 1 and 2. Has the extent of hog damage and its effect on foliar cover
changed from 2002-2012? Are hogs preferentially disturbing a particular zone of the slope? This
study consisted of a completely randomized design with repeated measures. Each seepage slope
site represented a replicate within which there were three fixed zones along a topographic
gradient. Main effects were zone (lower, middle, and upper), year (2002, 2010, and 2012), and
hunting access (open or closed). Significance of main effects and interactions were evaluated
using a repeated measures one-way ANOVA (α = 0.10) in a generalized linear mixed model
(GLIMMIX; SAS 9.3; SAS Institute 2011). Dependent variables were total foliar cover, forb
cover, grass cover, woody vegetation cover, A. stricta cover, and percent hog-disturbed plots.
Seepage slope sites were considered a random effect. Data was square root transformed to meet
assumptions of normality. Analysis of Eglin AFB data from 2003-2005 was previously published
by Engeman et al. (2007). Because the additional plots established by Eglin AFB were not
stratified by zones, analysis of Eglin AFB plots evaluated main effects of year (2007, 2010, and
2011) and hunting access (open or closed) on percent hog disturbed plots. Tukey–Kramer tests
were used for post hoc comparisons (α = 0.05) to analyze differences between means (Zar 2010).
Research question 3. Does hog removal or do environmental variables account for
changes in hog disturbance? Correlations and a multiple regression were used to determine
relationship between precipitation, hog removal, vegetation variables, and percent hog-disturbed
plots. UF and Eglin AFB data were combined for all Pearson correlations. The first correlation
was run (CORR; SAS 9.3; SAS Institute 2011) to detect correlation between percent of hog31
disturbed plots and number of hogs removed annually by trapping efforts. In addition, Pearson
correlation analyzed the relationship between total vegetation cover and: 1) rainfall during the
previous 11 months; and 2) time since last burn. Pearson correlation was then used to analyze the
relationship between percent of disturbed plots and total rainfall. The rainfall totals were from
the following time periods: 1) the 11 months prior to vegetation surveys; 2) Jan-May of each
vegetation survey year; 3) March-June of the year prior to vegetation surveys; and 4) August
through November of the year prior to vegetation surveys. Rainfall totals from the previous
spring (March-June) and fall (August-November) were investigated because hog breeding peaks
in the spring and fall in Florida (Giuliano 2009). Pearson correlation was used to evaluate the
relationship between number of hogs removed annually and rainfall totals in the previous spring
and fall. Pearson correlation was also used to identify correlation between time since burn and:
1) percent cover hog damage; 2) woody species cover; and 3) A. stricta cover. A multiple
regression was used to determine the relative importance of independent variables (including
percent hog-disturbed plots, time since burn, and rainfall totals from the previous AugustNovember) in determining total vegetation cover.
Results
Has the extent of hog damage and its effect on foliar cover changed from 2002-2012?
No clear long-term pattern in the percentage of plots disturbed by hogs was observed (Figure 33). For UF study slopes, there was no difference (p=0.6474) in the percent disturbed between
2002 and 2010 (Table 3-1). However, there was a decline in disturbed plots in 2012. As a result,
there was a significant difference in the percent disturbed from 2002 to 2012 (p=0.0387) and
2010 to 2012 (p=0.0034). There were significantly more hog-disturbed plots in areas open to hog
hunting (p=0.0222) compared to areas closed to hog hunting (Table 3-2). Although not stratified
by slope position, plots established and monitored by Eglin AFB showed an increase in the
32
percent disturbed from 2007 to 2010 (p<0.0001) and from 2007 to 2011 (p<0.0001) (Table 3-1).
Percent disturbed did not differ between 2010 and 2011 (p=0.7938), nor did it differ between
areas open and closed to hog hunting (p=0.6195).
For the most part, total vegetation cover reflected intensity and frequency of feral hog
damage and recovery rate. There was a significant interaction of year with zone (Table 3-4). For
UF slopes, vegetation cover declined from 2002 to 2012, but not in all zones (Figure 3-4). There
was significantly less total cover in the upper zone in 2010 (p=0.0020) and 2012 (p=0.0044)
compared to 2002. There was, however, no difference by year in lower or middle zones. In 2002,
total vegetation cover was significantly less in the lower zone than the upper zone (p=0.0443),
but indistinguishable from the middle zone (p=1.00). The total cover in the three zones did not
differ significantly in 2010 or 2012. There was no difference in total vegetation cover at sites in
areas open and closed to sport hunting (Table 3-2).
The mean cover of functional groups and A. stricta varied depending on zone and was
consistent across years (Figure 3-5; Table 3-4). Forb cover in the upper zone was significantly
less than cover in the lower zone (p=0.0006) and the middle zone (p=0.0002). Grass cover was
significantly less in the lower zone than the middle (p<0.0001) or upper zone (p<0.0001). The
same relationship was observed for A. stricta, with less cover in the lower zone than the middle
(p<0.0001) and upper zones (p<0.0001). In contrast, woody species cover was highest in the
lower zone and differed significantly (p=0.0010) from that of the middle zone. Woody cover in
the upper zone was also greater than the middle zone, although this difference was marginally
insignificant (p=0.0621).
Only cover of woody species and A. stricta differed during the ten years of this study.
Cover of forbs and grasses did not change significantly through time or by sport hunting access
33
(Table 3-4). Woody species cover increased significantly from 2002 to 2012 (p=0.0567) (Figure
3-6). A. stricta cover decreased significantly from 2002 to 2012 (p=0.0310) (Figure 3-7).
Are hogs preferentially disturbing a particular zone of the slope? Feral hog
disturbance was more extensive in the lower, wetter zone of seepage slopes. The percentage
disturbed differed significantly among the three zones (Table 3-1) and consistently across years.
Significantly more plots in the lower zone (50.92%) were disturbed when compared to the
middle (11.02%; p<0.0001) and upper zones (5.07%; p<0.0001). The percentage disturbed in the
middle and upper zones did not differ (p=0.1971).
Does hog removal or do environmental variables account for changes in hog
disturbance? When data from UF and Eglin AFB were combined, there was no correlation
between percentage of plots disturbed by hogs and number of hogs removed annually
(R2=0.0008; p=0.9111) or with the number of hogs removed the previous year (R2=0.0004;
p=0.8149) (Table 3-3). Total vegetation cover at sites, including those monitored by Eglin
managers, was found to have a weak, but significant negative correlation (R2=-0.0558;
p=0.0004) with total rainfall from the previous 11 months. Total vegetation was weakly
correlated (R2=0.0473; p=0.0034) to time since burn. The relationship between the percent of
hog-disturbed plots and total rainfall varied for the four time periods that were investigated.
There was no correlation (R2=0.0061; p=0.2501) between the percent of hog-disturbed plots and
total rainfall from the previous 11 months, from July through May of the sampling year (Table 33). Similarly, there was no relationship (R2=0.0036; p=0.3816) between the percent disturbed
and total rainfall during the five months prior to sampling, January through May. The percentage
of hog-disturbed plots also had no relationship (R2=0.0007; p=0.6951) with total rainfall from
March to June of the previous year, around the time of spring breeding. There was however, a
34
significant relationship (R2=0.0364; p=0.0047) observed with total rainfall from August through
November of the previous year, around the time of fall breeding. The number of hogs removed
was not correlated (R2=0.0335; p=0.9498) with rainfall during this fall time period. The number
of hogs removed from Eglin AFB annually was more strongly correlated (R2=0.7865; p=0.0635)
to rainfall during the previous March through June. Percent cover of hog damage in plots
monitored by Eglin AFB showed no significant relationship with time since burn (R2=0.0043;
p=0.4316). The foliar cover of woody species also had a significant positive relationship
(R2=0.1484; p<0.0001) with time since burn. A. stricta cover was not related (R2=0.0081;
p=0.2530) to time since burn. The multiple regression revealed percent hog-disturbed plots had
the most significant influence (p=0.0003) on total vegetation cover followed by rainfall totals
from the previous fall months (p=0.0054) and time since burn (p=0.0208).
Discussion
Has the extent of hog damage and its effect on foliar cover changed from 2002-2012?
This chapter offers insight concerning hog disturbance and vegetation dynamics in seepage
slopes on Eglin AFB. There was fluctuation in the percentage of disturbed plots over this tenyear period. Engeman et al. (2007) attributed the steady decline in hog disturbance from 2003 to
2005 to hog trapping. However, our data indicates hog disturbance was lower in 2002 and
increased the year hog trapping was initiated. In fact since hog trapping began, there were
periods of increased hog disturbance. Engeman et al. (2007) also indicated that there was more
hog disturbance in areas closed to hog hunting. On the contrary, our results indicate a higher
percentage of hog-disturbed plots in areas open to hunting. This discrepancy may be due to
difference in methods and/or sampling plots between the UF and internal Eglin monitoring
efforts. Our results do however indicate that more extensive monitoring is necessary to draw
conclusions on the effects of hog trapping on hog disturbance.
35
There is potential for hog disturbance to facilitate an increase in woody vegetation cover.
There was an increase in woody cover in the lower and upper zones where shrubs can encroach
from the bordering baygall and uplands respectively in the absence of fire. We found that hog
disturbance that disrupts the continuity of the herbaceous ground cover has the potential to
inhibit fire spread (Chapter 2) because the horizontal continuity of fuels can determine the extent
of fire spread (Brooks et al. 2004; Nader et al. 2007). If hogs migrate following available food,
recently disturbed areas are left to recover in terms of foliar cover. There may be more potential
for hog disturbance to reduce fire spread in places where prescribed fire is infrequent. If
vegetation has time to recover pre-disturbance foliar cover prior to prescribed fire, then it is more
likely that the fire will spread through the slope. While if hog disturbance occurs too frequently
for foliar cover to recover prior to a prescribed burn, then fire spread would be less likely,
providing opportunity for increased woody vegetation cover.
Although there was no change in total foliar cover of grasses, there was a decline in foliar
cover of A. stricta. A. stricta does not recover from intense soil disturbance due to a shallow root
system (Outcalt & Lewis 1990). As a disturbance sensitive species, it may be unable to recover
with continued hog disturbance. Chapter 5 indicates frequent experimental disturbance results in
a decline in A. stricta cover. In seepage slopes on Eglin AFB, more ruderal species, including
Dichanthelium spp., may recolonize disturbed areas more quickly and contribute to the
maintenance of grass cover (Chapter 5). This decline is important because A. stricta acts as a fine
fuel source to carry fire (Noss 1989; Haywood & Harris 1999). Further monitoring is necessary
to determine if this decline is continuing.
There was a significant difference in total forb and grass cover by position on slope. We
found less forb cover in the upper zone than the other two zones, while grass cover in the lower
36
zone was less than the other two zones. This represents the hydrologic gradient (Clewell et al.
