2006-Alligator-popul.. - University of Hartford

SEPTEMBER2006
NATURALIST51(3):346-351
THE SOUTHWESTERN
SEASONAL ACTIVITY, RELATIVE ABUNDANCE, AND SIZE-CLASS
STRUCTURE OF THE AMERICAN ALLIGATOR
IN A HIGHLY
(ALLIGATOR MISSISSIPPIENSIS
DISTURBED INLAND LAKE
WILLIAM I. LUTTERSCHMIDT* AND DENNIS K. WASKO
Department of Biological Sciences,Sam Houston State University,Huntsville, TX 77341
edu
*Correspondent:lutterschmidt@shsu.
We
Raven,
found
the observed
0.19
ported
class
of
were
0.6
5 m
mostly
populations
and
dance,
size-class
turbed
inland
coastal
alligator
also
Los
structure
of
una
La
en
Sin
del
temperatura
aire
actividad
para
hectarea.
is limited
information
activity,
and
sup
in the
the
size
lake
and
on
inland
relative
abun
and
(85 ha)
studies
dis
highly
with
comparisons
indice
de
del
de Texas,
relativamente
futuros
estos
a 5m
la mayoria
de
pequefio
este
eran
de
la orilla.
ofrece
estudio
las clases
(85 ha)
y comparaciones
todas
de
tamafno
Ya que
existe
informaci6n
de
y sumamente
con
las regiones
poblaciones
The American
(Alligator mississip
alligator
reptile
piensis) is the largest North American
(record size = 5.84 m; Conant and Collins,
1998) and has received a great amount of con
alligators were
servation interest. Historically,
the southeastern United
common
throughout
States until pressures of industry, agriculture,
and hunting greatly reduced breeding popu
lations in the first quarter of the 1900s (Mc
and Brown, 1948;
Ilhenny, 1935; Middleman
de
de
perturbado.
costeras
Esta
de
a 0.19
del
de
informaci6n
en
de
a 0.9 m.
lago
y
los
las poblaciones
un
puede
la
cocodrilos
abundancia
estacional,
habitan
de
crias.
la temper
0.6
de
tamafno
limitada
la actividad
eran
que
perimetro
2001.
los meses
observada
mantiene
del
que
efectiva
la variaci6n
informaci6n
los cocodrilos
en
fue
del
lago Raven,
observados
y fue mas
a la clase
y pertenecian
usaban
encontrado
el lago Raven
que
el
el anio en
todo
de
a agosto
2000
los individuos
el 37%
y explic6
estimo
juveniles
cocodrilos
que
de
significativo
pron6stico
del
es de
en el lago
mississippiensis
Alligator
septiembre
mayor
el 47%
los cocodrilos
de poblaci6n
en
estudios
un
brind6
de
cocodrilos
septiembre,
Un
Muchos
y estructura
en
de
encontramos
interior
nos
agua
observamos
lativa
of
37%
Raven
and
the perimeter
future
tem
Water
that Lake
juveniles
small
and
explained
the seasonal
cocodrilo
El numero
y enero.
diciembre
embargo,
del
Texas,
estos
de
la actividad
estacional.
Tambien
de Walker,
la actividad
que
incidencia
del
on
for
useful
interior
poblaci6n
condado
Park,
indicaron
y septiembre.
atura
por
State
Huntsville
con menor
be
of
a relatively
inhabiting
might
being
there
information
offers
alligators
information
Because
shoreline.
this study
regions
hatchlings.
and
of
in June
surveyed
estimated
alligators
all
were
2001.
occurrence
lowest
temperature
air
index
these
used
the
to August
populations.
resultados
junio
of
2000
were
individuals
over
A population
most
with
of alligators
activity
alligators
the
from
RESUMEN-Monitoreamos
Raven,
found
in Texas,
This
lake.
numbers
mississipiensis)
(Alligator
September
Raven,
the observed
activity.
with
hectare,
to 0.9 m. We
of
alligator
from
in Lake
greatest
of alligator
in seasonal
per
Texas
County,
activity
47%
predictor
variation
alligators
observed
alligator
The
in September,
a significant
of American
population
year-round
and January.
However,
was
perature
of
exhibited
in December
September.
inland
Park, Walker
State
Huntsville
alligators
alligators
an
monitored
ABSTRACT-We
in Lake
lago
re
interior
ser uitil para
cocodrilos.