2009) inherent to the slopes. The upper zones of seepage slope are generally drier and dominated
by characteristic grass species including A. stricta, C. aromaticum, and Muhlenbergia expansa
(Lam.) Trin (Cutover muhly). The lower zones have longer hydroperiods and are typically
dominated by forb species including Sarracenia spp., Drosera spp., Xyris spp., Eriocaulon spp.,
Pleea tenuifolia Michx. (Rush Featherling), and Lophiola aurea Ker Gawl (Golden Crest).
(FNAI 2010).
Are hogs preferentially disturbing a particular zone of the slope? Hogs commonly
forage in and disturb wet or mesic habitats (Bratton 1975; Wood & Brenneman 1980; Saunders
& Kay, 1991, Mitchell & Mayer 1997; Welander 2001; Chavarria et al. 2007) where it is easier
to root and food is abundant (Mitchell et al. 2007a). Our research indicates that hogs are
preferentially disturbing areas in the lower, wetter portions of the seepage slopes, closer to the
seepage streams. This result suggests that degradation associated with hog disturbance, including
erosion (Bratton 1975; Howe & Bratton 1976), plant mortality (Bratton 1974; Bratton 1975;
Johnson 2001), and increased plant available nitrogen resulting from excrement deposition
(Ruess & McNaughton 1987), is not uniform across the landscape. Preference of hogs to root in
this portion of seepage slopes has serious implications for the persistence of listed plants (e.g.,
Sarracenia spp.) that inhabit the lower zone. Engeman et al. (2007) found hog disturbance to be
negatively correlated with the presence of Sarracenia leucophylla Raf. and Sarracenia flava L.
among other species of interest. This lower zone is also important refugia for rare and endemic
amphibians (e.g., Hyla andersonii (Pine Barrens tree frog), Eumeces anthracinus pluvialis
(Southern Coal Skink), and Rana okaloosae (Florida bog frog)) (Enge 1997). If hog impacts are
37
to be managed in seepage slopes on Eglin AFB, it is important to focus on protecting the lower
zone microhabitat (Mitchell et al. 2007a).
Does hog removal or do environmental variables account for changes in hog
disturbance? Although the number of hogs removed each year from Eglin AFB fluctuated, there
was no correlation with the percent of disturbed plots surveyed that year or the subsequent year.
The number of hogs trapped would not affect the amount of hog disturbance if there were a
density-dependent increase in recruitment (Hanson et al. 2009), but this was not quantified in our
study. It appears that other factors, in addition to hog trapping, may be driving the changes in the
amount of hog disturbance. Extrinsic variations in food availability (i.e., variation in food
availability due to environmental factors not related to the effects of density dependent animal
foraging) have been shown to influence hog population dynamics. Rainfall, an extrinsic factor,
can drive forage biomass and thus hog abundance (Choquenot 1998; Sweitzer et al. 2000). In our
study, rainfall totals from the months of the previous fall were significantly correlated with the
number of hog-disturbed plots. In Florida, feral hogs can breed throughout the year, but breeding
peaks in the spring and fall (Giuliano 2009). Greater precipitation during peak breeding months
may contribute to greater reproductive success and correspond to an increase in hog disturbance
the following year, after sows farrow. The correlation between rainfall during the previous spring
months and the number of hogs removed from Eglin AFB by trapping efforts also supports this
hypothesis. These results are consistent with research that found higher reproductive rates in hog
populations in response to abundant forage (Coblentz & Barber 1987), while drought and limited
forage delayed breeding, reduced reproductive rates (Giles 1980), and increased mortality
(Woodall 1983; Massei et al. 1997). Lower densities of hogs then corresponded to less hog
disturbance (Hone 1995; Sweitzer 1998; Hone 2002; Sweitzer & Van Vuren 2002).
38
Patterns in hog foraging and disturbance can also reflect movement between community
types following mast crop (Howe & Bratton 1976, Bratton et al. 1982). Hogs prefer to feed on
hard mast when it is available (Henry & Conley 1972). Heavy rooting for tubers and roots may
increase during years of poor mast because preferred acorns are not available (Bratton et al.,
1982). After extensive searching, we concluded that information concerning the timing of hard
mast (Quercus spp.) on Eglin AFB has not been recorded. It is known that P. palustris
experienced the first large mast event since 1996 in the winter of 2011 (pers. comm. 2012 Eglin
AFB land manager). Roots of longleaf pine seedlings are a preferred food of hogs (Wood &
Brenneman 1980). This mast event in 2011 increased availability of seedlings in the upland
habitat and may have contributed to the decline in hog disturbance observed in the seepage
slopes in 2012. Bratton et al. (1982) indicate that vegetation may have time to recover from
heavy rooting during good mast years when acorns are plentiful, but will be rooted again when
mast crops fail. Longer trend data, including data on mast crops, is needed in order to identify a
stronger trend in the amount of hog disturbance and its potential drivers.
The percent cover of hog disturbance was unrelated to time since burn, although a variety
of research indicates that herbivores (Vinton et al. 1993; Coppedge & Shaw 1998; Vermeire et
al. 2004) as well as omnivores (e.g., Chen caerulescens (Snow Geese)) (Brennan et al. 2005)
may selectively forage in recently burned vegetation. This could be due to preference for plants
with increased nutritional value (Lemon 1946; Pearson et al. 1972; Shindler et al. 2004) and/or
new growth resulting from the reduction of shade (Copeland et al. 2002). Lack of correlation
between hog rooting and seepage slope burning is potentially a result of one of four scenarios: 1)
hogs show no preference to forage in areas that burned, possibly because the ground is drier and
more difficult for rooting; 2) hogs are foraging for a food source (e.g., invertebrate) that is
39
unaffected by fire; 3) fire spread in seepage slopes is not extensive enough for differences to be
significant; or conversely 4) fire spread in seepage slopes is frequent, uniform, and complete,
leaving no unburned areas from which to differ.
Our results indicated that woody vegetation cover had a significant positive correlation
with time since burn. While fire halts woody recruitment, it is not always effective at reducing
the cover of woody species once they become dominate in seepage slopes. Perennial shrubs
resprout and persist in communities adapted to frequent fire. Mortality from fire only occurs in
the very small shrubs (Olson & Platt 1995). Low mortality and resilient resprouting of
established shrubs after burning may necessitate further management actions to reduce shrub
densities (Olson & Platt 1995; Drewa et al. 2002).
These data also indicate that total vegetation cover is a function of intensity and
frequency of feral hog disturbance and to a lesser extent, rainfall, and time since burn. Over the
past five years, Eglin AFB land managers have focused on increasing fire frequency in seepage
slope areas. Yet even with almost annual prescribed fire (pers. comm. 2013 Eglin AFB land
manager), there was only weak correlation between total vegetation cover and time since burn.
While it is likely that many factors affect total vegetation cover, percent of disturbed plots was
relatively the most important factor in our analysis. Recovery rate of vegetation was not
quantified, but is none-the-less important to total vegetation cover. Recovery via perennial
regrowth, vegetative spread, seedling establishment, and germination requires favorable
environmental conditions, such as adequate soil moisture (Outcalt et al. 1999). Even though there
is less hog disturbance in the drier portion of the slopes, there is lower soil moisture and thus less
recruitment, recovery, and ultimately less cover. This region experienced exceptional drought
conditions (D4) in June of 2011 and was abnormally dry in June of 2012 (D0) (National Drought
40
Mitigation Center 2013). The middle and lower zones of the slope experience longer
hydroperiods and may have had an advantage recovering due to increased soil moisture. No
difference in total cover by year in the lower or middle zones supports this idea.
Significantly less total vegetation cover in the lower zone reflects hog preference for the
wetter portions of the slopes. The total cover in the three zones did not differ significantly some
years. One explanation could be that in years of heavier hog use, disturbance is more uniform
across the slope, while in years of light hog use, there is not enough disturbance for the
difference to be significant. Although hog use as measured by percent plot disturbed was greater
in areas open to hunting, this was not reflected in average total vegetation cover.
Summary
Hog management must involve long-term strategies. As with any ecological data with
inter-annual variation, long-term data are necessary (Krebs 1991; Troeng & Rankin 2005)
because there is the risk of drawing erroneous conclusions from short-term data. Our results
indicate that monitoring seepage slope vegetation and other environmental variables must
continue in order to understand the variability of feral hog disturbance and long-term
consequences. Annual adjustments should be made to management plans depending on
environmental factors, so that hog trapping is increased in years of: 1) mast failure when hogs
are forced to forage for other food sources (Bratton et al., 1982); or 2) high rainfall when hog
densities have the potential to increase (Coblentz & Barber 1987). We recommend that the lower
portions of seepage slopes be protected (e.g., using fence exclosures) because feral hogs target
this area where rare, endemic flora and fauna are found. Due to the increase in woody vegetation
cover we observed, there should also be a concerted effort to ensure fire spreads downslope into
seepage slope areas where fire may not spread unassisted. Larger, more established woody
vegetation may necessitate cutting or clearing by hand. Active management must continue and
41
address multiple facets in order to prevent further degradation of ecosystem structure and
function.
42
Table 3-1. Repeated measures ANOVA results for the percentage of hog disturbed plots over
time by zone on Eglin AFB from 2002 to 2012.
Plots
Percentage disturbed (%)
2002
2010
2012
Variable
Num df
df
F
P
UF
Eglin AFB
27.88
2007
31.13
30.40
2010
59.60
15.98
2011
57.51
Year
Zone
Year x Zone
2
2
4
122
122
122
5.54
47.63
0.46
p=0.0050
p<0.0001
p=0.7659
Year
2
38
32.76
p<0.0001
Table 3-2. Repeated measures ANOVA results for the percentage of hog disturbed plots over
time by hunting access on Eglin AFB from 2002 to 2012.
Plots
Open
Closed
Variable
Num df
df
F
P
Hunting Hunting
UF
38.25
16.67
Access
1
122
5.37
p=0.0222
Year x Access
2
122
1.98
p=0.1430
Eglin AFB
51.77
47.30
Access
Year x Access
1
2
38
38
0.45
1.00
p=0.5051
p=0.3781
Table 3-3. The number of hogs removed from Eglin AFB from October 1 of the previous year to
September 30 of that calendar year. The percentage of hog disturbed plots and rainfall
totals are also indicated.
Year
Hogs
Percentage
Rainfall
Rainfall
Rainfall
Rainfall
Removed Hog Disturbed Previous
JanuaryPrevious
Previous
by
Plots (%)
July-May May (in)
MarchAugustTrapping
(in)
June (in)
November
(in)
2002
0
27.88
UF
32.92
16.37
18.86
11.58
2003
0
47.62 Eglin
59.63
26.04
29.22
20.56
2004
439
36.11 Eglin
50.05
13.66
35.13
15.47
2005
189
34.92 Eglin
80.37
45.07
26.34
26.72
2007
68
31.13 Eglin
35.77
10.21
19.95
19.93
2010
261
44.75 Both
76.25
27.74
18.35
29.54
2011
106
57.51 Eglin
38.58
15.38
17.22
18.26
2012
110
15.98
UF
54.88
20.65
10.6
14.15
43
Table 3-4. Repeated measures ANOVA results for the cover of total vegetation, functional
groups, and A. stricta by zone and access on Eglin AFB in 2002, 2010, and 2012.