Joanen and McNease,
1970; Vaughn, 1991; Al
1999). Populations
in
trichter and Sherman,
the United States became so rare and disjunct
that A. mississippiensis was listed as an endan
gered species in 1969 (Altrichter and Sher
man, 1999). However,
through proper man
in the United States re
agement, populations
covered, allowing the species to be federally
in 1977. Current
to "threatened"
down-listed
ly, the American
alligator ranges from coastal
September
2006
Lutterschmidt
and Wasko-American
North Carolina
to the southern extreme of
Florida (including
the Florida Keys) and ex
tending westward to south-central Texas (Co
nant and Collins, 1998).
Several studies document
the current pop
ulation growth and status of A. mississippiensis
in different regions of the United States (e.g.,
Brandt, 1991; Taylor et al., 1991; Hayes-Odum
et al., 1993; Altrichter
and Sherman,
1999).
This has been possible through historical data,
long-term research, and current monitoring
programs. For example, Brown (1950:224) stat
ed that "the alligator has had a much broader
in Texas in the past than it does
distribution
at the present time. Its present distribution
is
restricted to those localities which are more or
less inaccessible
to man." Such historical rec
ords have helped identify the time of popula
tion decline, and current studies are allowing
to investigate the recovery time of
biologists
in the absence of uncon
stable populations
trolled hunting pressures.
Altrichter and Sherman
(1999) recently in
status of A. mississip
vestigated the population
piensis on the Welder Wildlife Refuge of San
Patricio County, Texas, 20 years after census
work in 1978 (Smith, 1979). They reported a
size since
30.6% overall increase in population
1978, but noted decreasing numbers of alliga
tors at 2 sampling sites (Tule Lake and Aransas
River) within
the refuge. This study helped
document how drought forced alligators from
the refuge into the river during the 1978 cen
sus. Although
this study is amajor contribution
in monitoring
the status of A. mississipiensis, it
was conducted
only between 3 April and 27
May of the same year and offers no informa
or activity due to
tion on seasonal occurrence
limited sampling period.
a small population of A. mis
We monitored
sissippiensis in Lake Raven for one year to in
vestigate seasonal activity, relative abundance,
occur
size-class structure, and distributional
rence within a relatively small (85 ha) and dis
turbed inland lake. This survey is of interest
because: 1) there is limited information on in
in Texas and no in
land alligator populations
formation on the Lake Raven population,
2)
Lake Raven most likely supports a coastally iso
lated alligator population within the SanJacin
to River drainage system, 3) Lake Raven is ap
proximately 333 km from the Welder Wildlife
Refuge and might offer an interesting compar
in a disturbed
alligator
inland
lake
347
s
FishingPier
BoatRamp
LakeSwimming
Area
*
BeginningSampiingPoint
^A AliigatorLocalities
A Haichiing
AiligatorLocaitly
l
A X
A
FIG.
285 m
indicating
1-Map
of American
bution
sis) observed
the
in Lake
Raven,
and
localities
alligators
distri
mississipien
(Alligator
County,
Walker
Texas.
and 4) the
ison with this coastal population,
is subject to high dis
Lake Raven population
turbance associated with urbanization and rec
reation.
of water
located
shed
of Walker
County,
a maximum
km of
Branch,
pin)
flow
end
of
(Fig.
Lake
into
lake and
the
and
with
Conroe
and
north
exits
East
and
Alli
Chinqua
the
also
10.53
Branch,
Little
from
Creek
connects
Lake
water
lake has
1). The
(Prairie
Raven
Branch
85.02-ha
approximately
and
creeks
Big Chinquapin,
to entering
prior
is an
Jacinto
Texas
Four
Prairie
northeast.
within
of 8.5 m
depth
shoreline.
gator
Raven
the San
Site-Lake
METHODS-Study
body
the south
Sandy
Creek
the SanJacinto
River.
Lake
and
Raven
receives
bance
from
public
city
the park
within
use
swimming,
sites. No
and
over
of
and
the
and
distur
communities,
With
220
on
camping
year,
per
of boating,
lake are without
in
campsites
visitors
450,000
consist
this population
Park
State
pressure
urban
of Houston.
and
Huntsville
public
pressures
biking,
areas
subjecting
within
surrounding
the
cluding
is located
tremendous
fishing,
shoreline
camp
public
access,
of American
to
alligators
high disturbance.
Sampling
sus of
conducted
Regimes-We
alligators
for
one
year
a monthly
(September
cen
2000
to
348
August 2001) using methods
and
vol. 51, no. 3
The Southwestern Naturalist
Sherman
(1999).