Percent Mean Cover (%)
2002
2010
2012
Variable
Num
df
F
P
df
Total vegetation
78.75
74.57
77.48
Year
Zone
Year x Zone
Access
Year x Access
Forb
23.69
18.90
22.97
Year
Zone
Year x Zone
Access
Year x Access
Grass
44.09
38.39
39.98
Year
Zone
Year x Zone
Access
Year x Access
Woody
16.53
19.31
21.03
Year
Zone
Year x Zone
Access
Year x Access
Aristida stricta
25.91
16.89
21.63
Year
Zone
Year x Zone
Access
Year x Access
44
2
2
4
1
2
122
122
122
122
122
2.59
5.17
3.76
0.04
0.08
p=0.0794
p=0.0070
p=0.0064
p=0.8405
p=0.9263
2
2
4
1
2
122
122
122
122
122
1.86
11.25
0.69
0.33
0.32
p=0.1593
p<0.0001
p=0.6017
p=0.5652
p=0.7288
2
2
4
1
2
122
122
122
122
122
1.56
21.34
0.50
0.42
0.78
p=0.2144
p<0.0001
p=0.7366
p=0.5169
p=0.4592
2
2
4
1
2
122
122
122
122
122
2.94
6.85
1.09
3.35
0.26
p=0.0567
p=0.0015
p=0.3637
p=0.0696
p=0.7685
2
2
4
1
2
122
122
122
122
122
3.58
56.34
1.84
0.39
0.53
p=0.0310
p<0.0001
p=0.1249
p=0.5335
p=0.5908
45
Figure 3-3. Percent hog-disturbed plots from 2002 to 2012. Analysis included both UF and Eglin
AFB data.
Figure 3-4. Total foliar cover by year. Bars are mean foliar cover ±1 SE, the P-values can be
found in Table 3-4, and different letters above bars indicate significant difference (α =
0.05) in foliar cover following Tukey–Kramer post-hoc tests.
46
Figure 3-5. Foliar cover of functional groups and A. stricta by zone (i.e., position on slope). Bars
are mean foliar cover ±1 SE, the P-values can be found in Table 3-4, and different
letters above bars indicate significant difference (α = 0.05) between zones following
Tukey–Kramer post-hoc tests.
Figure 3-6. Foliar cover of woody species through time. Bars are mean foliar cover ±1 SE and
the P-values can be found in Table 3-4.
47
Figure 3-7. Foliar cover of Aristida stricta through time. Bars are mean foliar cover ±1 SE and
the P-values can be found in Table 3-4.
48
CHAPTER 4
EFFECTS OF INTENSITY OF FERAL HOG DISUTRBANCE ON VEGETATION
DYNAMICS
Background
Herbivores have the potential to greatly modify their habitat (Bratton 1975; Tardiff &
Stanford 1998) in a manner that can be either beneficial or detrimental to ecosystem function and
structure depending on the resident plant species. Herbivory can benefit some plant species when
dominant plants are targeted and the competitive interaction between plants is reduced. Thus,
competitive dominance is prevented and species diversity enhanced (Lubchenco 1978).
Herbivores can also increase spatial and temporal variation in the resources with localized
excrement deposition and soil disturbances resulting from digging (Tardiff & Stanford 1998),
trampling, and wallowing. These activities create competition-reduced gaps that can also have a
positive effect on seedling survival (Silvertown & Bullock 2003). On the contrary, herbivory
targeting competitively inferior species or that is particularly intense can reduce species
diversity. This disturbance may result in the dominance of only a few tolerant species
(Lubchenco 1978). Selective herbivory can also reduce preferred plant species (Bratton 1974;
Recher & Clark 1974; Challies 1975; Howe & Bratton 1976). Depending on the system, species,
and herbivory regime, herbivore effects on plant community composition and diversity can vary
greatly (Maschinski & Whitham 1989).
The modern, Pinus palustris Mill. (longleaf pine) savanna ecosystem of the southeastern
United States came into existence in the early Pleistocene, approximately 2 million YBP. Plant
species evolved under grazing and browsing disturbance from megaherbivore including horses,
tapirs, peccaries, proboscideans, ground sloth, and various ruminants. Modern characteristics of
the P. palustris (i.e., masting, strong taproot and wood) can be interpreted as adaptations to
defend against these megaherbivores (Means 2006). Other modern vegetative characteristics of
49
this system still reflect coevolution with megafauna, such as seeds of herbaceous species being
viable after being digested by livestock (Janzen 1984). However, disturbance resulting from
large herbivores ended after the mass extinction of these animals at the end of the Pleistocene,
approximately 11,000 YBP (Grayson & Meltzer 2002). The Bison bison (American buffalo)
migrated into the Gulf Coastal Plain in the late sixteenth century, but populations were extirpated
in the early nineteenth century (Rostlund 1960). After the Pleistocene extinction of the
megafauna, it is likely that vegetation experienced an ecological release greater than the
influence extant megafauna had on savanna plant characteristics or distributions (Means 2006).
One thing that has not changed through time in this system is the dependence of the P.
palustris-grasslands on fire (Watts 1971; Komarek 1965, 1968, 1974; Myers 1990; Stout &
Marion 1993; Means 2006). Lightning-ignited fire was a major component of the climate,
enough so to frequently burn large expanses and to shape the evolution of pyrophytic vegetation
long before aboriginal peoples (Komarek 1965). Paleoindians were not widespread in the
southeast until 8,500-5,000 BP. It can be inferred that by that time, human-ignited fire
contributed to an increased frequency of fires preventing the pine savanna from succeeding to
broad-leaved forest (Watts 1971). Native Americans burned the forests frequently for cultivation,
hunting, reducing insects, and defense (Williams 1992; Kay 2007). Vegetation has evolved to
recover rapidly after the disturbance of high frequency (1-3 years), low intensity fire. Some
examples of fire-evolved traits include: the early growth of longleaf pine in a grass stage (Keeley
& Zedler 1998); fire induced germination (Wiggers 2011), vigorous resprouting of shrubs
following fire (Olson & Platt 1995); and increased flowering, seed production, and germination
of dominant matrix grasses after growing season fire (Shepherd et al. 2012). It seems that fire has
50
been the principal selective agent in the evolution of longleaf pine savannas of the southeast
(Frost et al. 1986).
Introduced herbivores, including feral hogs, present substantial potential for degradation
to native plant communities resulting from herbivory and altered disturbance regimes (Singer et
al. 1984; Van Vuren & Coblentz 1987; Hobbs & Huenneke 1992; Kotanen 1995). We propose
hogs could either replace the role of now-extirpated megaherbivores or represent a novel
disturbance that results in ecosystem change. In this chapter, we investigate the short-term
floristic response after different intensities of hog disturbance in seepage slopes. We examine the
effect of disturbance intensity on cover, richness, and species composition over a three-year
period. We hypothesize that over the short-term high hog disturbance intensity reduces cover,
species richness, and alters species composition when compared to undisturbed vegetation.
Methods
Study Area
Eglin AFB is located in the western Florida Panhandle, Walton County, USA
(30°29′22″N 086°32′32″W). Seepage slope sites are restricted to the loamy sand soils of the
eastern portion of the AFB. It is here where gently rolling erosional hills have been cut into the
remains of a Pliocene-Pleistocene age delta plane. These dendritic drainage systems have relief
of up to about 30 m and areas of steeper slope along creeks (NRCS 1989). The relatively higher
clay content of these soils, compared to the deep sands of xeric P. palustris sandhills, results in
more mesic conditions. This mesic upland community is classified as Upland Pine Forest (FNAI
1990).
Seepage slopes are grass and sedge dominated wet prairies located on hillsides in the
upland pine community (Figure 4-1) (FNAI 1990). The nutrient poor soils are kept saturated by
the downslope seepage of groundwater above an impermeable layer of clay, organic matter, or
51
rock. Seeps occur along hillsides of these dendritic systems of streams where the impermeable
layer intersects the hillside. During times of drought, seepage slopes may dry out due to this
dependence on groundwater (FNAI 2010).
A diverse suite of herbaceous species inhabit this wet prairie community. Aristida stricta
Michx. (Wiregrass), Ctenium aromaticum (Walter) A. Wood (Toothachegrass), Muhlenbergia
expansa (Lam.) Trin. (Cutover Muhly), and Rhexia spp. (Meadowbeauties) can be found upslope
in the drier soils, while Rhynchospora spp. (Beaksedges), Xyris spp. (Yelloweyed Grasses),
Eriocaulon spp. (Pipeworts), and Lycopodiella spp. (Club Mosses) inhabit the wetter portions of
the slopes (FNAI 2010). Carnivorous plants that acquire nutrients by trapping insects (Schnell
1976) can be abundant, including Sarracenia spp. (Pitcherplants), Drosera spp. (Sundews),
Pinguicula spp. (Butterworts), and Utricularia spp. (Bladderworts). Shrubs are maintained at a
reduced height by frequent fire that burns down slope from the uplands. Cliftonia monophylla
(Lam.) Sarg. (Black Titi) and other shrubs invade from the shrub bogs or baygalls along the
seepage creek, outcompeting the heliophytic, herbaceous vegetation in the absence of frequent
fire (FNAI 2010).
Data Collection
Research questions 1, 2, and 3. Does the difference in foliar cover between disturbance
intensities persist over time? Does species richness differ in areas of varying disturbance
intensity? Does species composition differ in areas of varying disturbance intensity? The
intensity of feral hog foraging and wallowing activity varies within seepage slopes on Eglin
AFB. In 2010, 20 independent, replicate seepage slope sites with the varying levels of hog
disturbance were randomly chosen on Eglin AFB (Figure 4-2). Three hog disturbance intensities
were identified based on percent of uprooted vegetation and total vegetation cover. The
intensities were: 1) undisturbed (control), with less than 3% uprooted and total cover of 7552
100%; 2) moderate, with <50% uprooted and total cover of 50-75%; and 3) high, with >50%
uprooted and total cover of 0-50%. Potential plot locations were restricted to flat, open slopes to
minimize the influence of aspect. Plots were placed within the middle zone of the slope to
minimize the effect of the moisture gradient and restricted to the area of the slope with different
intensities of hog damage within close proximity. At each seepage slope site, two locations were
randomly selected for placement of wire fence exclosures (6m in circumference and 1m in
height) within each disturbance intensity (control/undisturbed, moderate, and high). Exclosures
prevented subsequent hog damage. Within each exclosure one 1 m2 permanent plot was
established. Adjacent to each exclosure (approximately .5 m downslope) an open 1 m2plot was
permanently marked (Figure 4-3). This resulted in approximately six open and six exclosure
plots at each site. The two subsamples accounted for within site variability. Two of the 20 sites
only contained two disturbance intensities due to the nature of hog disturbance at the time of plot
initiation. Between 2010 and 2013, eight exclosure plots were compromised by hog disturbance
and removed from the study.
In August of 2010 and 2012, total foliar cover and cover of functional groups was
determined using a modified Daubenmire scale (0= 0%, 0.5-5 = 3%, 5-10 = 8%, 10-15 = 13%,
15-25 = 21%, 25-50 = 38%, 50-75 = 68%, 75-95 = 88%, 95-100 = 98%) within 1 m2 plots inside
and outside exclosures. Total number of species present was recorded. Due to high species
richness, percent cover by species was estimated within ¼ of each 1 m2 plots. Species
nomenclature was based on Wunderlin & Hansen (2008). In August of 2013, total foliar cover
only was estimated in 1 m2 plots inside and outside exclosures.
Wildlife cameras with motion sensors recorded hog activity at 21 seepage slopes (Figure
4-4). Hog observations were tallied independent of size of group or length of time spent foraging
53
at site. Cameras were checked every two to three months to download images and replace
batteries. Over the course of the four-year study, cameras were either stolen (7), burned in
prescribed fire (1), or eventually became inoperable after prolonged exposure to the elements (4).
These losses, in combination with inaccessibility of areas due to military operations and removal
of cameras for prescribed burns, resulted in limited and intermittent data acquisition. This data
represents a conservative estimate of hog activity.
Data Analyses
Research questions 1 and 2. Does the difference in foliar cover between disturbance
intensities persist over time? Does species richness differ in areas of varying disturbance
intensity? The experimental followed a randomized, split plot repeated measure design. Within
each seepage slope site, initial intensity of hog disturbance (control, moderate, and high)
represented whole plots and exposure to subsequent hog disturbance (exclosure vs open) were
split plots. Significance of main effects and interactions (α= 0.10) were evaluated using a
repeated measure one-way ANOVA (PROC GLIMMIX; SAS 9.3; SAS Institute 2011) to
determine significance of recovery with and without potential for further hog disturbance. Means
were arcsine square root transformed when necessary to meet assumptions of ANOVA (Zar
1998). Year, intensity, and exclosure were the main effects. Site was considered a random effect.
Tukey–Kramer tests were used for post hoc comparisons (α= 0.05) to analyze differences
between means. The average number of hog occurrences per month was calculated from the
camera data.
Research question 3. Does species composition differ in areas of varying disturbance
intensity? NMS was used to better understand temporal changes in species of interest with
respect to disturbance intensity and edaphic conditions. We removed all species that occurred in
less than 5% of the plots and grouped all Andropogon spp. (Bluestems), Asteraceae spp. (Asters),
54
Eriocaulon spp., Rhexia spp., Rhynchospora spp., Sarracenia spp., Scleria spp. (Nutrushes), and
Xyris spp. Moderate disturbance intensity plots were removed to reduce overlap with high
intensity and control plots. A Bray-Curtis distance measure was used for the ordination in PCORD. Monte Carlo tests were used to determine if the ordination results were significantly
different from random data. Then we used MRPP to test for differences within and between
disturbance intensity groups (McCune & Grace 2002).
Results
Foliar Cover
Does the difference in foliar cover between disturbance intensities persist over time?
Hog disturbance intensity significantly altered vegetation cover over time (Figure 4-5; Table 41). Foliar cover in high disturbance plots (inside and outside exclosures) increased significantly
(p<0.0001) from 2010 to 2013. There was no significant change in cover in control (p=0.8185)
or moderate (p=0.9994) disturbance plots. Total foliar cover differed among all intensities in
2010 (Year 1): high intensity plots had less cover than control plots (p<0.0001) and moderate
disturbance plots (p<0.0001); while control plots had greater total foliar cover than moderate
(p<0.0001). By 2013, moderate disturbance plots still differed significantly from control
(p=0.0670) and high (p=.0609) disturbance plots. Cover in high disturbance plots also continued
to be less than control (p<0.0001) plots. Initially in 2010, there was no difference (p=0.51) in
foliar cover inside (68.55%) and outside (64.22%) exclosures (Figure 4-6). By 2012, foliar cover
increased both inside (p<0.0001; 83.02%) and outside (p=0.0011; 73.04%) exclosures. However
by 2013, continued, albeit less intense and frequent, hog activity resulted in significantly greater
(p<0.0001) total foliar cover inside (77.19%) compared to outside (68.39%) exclosures.
Cover of woody species increased in plots inside and outside exclosures in all disturbance
intensities from 2010 to 2012 (Figure 4-7; Table 4-1). Initially, high disturbance plots did not
55
differ from moderate disturbance plots (p=0.9952), but had less woody cover than control plots
(p=0.0553). Over time, the only significant change (p<0.0001) took place in high disturbance
plots, where cover of woody species more than doubled. There were no significant differences
between the three disturbance intensities by 2012: high and control (p=0.9995); high and
moderate (p=0.9060); and control and moderate (p=0.9781). There was also no significant
difference between plots inside and outside exclosures.
Intensity of disturbance was a determining factor in foliar cover of forbs (Table 4-1).
Control plots had higher forb cover (34.01%) than high (19.34%; p<0.0001) and moderate
(25.04%; p=0.0207) disturbance plots. There was no difference (p=0.1573) between high and
moderate disturbance plots. Forb cover inside and outside exclosures did not differ significantly.
Foliar cover of grass species changed over time with disturbance intensity (Figure 4-8;
Table 4-1). High disturbance plots (inside and outside exclosures combined) had less grass cover
than control (p<0.0001) and moderate (p=0.0002) disturbance plots in 2010. Control plots did
not differ from moderate (p=0.7272) disturbance plots. By 2012, high disturbance plots still had
significantly less grass cover than control (p=0.0404) and moderate (p=0.0139) disturbance
plots. There was still no difference (p=0.9991) between control and moderate disturbance plots.
Initially in 2010, there was no difference (p=0.4491) in grass cover inside (35.71%) and outside
(32.08%) exclosures (Figure 4-9). By 2012, foliar cover increased both inside (p<0.0001) and
outside (p<0.0001) exclosures. This increase in grass cover resulted in significantly greater
(p=0.0006) grass cover inside (53.27%) compared to outside (43.97%) exclosures.
The cover of A. stricta differed by disturbance intensity with no effect of time (Table 41). High disturbance plots had the lowest cover (7.58%) of A. stricta. The high disturbance plots
had significantly less cover than the control (22.42%; p<0.0001) and moderate (16.51%;
56
p=0.0224) disturbance plots, while control and moderate disturbance plots did not differ
(p=0.1602). Exclosures had greater A. stricta cover (16.27%; Table 3-1) than outside exclosures
(14.74%) throughout the study.
Richness
Does species richness differ in areas of varying disturbance intensity? Species
richness was dependent on time since disturbance, but not disturbance intensity (Table 4-1).
There was no difference in species richness among disturbance intensities at the start of the study
(high and control, p=0.1617; high and moderate, p=0.7780; control and moderate, p=0.8711).
The number of species per plot increased significantly from 2010 to 2012 for all intensities (high,
p<0.0001; moderate, p<0.0001; control, p=0.0017). Still at the end of the study, no significant
difference in richness was detected between the intensities (high and control, p=1.0; high and
moderate, p=0.9975; control and moderate, p=0.9925). There also was no difference between the
species richness inside (15.16) and outside (15.36) exclosures.
Camera Detection of Hog Activity
Hog detection by the cameras fluctuated a great deal, but there was a decreasing trend
from 2010 until the end of 2011 (Figure 4-10). The average number of hog occurrences detected
by cameras was lower in the summer months. Overall, hog occurrences increased from 2012 to
the highest numbers in the fall of 2013.
Vegetation Composition
Does species composition differ in areas of varying disturbance intensity? MRPP
indicated that there were significant differences in vegetation composition depending on soil type
when all seepage slope sites were included in the analysis (T=-8.1775; A=0.0349; p<0.0001).
The subsequent multivariate analyses focused on 10 sites located on the Leefield-Stilson Loamy
57
Sands soil type. This soil type was selected in an attempt to reduce variability and retain the
largest sample size possible.
NMS ordination and MRPP revealed differences in species composition. Vegetation
composition at sites located on the Leefield-Stilson Loamy Sands soil type differed between high
disturbance and control plots when years were combined (T=-56.9861; A=0.3296; p<0.0001;
Figure 4-11). A two-dimensional ordination was optimal (stress = 6.1094) with axis 1 explaining
82.7% and axis 2 explaining 15.9% of the variation. Monte Carlo test determined that the
ordination differed from random data (p=0.0164). Comparable results were observed when years
were analyzed individually. Both of these analyses had three-dimensional solutions. When 2010
data was analyzed separately (stress=17.8216), axis 1 explained 11.9%, axis 2 explained 24.4%,
and axis 3 explained 37.6% of the variation observed. When 2012 data was analyzed separately
(stress=19.0722), axis 1 explained 20.0%, axis 2 explained 20.6%, and axis 3 explained 31.5% of
the variation observed. This analysis revealed that the difference between disturbance intensities
and separation between groups decreased over time as the high intensity disturbance recovered
(2010: T=-16.6659, A=0.0616, p<0.0001; 2012: T=-8.8013, A=0.0284, p<0.0001, Figure 4-12a
and 4-12b).
Discussion
Does the difference in foliar cover between disturbance intensities persist over time?
Three years of field observations suggest that heavily hog-disturbed areas differ from
undisturbed areas in both cover and composition. The most obvious change resulting from hog
disturbance is loss of total foliar cover (Bratton 1974, 1975; Challies 1975; Cuevas et al. 2010).
Even after three years of recovery inside exclosures, high intensity disturbance plots still have
significantly less cover than moderately disturbed and control plots. Although theoretically
successional communities maintained by frequent disturbance should recover more quickly than
58
"climax equivalents" (Baron 1982), these results indicate that more time is necessary before
heavily disturbed seepage slopes areas are indistinguishable in terms of total cover.
Total cover increased over time both inside and outside exclosures due to reduced hog
foraging. This region experienced moderate to exceptional drought conditions (D1-D4) from
October 2010 to June 2012 (National Drought Mitigation Center 2013). We discussed previously
in Chapter 3 that rainfall drives forage biomass and can influence hog abundance (Choquenot
1998; Sweitzer et al. 2000). Drought and limited forage delays breeding, reduces reproductive
rates (Giles 1980), and increases mortality (Woodall 1983; Massei et al. 1997). A drought and
subsequent decline in hog numbers corresponds to less hog disturbance outside exclosures and
likely explains the observed increase in cover outside exclosures. Our camera data also
substantiates this idea with fewer hog occurrences (i.e., images) from 2011 through 2012.
However with an increase in hog activity and disturbance in 2013, our exclosure study showed
that continued hog disturbance outside exclosures was enough to result in significantly lower
total foliar cover compared to inside exlosures.
Hog foraging has the potential to have different effects on woody cover at different
scales. Uprooting vegetation, including woody species, reduces cover at the small scale. In the
absence of intense and/or frequent hog disturbance during our study, the cover of woody species
increased significantly over time in heavily disturbed plots as vegetation recovered. At the large
scale, hog rooting reduces herbaceous cover that acts as fuel to carry fire (Chapter 2). Woody
species then have an opportunity to become established with reduced fire spread from the
uplands, increasing woody cover at the large scale. This idea is discussed in further detail in
Chapter 2. The effect of exclosures on woody cover would likely have been significant given
59
more time or if hog disturbance had been heavier in seepage slopes in 2012 (Mitchell et al.