Counts
at dusk
began
by eye
shining alligators while we canoed the perimeter of
the lake in a counterclockwise direction at approxi
mately
5 km/h
time
to complete
10.84,
10 to 15 m
dense
a survey
n = 12). Canoeing
transportation
and
aquatic
shines
by directing
the water
and
an eye
proached
the
made
of
or
minutes
to sample
(SE =
areas
light
beam
over
an
arc
the alligator
spotted,
of
was
250
mv
M
20~
of
with
at the surface
30
0
25
Mean
a silent means
O AirTemperature
|+ Water
Temperature >/
Lp IndviduaCount.d /
:2
20
~
~~~~~~~~~~~~~20
0
1
15
A Q-beam
(million-candle
for spotting
alligator
eye
shoreline
was
103.8
, 40
0 35
35
C00
0 30
C*~~~~~
10 *.
- ~
~
~
~
~
~ ~
1
of
1500.
Sampling Month
ap
to estimate its body size. Cruising speed
was maintained
estimate
used
the
shine
us
allowed
was
spotlight
was
the shoreline.
provided
vegetation.
power)
When
from
35
45
similar to Altrichter
toward
an alligator
distance
snout-to-eye
the animal
submerged.
(in
Both
FIG. 2-The
an accurate
until
inches)
was
air and water
temperatures and general weather conditions were
recorded to correlate with activity.
General
habitat
use of each
alligator
was
number
of American
alligators
(Alli
gator mississipiensis) observed during each monthly
sample.
Air
and water
temperatures
are
indicated
by
white and black circles, respectively.
recorded
21 (47%) of the 44 alligators observed during
this sample were hatchlings. Hatchlings were
from the shoreline
was estimated.
Al
their distance
in September
and were all lo
only observed
as either
or ac
ligators also were
identified
inactive
cated in the middle arm of the lake, where Big
tive (i.e., demonstrating
or terrestrial
loco
aquatic
Creek enters the lake (Fig. 1).
Chinquapin
motion).
Much of this area had dense, floating vegeta
total length
in cm, was
The
(TL) of each alligator,
tion that included water lotus (Nelumbo lutea)
calculated
in inches,
by estimating,
the distance
all
and hydrilla (Hydrilla verticillata). Although
to eyes (X) and using
from
its snout
the following
water
within
the
sev
hatchlings
were
observed
equation:
eral meters from shore, they used this dense
TL (cm) = X inches X 12 X (2.54 cm/ I inch)
floating vegetation much like a terrestrial hab
to eye
because
the distance
from snout
multiplying
itat. The second highest count of alligators
in inches
(X) by 12 approximates
total
length
(24) was observed in June, which is similar to
(Mcllhenny, 1935). This metric is commonly used in
in
the 23 non-hatchling
alligators observed
wildlife
to estimate
surveys
body size in feet and gen
September.
eralize
size classes among
alligators
(G. Creasy, Wild
counts of alligators were signifi
Monthly
Parks
life Biologist,
Texas
and Wildlife,
pers.
candy affected by season. Counts in September
comm.).
(44), December
(5), and January (3) account
Data Analyses-We
the effects
of both
investigated
ed for most of the seasonal variation (Fig. 2).
on alligator
water
and air temperature
activity using
A Kolmogorov-Smirnov
goodness of fit (Zar,
simple linear regression. Descriptive statistics were
to define
in this popula
used
1999) indicated that monthly alligator counts
size classes observed
was used to examine
tion, and an analysis of variance
Z
differed significantly
(Kolmogorov-Smirnov
in each
differences
the numbers
of alligators
among
= 1.704, df = 11, P = 0.006). However, if we
size class. An
index
of population
size was deter
assume that the observation
in September was
calculated
mined
from
the mean
sampling
effort
biased by the 21 hatchling alligators, only the
from the sampling
effort of each of the 12 months.
winter months December
and January showed
We used SigmaStat
Scientific
2.03
Software,
(Jandel
the greatest reduction in activity and did not
SPSS Science,
and SPSS 8.0 (SPSS Inc., 1998)
1997)
differ significantly from other monthly counts
for all statistical
of normality
analyses. Assumptions
Z = 0.949,
of alligators (Kolmogorov-Smirnov
were met
and equal variance
to analyses.
All
prior
df = 11, P = 0.329). Thus, if we consider only
were
statistical
to be signif
considered
comparisons
adult alligator counts (congruent with meth
icant at P < 0.05.
ods that typically exclude hatchlings for pop
RESULTSAND DISCUSSION-Seasonal Activity
ulation size analysis), there is no effect of sea
We found alligator activity to be greatest dur
son on activity in this inland alligator popula
tion.
ing the month of September
(Fig. 2). However,
as on
land,
in water.
on
floating
If alligators
or above-water
were
observed
structure,
in
or
the water,
September
2006
Lutterschmidt
and Wasko-American
alligator
30
in a disturbed
no 22
L 0
--
V0 25
0)
Air Temperature
Water Temperature
a
18
inland
3
lake
22
349
15
I
13
U
20~
*~
20
0
0
0
.5 15
0
.