2007b; Taylor et al. 2011).
Although hogs are omnivorous (Taylor & Hellgren 1997), previous studies have found
herbaceous vegetation can account for the greatest proportion of the diet of feral hogs (Challies
1975; Everitt & Alaniz 1980; Coblentz & Baber 1987; Cuevas et al. 2010). While little is known
about the diet of feral hogs in this system, our findings indicate that either direct consumption or
indirect soil disturbance associated with rooting results in greater forb and grass cover in
undisturbed areas. The observed increase in graminoid cover was greater inside the protection of
exclosures where ruderal species (e.g., Dichanthelium spp.) quickly recolonize disturbed soil
(Rodgers & Provencher 1999). In the absence of hog foraging, other studies have observed
similar increases in herbaceous cover (Siemann et al. 2009; Cole et al. 2012).
Aristida stricta is a dominant perennial bunchgrass in the longleaf pine system and
considered a keystone species (Noss 1989). This native understory species is critical for carrying
frequent, low-intensity fire (Clewell 1989; Haywood & Harris 1999). Hog rooting has been
found to have little impact on native perennial bunchgrasses in coastal meadows of California
because these species are deep rooted and difficult to uproot (Kotenan 1995; Cushman et al.
2004). However, A. stricta has a shallow root system and is notorious for not recolonizing after
intense soil perturbations (Outcalt & Lewis 1990; Huffman & Judd 1998). It was no surprise that
A. stricta cover was significantly lower in high intensity than control plots. Although it may
seem concerning that A. stricta cover was reduced in plots outside exclosures, the difference
between inside and outside exclosures was within one cover class and present from the beginning
of the study. Again with 2012 being a particularly dry year, hog disturbance was negligible
compared to years with greater rainfall (Chapter 2). Future periods of heavy rainfall, like that of
60
2013, could potentially be conducive to hog rooting with greater effect on A. stricta cover and
thus fire spread.
Does species richness differ in areas of varying disturbance intensity? The effects of
hog rooting on diversity varies depending on plant community type. Hog disturbance results in
increased plant species richness in coastal grasslands in California (Kotanen 1995, 1997;
Cushman et al. 2004), while it can reduce plant species richness in Grey Beach Forests of the
Great Smoky Mountains National Park (Bratton 1974, 1975; Howe & Bratton 1976; Bratton et
al. 1982), floodplain marshes of Central Florida (Arrington et al. 1999), and mixed pinehardwood forests in Texas (Siemann et al. 2009). The vegetative response depends on whether
the community is adapted to that particular disturbance regime (Hobbs & Huenneke 1992;
Doupé et al, 2010). Similar to studies in other systems (Doupé et al. 2010; Taylor et al. 2011;
Cole et al. 2012), our study did not find that intensity of hog rooting affected plant species
richness. It is hypothesized that because seepage slope vegetation is subject to stress from
periods of prolonged soil saturation, there is less potential for competitive exclusion; thus soil
disturbance has a less pronounced effect on diversity (Huston 1979). Another possibility could
be that this study was too short-term to detect this community level change (Taylor et al. 2011).
The change we observed in richness was an increase over time with recovery and increase in
cover. More time may be necessary to detect an exclosure effect.
Does species composition differ in areas of varying disturbance intensity? Our results
from the NMS ordination analyses indicate significant differences between: 1) plant assemblages
on differing soil types within the same community; and 2) undisturbed (controls) assemblages
and those that are recovering from high intensity hog disturbance. Although the difference
between disturbance intensities and similarity within groups decreased over time as vegetation
61
recovered in high hog disturbance, the composition of these groups remained significantly
different even when only one soil type was included. It may take more than three years without
disturbance by hogs for heavily hog disturbed areas to succeed from an early successional state
(Kirkman et al. 2004).
It could be contested whether or not it is beyond the scope of this study to conclusively
state that the difference in plant composition (and abiotic components) of disturbed and
undisturbed areas is a direct result of hog disturbance. Siemann et al. (2009) suggest that hog
disturbance is not random, but targets a particular biotic or abiotic component. Thus
observational studies cannot quantify effects of hogs on vegetation because the area was likely
inherently different from undisturbed areas. On the contrary, Arrington et al. (1999) indicate that
years of observational data of vegetation composition showed areas of hog rooting were similar
to undisturbed areas before rooting occurred. We followed similar protocol to Cuevas et al.
(2010) and set up rooted plots in close proximity to undisturbed plots in an attempt to reduce
abiotic and biotic variability. Field observations over the course of this study validate the
observations of Arrington et al. (1999) as undisturbed areas were converted to heavily rooted
areas overnight, indicating little difference between the pre-disturbance states of undisturbed and
heavily disturbed plots.
Although domestic hogs (Sus scrofa) were first introduced over 400 years ago, no record
of large numbers of hogs on Eglin AFB exists prior to domestic herds that were free-range until
1960 (Mayer & Brisbin 1991). Considering that hogs have been in this area for a substantial
amount of time, there is no definitive way to determine if undisturbed areas were different from
disturbed areas prior to the presence of hogs. Hogs could be acting as ecological engineers, as
rooting is known to exacerbate abiotic differences (e.g., increase depth of depression and water
62
retention), perpetuating biotic differences (Cuevas et al. 2010). However this study did not
discover that hog rooting created unique plant associations (e.g., Lachnanthes caroliana (Lam.)
prairies) as other research in peninsular Florida has indicated (Huffman & Judd 1998).
Summary
The resilience of a system describes the amount of disturbance that system can withstand
and still remain in the same state (Holling 1973). There is not enough data to indicate that hog
disturbance has pushed seepage slopes on Eglin AFB past a threshold to an alternative stable
state. Feral hogs could be ecologically equivalent to extinct megaherbivores in regards to soil
disturbance just as it is hypothesized that an intermediate level of feral hog disturbance replaces
activities of the extirpated Ursus arctos L. (grizzly bear) in California (Work 1993) Still, it is
dangerous to interpret these results as if feral hog disturbance is ecologically benign. State
changes in ecosystems occur when there are changes in the key variables that affect the stability
of the system. The popular heuristic of a ball in a cup can be used to illustrate this change, where
changes in the shape of the cup correspond to alteration of both stability (return time) and
resilience (width of stability domain) (Gunderson 2000). Hog disturbance has likely reduced the
resilience of the seepage slope community on Eglin AFB and thus it may be approaching a
threshold even though a threshold has not yet been crossed.
63
Table 4-1. ANOVA results for the foliar cover of total vegetation, functional groups, and A.
stricta and species richness by disturbance intensity and exclosure on Eglin AFB.
Variable
Num df
df
F
P
Total Vegetation
Year
Intensity
Year x Intensity
Exclosure
Year x Exclosure
Inten. x Exclos.
Yr. x Inten. x Exclos.
Woody
Year
Intensity
Year x Intensity
Exclosure
Year x Exclosure
Inten. x Exclos.
Forb
Year
Intensity
Year x Intensity
Exclosure
Year x Exclosure
Inten. x Exclos.
Grass
Year
Intensity
Year x Intensity
Exclosure
Year x Exclosure
Inten. x Exclos.
Yr. x Intens. x Exclos.
Aristida stricta
Year
Intensity
Year x Intensity
Exclosure
Year x Exclosure
Inten. x Exclos.
Richness
Year
Intensity
Year x Intensity
Exclosure
Year x Exclosure
Inten. x Exclos.
2
2
4
1
2
2
4
287.7
55.54
304.5
80.51
287.4
64.35
201.8
17.34
75.12
14.60
27.02
2.61
0.89
0.75
<0.0001
<0.0001
<0.0001
<0.0001
0.0800
0.4143
0.5592
1
2
2
1
1
2
2
157.5
54.30
157.5
152.7
152.7
152.7
152.7
20.76
1.75
6.65
1.47
1.26
1.39
0.15
<0.0001
0.1836
0.0017
0.2276
0.2625
0.2519
0.8571
1
2
2
1
1
2
2
156.9
53.88
156.9
152.2
152.2
152.2
152.2
0.68
10.61
1.05
0.0
1.04
0.21
0.17
0.4101
0.0001
0.3522
0.9919
0.3097
0.8118
0.8442
1
2
2
1
1
2
2
159.8
55.22
159.8
153.9
153.9
153.9
153.9
72.53
15.43
4.70
14.5
2.73
0.42
0.26
<0.0001
<0.0001
0.0103
0.0002
0.1006
0.6583
0.7735
1
2
2
1
1
2
2
153.0
53.06
153.0
151.0
151.0
151
151
0.22
10.44
2.11
4.54
0.24
1.4
0.72
0.6408
0.0002
0.1250
0.0347
0.6262
0.2506
0.4891
1
2
2
1
1
2
2
158.0
54.19
158.0
152.7
152.7
152.7
152.7
89.34
1.08
3.47
0.55
1.05
0.58
2.16
<0.0001
0.3471
0.0336
0.4582
0.3063
0.5590
0.1193
64
65
Figure 4-3. Exclosure and open plot pairs were located in the middle zone of the slope.
Figure 4-4. The location of seepage slope sites with motion detecting wildlife cameras.
66
Figure 4-5. Total foliar cover by intensity by year inside and outside exclosures on Eglin AFB.
Bars are mean foliar cover ±1 SE and different letters above bars indicate significant
difference (α = 0.05) in foliar cover following Tukey–Kramer post-hoc tests.
Figure 4-6. Total foliar cover by exclosure by year on Eglin AFB. Bars are mean foliar cover ±1
SE and different letters above bars indicate significant difference (α = 0.05) in foliar
cover following Tukey–Kramer post-hoc tests.
67
Figure 4-7. Foliar cover of woody species by intensity by year inside and outside exclosures on
Eglin AFB. Bars are mean foliar cover ±1 SE and different letters above bars indicate
significant difference (α = 0.05) in foliar cover following Tukey–Kramer post-hoc
tests.
Figure 4-8. Foliar cover of graminoid species by intensity by year inside and outside exclosures
on Eglin AFB. Bars are mean foliar cover ±1 SE and different letters above bars
indicate significant difference (α = 0.05) in foliar cover following Tukey–Kramer
post-hoc tests.
68
Figure 4-9. Foliar cover of graminoid species by exclosure by year on Eglin AFB. Bars are mean
foliar cover ±1 SE and different letters above bars indicate significant difference (α =
0.05) in foliar cover following Tukey–Kramer post-hoc tests.
Figure 4-10. Average number of hog occurrences per month in seepage slopes with cameras on
Eglin AFB from June 2010 through December 2013.
69
Control
High
Figure 4-11. Two-dimensional nonmetric multidimensional scaling ordination showing cover by
species in high disturbance intensity and control plots on Eglin AFB. Samples from
2010 and 2012 are combined. Each point represents a plot and the distance between
points is proportional to the difference in species composition.
70
A.
B.
Control
Control
High
High
Figure 4-12. Three-dimensional nonmetric multidimensional ordination showing cover by
species in high disturbance and control plots on Eglin AFB in A) 2010 and B) 2012.
Each point represents a plot and the distance between points is proportional to the
difference in species composition. By 2012, high disturbance intensity plots are
recovering and separation between groups has decreased.
71
CHAPTER 5
EFFECTS OF EXPERIMENTAL SOIL DISTURBANCE ON VEGETATION DYNAMICS
Background
Disturbances are environmental fluctuations or destructive events that cause abrupt
changes in the community or population structure of natural communities. They can be biological
or physical in nature and differ in magnitude and distribution over time. The irregular nature of
these events can produce heterogeneous or patchy effects (Sousa 1984; Pickett & White 1985;
Moloney & Levin 1996). It is generally accepted that disturbances keep natural communities in a
state of nonequilibrium, maintaining diversity by preventing competitive equilibrium. A dynamic
balance is reached between the frequency of disturbance and rate of competitive displacement
when competitive equilibrium is prevented and thus a stable level of diversity is maintained
(Connell 1978; Huston 1979). The intermediate disturbance hypothesis (IDH) proposes that high
diversity results from an intermediate disturbance regime. Longer-lived species are lost when
disturbance is too great or too often, while ruderal species are lost when disturbance is
insignificant or too infrequent (Connell 1978; Petraitis et al. 1989; Reynolds 1993; Shea et al.
2004). Therefore the diversity pattern observed in disturbed environments is unimodal (Shea et
al. 2004). An alternative hypothesis to the IDH is that diversity and richness are independent of
disturbance (Mackey & Currie 2001).
Glitzenstein et al. (2003) hypothesized that in the longleaf pine ecosystem the open,
herbaceous ground cover is maintained with maximum species diversity when burning occurs as
frequently as fuels allow. The authors referred to this as the "Most Frequent Fire Hypothesis"
(MFFH). This idea is supported by other studies that found frequent fire (i.e. one to two years
interval) necessary to achieve reduced woody cover and dominance of diverse herbaceous
72
vegetation (Walker & Peet 1983; Waldrop et al. 1992; Brockway & Lewis 1997; Haywood &
Grelen 2000; Glitzenstein et al. 2003; Glitzenstein et al. 2012).
Multiple types of disturbances with different characteristics often interact to make up the
disturbance regime of a community (Collins 1987) and may result in complex patterns not
described by the IDH (Collins & Barber 1985) or other hypotheses. Alternatively, it may be the
type and characteristics of the combined disturbances that determine effect on post-disturbance
community trajectory (Collins 1987; Paine et al. 1998; Platt et al. 2002). Grassland disturbance
regimes may include combinations of fire, grazing, and soil disturbance by animals where the
effect of each disturbance is variable depending on scale, frequency, and intensity (Collins
1987). For example, one disturbance could reduce competitive superiority (e.g., grazing), while
another could increase habitat heterogeneity (e.g., soil disturbance). Still, community response is
dependent upon the historical disturbance regime (Collins & Barber 1985). It is ultimately
divergence from the historical disturbance regime that can leave native species ill adapted for
successful recovery (Denslow 1980; Hobbs & Huenneke 1992) and may result in extirpation or
even extinction (Connell 1978).
All over the world, disturbance regimes are changing due to climate change,
anthropogenic modifications, and biological invasions (Shea et al. 2004). Introduced species of
both plants and animals have the potential to alter disturbance regimes. These species can
increase or reduce the frequency and/or intensity of disturbances, like fire or erosion, which can
quickly result in mortality of a significant portion of the community. Introduced feral hogs
demonstrate that the invader can also be the actual agent of disturbance (Mack & D'Antonio
1998). Regardless of mechanism, these alterations to the disturbance regime can cause dramatic
changes in composition and successional trajectory of the invaded community (Bratton 1975;
73
Mack & D'Antonio 1998). The purpose of Chapter 5 is to assess the short-term effect of four
experimental soil disturbance regimes, meant to simulate hog disturbance, on community
structure and composition in the seepage slope community. We examine the effect of disturbance
frequency and time since last disturbance on foliar cover, richness, and species composition over
a four-year period.
Methods
Study Area
This study was conducted on Eglin AFB located in Walton County, Florida, USA
(30°29′22″N 086°32′32″W). On the eastern portion of Eglin AFB, dendritic drainage systems
have cut into the remains of a Pliocene-Pleistocene age delta plane, leaving erosional hills with
relief of up to about 30 meters (NRCS 1989). Here the loamy sand soils allow for the formation
of seepage slopes. Along hillsides, the Pinus palustris Mill. canopy opens up into grass and
sedge dominated wet prairies (Figure 5-1). Nutrient poor soils are saturated by groundwater that
seeps downslope above an impermeable layer of clay, organic matter, or rock. Depending on the
size of the watershed, these wetlands may dry out during times of drought, as soil saturation is
dependent on groundwater (FNAI 2010).
The hydrologic gradient inherent to the slopes is reflected in the diverse herbaceous
vegetation. Upslope tends to be drier and dominated by Aristida stricta Michx. (Wiregrass),
Ctenium aromaticum (Walter) A. Wood (Toothachegrass), Muhlenbergia expansa (Lam.) Trin.
(Cutover Muhly), and Rhexia spp. (Meadowbeauties). The lower slope generally has a longer
hydroperiod and is dominated by Eriocaulon spp. (Pipeworts), Lycopodiella spp. (Club Mosses),
Rhynchospora spp. (Beaksedges), Xyris spp. (Yelloweyed Grasses), and a diverse suite of
carnivorous plants (FNAI 2010). Sarracenia spp. (Pitcherplants), Drosera spp. (Sundews),
Pinguicula spp. (Butterworts), and Utricularia spp. (Bladderworts) acquire nutrients by trapping
74
insects (Schnell 1976). The heliophytic, herbaceous vegetation of this community is dependent
upon frequent fire because without it, Cliftonia monophylla (Lam.) Sarg (Black Titi) and other
shrubs will move in from the shrub bogs or baygalls and dominate (FNAI 2010).
Data Collection
How does frequency and time since last disturbance effect foliar cover, richness and
species composition? In June 2010, two 1m2 plots were randomly selected and surveyed at 20
independent seepage slope sites on the eastern portion of Eglin AFB, Walton County, Florida,
USA (Figure 5-2). Plot locations were chosen in undisturbed vegetation on flat, open slopes in an
attempt to reduce the influence of aspect and reduce the variability of the hydrologic gradient.
Experimental disturbance treatments were applied using a shovel to turn the soil in each plot to a
depth of ~7-15 cm (Kotanen 1997). Wire fence exclosures (6m in circumference and 1m in
height) were erected to protect plots from future hog disturbance. Two disturbance treatments
were applied at 10 sites while another two disturbance treatments were applied at the remaining
10 sites (e.g., F1 and I1 at 10 sites; F4 and I4 at 10 sites). The disturbance treatments applied
were categorized as one of the following four classes.
1.
2.
3.
4.
F1- One plot at 10 of 20 sites was disturbed frequently (quarterly) for one year (2010).
F4- One plot at 10 of 20 sites was disturbed frequently (quarterly) for four years.
I1- One plot at 10 of 20 sites was disturbed infrequently (once), for one year (2010).
I4- One plot at 10 of 20 sites was disturbed infrequently (once annually) for four years
(June, 2010; September 2011; December 2012; and June 2013).
Prior to application of the four disturbance treatments, foliar cover was estimated in June
2010, September 2010, December 2010, and March 2011 before subsequent disturbances. A
modified Daubenmire scale (0= 0%, 0.5-5 = 3%, 5-10 = 8%, 10-15 = 13%, 15-25 = 21%, 25-50
= 38%, 50-75 = 68%, 75-95 = 88%, 95-100 = 98%) was used to estimate foliar cover of total
vegetation, forbs, grasses, woody vegetation, Aristida stricta, and Dichanthelium spp.
(Witchgrasses) in the 1m2 plot. A. stricta was chosen because it is considered a keystone species
75
(Noss 1989) due to its importance in fire spread (Clewell 1989; Haywood & Harris 1999).
Dichanthelium spp. were chosen because they may be less sensitive to disturbance. Foliar cover
for individual species was estimated for one quarter of each plot. Species nomenclature was
based on Wunderlin & Hansen (2008). Richness was also estimated. Plots were also surveyed in
June 2011, September 2011, March 2012, June 2012, September 2012, March 2013, June 2013
and September 2013 preceding disturbances. Undisturbed, control plots inside exclosures were
established in 2010 and surveyed in June 2010, June 2011 (total cover only), June 2012, and
September 2013.
Data Analyses
How does frequency and time since last disturbance effect foliar cover and species
richness? This experiment followed a balanced incomplete blocking design. Each seepage slope
site was considered a block with a subset (two of four) of all disturbance treatments present in
each block. Each disturbance treatment had equal replication within blocks. Repeated measure
one-way ANOVA (PROC GLIMMIX; SAS 9.3; SAS Institute 2011) was used to evaluate
significance of main effects and interactions (α= 0.10). Means were arcsine square root
transformed when necessary to meet assumptions of ANOVA (Zar 1998). Survey date and
disturbance treatment (frequency and time since last disturbance) were the main effects. Site was
considered a random effect. Tukey–Kramer tests were used for post hoc comparisons (α= 0.05)
to analyze differences between means. Additional ANOVAs were run to analyze data from
controls because the control plots were surveyed fewer times (once a year) than the experimental
plots. The data from wildlife cameras with motion sensors used in Chapter 4 was also used to
analyze annual frequency of hog occurrences at seepage slopes. The number of hog occurrences
per site per year was calculated from the camera data and averaged across cameras for each year.
76
How does frequency and time since last disturbance effect species composition?
NMS ordination was used to depict temporal changes in species in terms of disturbance regime,
burn events, and soil type. Species that occurred in less than 5% of the plots were excluded.
Andropogon spp. (Bluestems), Asteraceae spp. (Asters), Eriocaulon spp., Rhexia spp.,
Rhynchospora spp., Sarracenia spp., Scleria spp. (Nutrushes), and Xyris spp. were grouped. In
PC-ORD, a Sorenson (Bray-Curtis) distance measure was used in addition to shortest path for
data with high beta-diversity and partially disjunct outliers. Monte Carlo tests determined if the
ordination results were significantly different from random data. MRPP was additionally used to
test for differences within and between disturbance treatment groups (McCune & Grace 2002).
This method is effective for testing differences when sample sizes are small or distributions of
the data are unknown.
Results
Foliar Cover
How does frequency and time since last disturbance effect foliar cover? Disturbance
frequency and time since last disturbance affected total foliar cover across dates (Figure 5-3;
Table 5-1; Table 5-2). Pre-treatment total foliar cover among plots was indistinguishable (p=1.0).
There was a significant decline in total foliar cover after the initial disturbance treatments
(p<0.0001). Frequently disturbed plots (F1 and F4) continued to decline the first year of the
study (p<0.0001). Total foliar cover was significantly less in all treatment plots (F1, I1, F4, and
I4) than controls (p<0.0001) in June 2011. Although infrequently disturbed plots (I1 and I4)
began to recover, total foliar cover remained significantly lower after one year than pretreatment (p<0.0001). Plots that were not disturbed after the first year of the study (F1 and I1)
continued to increase in total foliar cover over the next three years. Only I1 plots recovered to
pre-disturbance cover (p=0.9991) and were indistinguishable from controls (p=0.9632). After
77
three years of recovery, cover for F1 plots remained significantly lower than cover predisturbance (p=0.0012) and the controls (p<0.0001). F1 plots were indistinguishable from I1
plots (p=0.8116) by September 2013. Total cover continued to decline after subsequent
disturbances when disturbed once a year (I4) for the next three years. After four years of annual
disturbance (I4), cover was 16.6%, significantly lower than pre-disturbance (p<0.0001) and the
controls (p<0.0001), but not different from F4 plots (p=1.0). Plots disturbed quarterly for four
years (F4) declined steadily to 8.60% and had the lowest cover out of all treatments except I4, at
the end of the study. Frequency of disturbance affected total cover, however these effects are
confounded by time since last disturbance.
Cover of functional groups also changed by disturbance treatment over time (Table 5-1;
Table 5-2). As expected, there was a decline in forb (Figure 5-4), grass (Figure 5-5), and woody
species (Figure 5-6) cover after the initial disturbances (p<0.0001). I1 plots recovered to predisturbance cover for forbs (p=1.0), grass (p=1.0); and woody (p=0.9994) by the end of the study
and did not differ from controls (forbs, p=0.9947; grass, p=1.0; woody, p=1.0). F1 plots had less
forb cover than pre-disturbance plots (p<0.0001), but did not differ from controls (p=0.2099).
Grass and woody cover in F1 plots recovered to levels indistinguishable from the pre-disturbance
levels (p=1.0) and controls (grass, p=0.9901; woody, p=1.0). Plots disturbed over the entire four
years (F4 and I4) experienced the greatest decline in cover by functional group. Forb and grass
cover was significantly less than pre-disturbance cover (p<0.0001) and controls (p<0.0001) by
2013 for F4 plots, while woody vegetation cover recovered to levels comparable to predisturbance levels (p=0.6209) and controls (p=0.2428). The same pattern was observed for I4
plots. Forb and grass cover did not recover to pre-disturbance levels (p<0.0001) or to that of
controls (p<0.0001). Woody cover in I4 plots remained lower than controls (p=0.0463), but did
78
not differ from pre-disturbance levels (p=0.9996). By 2013, cover of forbs did not differ between
F4 and I4 plots (p=1.0), but both treatments were significantly lower than I1 (p<0.0001). F1 plots
had forb cover that was indistinguishable from I1 plots (p=1.0), but significantly greater than I4
and F4 plots (p=0.0766; p=0.0030 respectively). By 2013, cover of grasses did not differ
between F4 and I4 plots (p=1.0), but both treatments were significantly lower than I1
(p<0.0001). F1 plots had grass cover that was indistinguishable from I1 (p=1.0), but significantly
greater than I4 plots and F4 plots (p=0.0002; p=0<0.0001 respectively). By 2013, cover of
woody vegetation was significantly greater in F1 and I1 plots than I4 (p=0.0917; p=0.0964
respectively). The cover of woody species did not differ between the remaining disturbance
treatments by the end of study (F1&F4, p=0.4372; F1&I1, p=1.0; I1&F4, p=0.4500; and I4&F4,
p=1.0).
Aristida stricta (Figure 5-7) and Dichanthelium spp. (Figure 5-8) varied in their
responses to the experimental disturbance treatments (Table 5-1; Table 5-2). Aristida stricta and
Dichanthelium spp. experienced a significant decline (p<0.0001; p=0.0310 respectively) in cover
after the first disturbance event. A. stricta cover continued to decline and was significantly
reduced by Sept. 2013 in all disturbance treatments (F1, p=0.0033; F4, p<0.0001; I4, p<0.0001)
except I1 plots (p=0.3859) and controls (p=0.9999). At the end of the study, A. stricta cover was
indistinguishable between controls and I1 plots (p=0.4033), but A. stricta cover was significantly
reduced in F1 plots (p=0.0044), I4 plots (p=0.0011), and F4 plots (p=0.0008) compared to
controls. All treatments were indistinguishable in terms of A. stricta cover by 2013 (F1& I1,
p=0.9811; F1&I4, p=1.0; I4&F4, p=1.0; F1&F4, p=1.0; F4&I1, p=0.9073; I1&I4, p=0.9278). It
should be noted that while I1 did not significantly differ from the other treatments, A. stricta
cover was double that observed in the other treatments. Also, A. stricta cover observed in
79
controls was double that of I1 plots. Dichanthelium spp. were able to recover to pre-disturbance
values in all disturbance treatments (I1, p=1.0; F1, p=0.9996; F4, p=0.7268; I4, p=0.9253) and
was not significantly different from controls (I1, p=0.6118; F1, p=0.9359; F4, p=0.7855; I4,
p=0.9979) by the end of the study. F4 plots had significantly less Dichanthelium spp. cover than
I1 plots (p=0.0263), but no other treatments differed by 2013 (F1& I1, p=1.0; F1&I4, p=0.4896;
I4&F4, p=1.0; F1&F4, p=0.1128; I1&I4, p=0.1856).
Cover of Sarracenia spp. declined significantly over the four-year study in treatment
plots (Figure 5-9; Table 5-1; Table 5-2). The effect of disturbance regime was significant. These
results indicate that soil disturbance has a negative effect on Sarracenia spp.
Camera Detected Hog Frequency
Our estimate for the number of hog occurrences at each camera site each year is
conservative considering the number of months that cameras were stolen, broken, or nonfunctional. Still, on average there were more than two hog occurrences at each site every year of
the study. Cameras were first set up in June 2010 and averaged 3.79 hog occurrences in 2010. In
2011, the average hog occurrences at camera sites were 2.29. Hog occurrences increased to 5.75
in 2012 and 5.13 in 2013.
Richness
How does frequency and time since last disturbance effect species richness? Richness
was also affected by the experimental disturbance treatments (Figure 5-10). Overall, a seasonal
pattern in richness was detected, with lower richness in March and higher richness in June. The
initial disturbance resulted in a significant decline (p<0.0001) in richness. Richness in I1 plots
increased until September 2011 when there was no significant difference (p=0.9903) compared
to pre-disturbance levels. Richness in F1 plots were indistinguishable from pre-disturbance
values (p=1.0) by June 2012, with values not significantly different from controls and I1
80
(p=0.9999). Until the end of the study, richness continued to increase and did not significantly
differ from pre-disturbance (I1 and F1, p=1.0) and controls (I1, p=0.9912; F1, p=1.0). Plots
disturbed for the duration of the study (F4 and I4) exhibited further declines in richness. I4 plots
regained pre-disturbance richness (p=0.1611) with values that continued to differ from the
controls (p=0.0005) by the end of the study. F4 plots never regained pre-disturbance richness
(p<0.0001) and richness was significantly lower than the control (p<0.0001), I1 (p<0.0001), and
F1 (p<0.0001), yet indistinguishable from I4 (p=1.0).
Vegetation Composition
NMS ordination and MRPP revealed increasing compositional change with increased
disturbance frequency and decreased time since last disturbance (T=-14.1016; A=0.0875;
p<0.0001; Figure 5-11). Vegetation composition in control plots and I1 plots clustered together
and changed relatively little over the four years. F1 and I4 treatments changed vegetation
composition, but the most extreme change in vegetation composition was observed in the F4
treatment. A three-dimensional ordination was optimal (stress = 18.8439) with axis 1 explaining
9.6%, axis 2 explaining 36.4%, and axis 3 explaining 25.4% of the variation. Monte Carlo test
determined that the ordination differed from random data (p=0.0164). When data were grouped
by time since disturbance (i.e., I1 & F1 compared to I4 & F4), MRPP showed that time since
disturbance is important in the clustering of groups in the NMS ordination (T=-12.2302;
A=0.03451; p<0.0001). MRPP also illustrated that soil does influence community composition
(T=-7.4020; A=0.0385; p<0.0001), although disturbance regime was most reflected in the NMS
ordination clusters.
81
Discussion
The results from these analyses demonstrate that the effect of disturbance on seepage
slope vegetation cover, richness, and composition was a function of frequency of occurrence.
Recovery of vegetation was, in turn, a function of time since disturbance. Frequent disturbance
for four years (F4) resulted in the lowest cover and richness and most altered composition, while
frequent disturbance for one year (F1) with three years of recovery resulted in cover, richness,
and composition that was similar to controls and plots only disturbed once (I1).
How does frequency and time since last disturbance effect foliar cover? Dominant
flora such as A. stricta, was negatively affected by soil disturbance treatments. This shallow
rooted (Outcalt & Lewis 1990) keystone species (Noss 1989) is important to carry fire (Clewell
1989; Haywood & Harris 1999). Hog foraging results in soil disturbance that is novel in type,
intensity, and frequency. Aristida stricta experienced significant declines in all treatments except
plots disturbed once (I1) and failed to reestablish in plots that were disturbed more than once.
The decline in A. stricta cover that occurred in 2013 in all treatments and controls could not be
explained by removing from the analysis plots that burned in prescribed fires during the spring of
2013. It is likely that some other unquantified variable affected cover. Still, these observations
substantiate those in Chapter 3 where A. stricta cover was significantly lower in high intensity
than controls. The loss of this keystone species is significant, especially considering that woody
species were less negatively impacted by disturbance regime. It seems that a positive feedback is
established through soil disturbance where A. stricta cover is lost and woody cover maintains,
both of which reduce fire spread (Chapter 2), ultimately leading to increased woody cover.
Unlike A. stricta, some Dichanthelium species are commonly found in early, secondary
successional communities. These grasses require bare soil to recolonize and capitalize on postdisturbance conditions with decreased competition and increased colonization opportunities,
82
increasing dramatically in terms of cover (Estes 2006). In plots that were disturbed during the
first year of the study (F1, I1), Dichanthelium species actually increased in cover from predisturbance cover, although this increase was not significant. Cover of Dichanthelium species
kept grass cover high even while other grasses, such as A. stricta, declined.
Sarracenia species are sensitive to hog disturbance (Johnson 2001). Trampling and
rooting, both the physical action and associated changes in soil moisture, negatively impact
survival of juvenile Sarracenia (Hermann 1995). Research has also shown that abundance of
white-top pitcherplants and yellow trumpets are both negatively correlated with areas of hog
disturbance (Engeman et al. 2007). Our results also indicate that Sarracenia species are sensitive
to soil disturbance as cover declined in all treatments, although cover did not decline in controls.
Our experimental soil disturbance results indicate that soil disturbance more than once a
year will reduce species important to the function and unique composition of the seepage slope
community. Motion sensor wildlife cameras detected a true frequency of hog foraging around
two to five hog occurrences at each site every year of the study. This conservative estimate of
true hog disturbance frequency in specific portions of particular slopes is greater than our
experimental frequency that resulted in degradation of the community in terms of cover of
important/keystone species. As a result, hogs appear to return to disturb seepage slopes
frequently enough to negatively impact the community composition, structure, and function.
How does frequency and time since last disturbance effect species richness? This
analysis did not find richness to vary unimodally across a gradient of disturbance frequencies,
but rather found that highest species richness resulted from the lowest experimental soil
disturbance frequency and longest time since soil disturbance. This richness was equivalent to
the observed pre-disturbance richness levels. In the P. palustris-A. stricta ecosystem, fire is a
83
large-scale disturbance that reduces aboveground competition, but does not reduce richness
(Kirkman et al. 2001). High diversity is achieved in P. palustris communities when frequent fire
limits hardwood species dominance (and competitive exclusion) and perennial life histories
allow species to persist once established (Brockway & Lewis 1997; Kirkman et al. 2001).
Seepage slope vegetation is also subject to prolonged soil saturation and thus this stress (Huston
1979). This stress combined with frequent disturbance from fire limits the potential for
competitive exclusion enhancing diversity (Kirkman et al. 2001). Also, oligotrophic conditions
slow the rate of recovery from disturbance due to the lack of available nutrients (Walker & Peet
1983), which explains why recurrent soil disturbance at any frequency reduces seepage slope
plant diversity temporarily. Species richness then has the opportunity to recover with increasing
time after the disturbance. This recovery of richness should not be the only metric used to
interpret recovery as differences in species composition is not taken into account (McLachlan &
Bazely 2001).
Deterministic factors of life history characteristics (Kirkman et al. 2004) and competitive
relationships of resident plant species influence the vegetation dynamics and recovery trajectory
after disturbance events. Many resident species in seepage slopes, like other successional
communities maintained by frequent disturbance, have life history traits that allow for
resprouting and vegetative expansion after fire. This response facilitates (when given three years
to recover) the community returns to the pre-disturbance composition (Halpern 1988). The NMS
and MRPP analyses revealed that continued disturbance (F4, I4), even at a low frequency of
once a year, alters species composition. Once extirpated from a site, species with limited
dispersal take longer to reestablish in disturbed sites then seeds of plants that are wind or animal
84
dispersed (Kirkman et al. 2004). Further research is necessary to determine if seepage slopes
with prolonged disturbance will recover in terms of species composition given more time
considering other studies in P. palustris savannas have found that some species are particularly
vulnerable to disturbance and do not reestablish even after extended periods of time (i.e. 65
years) (Kirkman et al. 2004).
Summary
A fundamental question in ecology is whether or not communities have an inherent
resilience to recover pre-disturbance structure and function, or if historical events create legacies
that allow for more than one alternative state. The successional trajectory and development of a
community after a disturbance event is dependent upon remaining vegetation and recruitment,
which in turn depends on dispersal limitations and differences in mechanisms of establishment
(Young et al. 2001). This experiment demonstrated: 1) development of a legacy effect in plots
that had continued disturbance for four years; and 2) differences among the ability of species to
recover and recruit after varying levels of disturbance. Frequent disturbance for four years (F4)
created a legacy that impaired foliar cover, richness, and altered species composition. Longerterm continuation of this study would be necessary to determine if a threshold has been crossed
to an alternative stable state.
85
Table 5-1. Repeated measures ANOVA results for the foliar cover of total vegetation, functional
groups, A. stricta, Dichanthelium spp., Sarracenia spp., and species richness by
disturbance frequency on Eglin AFB.
Variable
Num df
df
F
P
Total Vegetation
Time
11
375.5
90.95
<0.0001
Treatment
3
35.99
40.22
<0.0001
Time x Treatment
33
375.5
17.71
<0.0001
Forb
Time
Treatment
Time x Treatment
11
3
33
374.9
35.18
374.8
27.46
15.81
5.62
<0.0001
<0.0001
<0.0001
Grass
Time
Treatment
Time x Treatment
11
3
33
376
36.09
375.9
33.4
23.46
9.82
<0.0001
<0.0001
<0.0001
Woody
Time
Treatment
Time x Treatment
11
3
33
375.5
36.12
375.5
14.07
6.1
2.83
<0.0001
0.0022
<0.0001
Aristida stricta
Time
Treatment
Time x Treatment
11
3
33
376
36.33
375.9
17.19
11.3
4.01
<0.0001
<0.0001
<0.0001
Dichanthelium spp.
Time
Treatment
Time x Treatment
11
3
33
375.4
35.96
375.4
17.46
7.16
3.5
<0.0001
0.0008
<0.0001
Sarracenia spp.
Time
Treatment
Time x Treatment
11
3
33
377.9
36.44
377.9
3.17
2.75
0.99
<0.0001
0.0743
0.5497
Richness
Time
Treatment
Time x Treatment
11
3
33
375.9
36.34
375.9
68.68
16.49
6
<0.0001
<0.0001
<0.0001
86
Table 5-2. Repeated measures ANOVA results for the foliar cover of total vegetation, functional
groups, A. stricta, Dichanthelium spp., and Sarracenia spp., and species richness by
disturbance frequency on Eglin AFB with controls in 2010, 2012 and 2013. Total
vegetation cover was also surveyed in 2011.
Variable
Num df
df
F
P
Total Vegetation
Time
3
161.5
325.6
<.0001
Treatment
4
55.21
142.6
<.0001
Time x Treatment
12
161.4
54.86
<.0001
Forb
Time
Treatment
Time x Treatment
2
4
8
105.3
52.96
105.3
37.75
16.97
6.98
<.0001
<.0001
<.0001
Grass
Time
Treatment
Time x Treatment
2
4
8
106.7
54.12
106.7
25.57
11.84
12.95
<.0001
<.0001
<.0001
Woody
Time
Treatment
Time x Treatment
2
4
8
106.1
54.73
106.1
7.24
5.67
1.4
0.0011
0.0006
0.2200
Aristida stricta
Time
Treatment
Time x Treatment
2
4
8
105.8
54.63
105.7
48.68
7.44
5.7
<.0001
<.0001
<.0001
Dichanthelium spp.
Time
Treatment
Time x Treatment
2
4
8
106.9
55.52
106.9
0.68
2.54
2.9
0.6041
0.0441
0.0111
Sarracenia spp.
Time
Treatment
Time x Treatment
2
4
8
106.3
55.35
106.3
2.78
1.92
1.29
0.4446
0.3745
0.5518
Richness
Time
Treatment
Time x Treatment
2
4
8
107.7
55.87
107.7
5.48
12.05
6.98
0.0159
<.0001
<.0001
87
88
Figure 5-3. Total foliar cover by disturbance treatment over time on Eglin AFB. Different letters
above points indicate significant difference (α = 0.05) within each survey date
following Tukey–Kramer post-hoc tests. Dotted lines indicate annual disturbance
treatment.
89
Figure 5-4. Foliar cover of forbs by disturbance treatment over time on Eglin AFB. Different
letters above points indicate significant difference (α = 0.05) within each survey date
treatment.
90
Figure 5-5. Foliar cover of grasses by disturbance treatment over time on Eglin AFB. Different
treatment.
91
Figure 5-6. Foliar cover of woody species by disturbance treatment over time on Eglin AFB.
Different letters above points indicate significant difference (α = 0.05) within each
survey date following Tukey–Kramer post-hoc tests. Dotted lines indicate annual
disturbance treatment.
92
Figure 5-7. Foliar cover of A. stricta by disturbance treatment over time on Eglin AFB. Different
treatment.
93
Figure 5-8. Foliar cover of Dichanthelium spp. by disturbance treatment over time on Eglin
AFB. Different letters above points indicate significant difference (α = 0.05) within
each survey date following Tukey–Kramer post-hoc tests. Dotted lines indicate
annual disturbance treatment.
94
Figure 5-9. Foliar cover of Sarracenia spp. by disturbance treatment over time on Eglin AFB.
Dotted lines indicate annual disturbance treatment.
95
Figure 5-10. Species richness by disturbance treatment over time on Eglin AFB. Different letters
above points indicate significant difference (α = 0.05) within each survey date
treatment.
96
Figure 5-11. Three-dimensional nonmetric multidimensional ordination showing cover by
species by disturbance treatment on Eglin AFB in June 2010 and Sept 2013. Each
point represents a plot and the distance between points is proportional to the
difference in species composition.
97
CHAPTER 6
CONCLUSIONS
The seepage slope community is, for the most part, poorly understood despite its intrinsic
beauty and scientific value (Hermann 1995). This research addresses a data gap by quantifying
hog disturbance and its impact on this unique wetland habitat. Our results strongly suggest that
hog management should be viewed as a long-term management strategy that may need to be
adjusted on an annual basis depending on environmental factors (Bieber & Ruf 2005). Managers
should protect the lower portions of seepage slopes (e.g., using fence exclosures) where feral
hogs prefer to forage and where rare, endemic flora and fauna are found. Additionally, it is
evident that this novel soil disturbance reduces cover of A. stricta and alters vegetation
composition, likely indicating that ruderal species recolonize hog-created gaps where
competition is decreased (Cushman et al. 2004). This research indicates that the exclusion of
feral hogs from seepage slopes is the first step towards the conservation and/or restoration of this
endangered plant community. In some cases and where practical, it may be necessary to actively
restore portions of a slope with appropriate combinations of soil stabilization, direct seeding,
and/or outplanting of seedlings of native species. Considering that so little of the original extent
of this community is still intact (FNAI 1990), encroachment of woody species into remaining
seepage slopes is a serious threat. It may be necessary for prescribed fire crews to push fire
downslope into areas where fire may not spread unassisted or cut and clear these areas by hand.
Continued active management, including hog control and monitoring, is essential to mitigating
hog damage and preserving the ecological integrity of this rare resource.
98
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112
BIOGRAPHICAL SKETCH
Megan was born and raised in a small town south of Richmond, Virginia. She graduated
from the University of Virginia in 2002 with a Bachelor of Arts degree in biology. Megan went
on to earn a Master of Science degree in geological sciences from Virginia Polytechnic Institute
and State University in 2004. An internship working with the Bureau of Land Management and
Chicago Botanic Garden took her to the tiny town of Vale in eastern Oregon. This experience
fostered her interest in wetlands and restoration. After leaving Oregon, Megan went back east to
teach biology laboratories at Virginia State University in 2005. Restless for a job with more time
outside, she saw an opportunity to go back to school and conduct wetland research at the
University of Florida. In 2008, she became a fellow in the Adaptive Management of Water,
Wetlands, and Watershed Integrative Graduate Education and Research Traineeship (IGERT)
program that was funded by the National Science Foundation. Megan earned her Ph.D. in
interdisciplinary ecology from the University of Florida in May of 2014.
113

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