0
Alligators =0.363(Air)+ 7.368r0;
F = 1.581 df= 1,10 P) 0.237
>
-;
C 0 0
O
0
is
0
13
2E
r S
Alligators= 0.526(Water)+ 2.874; r2= 0373
F=5.942 df=1,10 P =0.035
0
10
15
20
25
Temperature
FIG. 3-Regression
temperatures
sissipiensis)
tical
shown.
fect
and
and American
activity.
results
Only
analyses
for both
water
explained
Regression
(C)
37%
of
Size Class (in)
between
air and water
alligator
(Alligator
equations
air and water
temperature
35
30
had
and
temperature
mis
statis
are
a significant
the variation
in activity.
ef
FIG.4-Mean number of American alligators (Al
ligatormississipiensis) observed in each size class cal
culated from the 12 monthly surveys. Black caps
above each gray bar represent standard errors. Black
circles above each standard error indicate the great
est number of alligators observed in each size class
and the respective month(s) in which these obser
vations were made. An asterisk indicates that a size
class differed significantly (P < 0.05) from other size
classes. The last bar (i.e., unknown size class) was not
tested (NT) in the anlalysis for differences among
size class. Numbers at the top of the graph indicate
the number of observations (n) for each size class.
Although
there was a nonsignificant
effect of
month or season on activity, most activity and
the activity season for this population
seemed
to range from March
to November, with ap
15 alligators
observed
each
proximately
month. This activity period ismost likely asso
ciated with warm water temperature
(>20'C).
Wood et al., 1985) was 0.19 alligators per hect
During many months of this activity period, wa
ter temperature was higher than air tempera
are in Lake Raven. The mean alligator density
ture. Linear regression analyses indicated that
recently reported on the Welder Wildlife Ref
uge was 0.62 alligators per hecztare (Altrichter
there was a significant relationship between wa
ter temperature and alligator activity, with wa
and Sherman, 1999). Big Lake (56.7 ha) of the
ter temperature explaining
Welder Wildlife Refuge was the largest of the
37% of the varia
sites sampled and supported 21 alligators in
tion in activity (Fig. 3). Like many reptiles oc
cluding 9 adults. Based on data from the Weld
curring in the subtropical region of the south
er Wildlife Refuge, Lake Raven (i.e., 85.02 ha)
ern United
States,
alligators
demonstrate
has a relatively low density. It might be of fu
yearlong activity. However, questions regarding
ture interest to investigate if the alligator den
seasonal activity and extended patterns of sea
sity of Lake Raven is correlated with its level of
sonal activity might be of interest because of
disturbance.
the increased thermal inertia of an alligator as
Size Class Distribution-We
estimated TL of
sociated with large body mass and the use of
each alligator
as described
by McIlhenny
an aquatic system as a thermal buffer.
(1935) to investigate size-class structure of the
Population Index-We
observed
a mean of
Lake Raven population. Seven size classes were
16.25 (SE = 3.46, n = 12) alligators in Lake
the size-class distribution of
index is used to describe
Raven each month. This population
alligators in Lake Raven. We found similar
most likely a conservative estimate because we
numbers of alligators in all size classes except
only counted alligators observed during our
for the size class that ranged from 0.6 to 0.9 m
defined sampling regime. Alligators that might
TL (Fig. 4). The mean monthly count of alli
have been submerged or on shore in dense
gators in this size class differed significantly (H
vegetation were missed for census. Based on
= 18.129, dlf = 6, P
0.006) from the other 6
the mean number of alligators observed each
by Taylor et al.
size classes. Data presented
our calculated
index of density (e.g.,
month,
350
71heSouthwestern Naturalist
vol. 51, no. 3
(1991) suggest that alligators do not become
reproductive until they reach a total length of
1.83 m; Altrichter and Sherman
(1999) classi
fied alligators <1.5 m as juveniles. Most of our
observations
(i.e., 5 of 7 size classes in our
study) represent counts of juvenile alligators.
It is possible
that the data presented
in the
"unknown
size class" (Fig. 4) might explain
our observed size-class structure. Fifty-nine of
the 195 alligators observed (30%) could not be
classified due to the alligators submerging pri
or to estimating
size. Thus, many of our un
classified alligators might have been reproduc
tive adults because larger reproductive
alliga
tors are more experienced
at avoiding close
human interaction by submerging.
the above estimates of relative
Although
abundance
and size-class structure are impor
tant for future work in Lake Raven and might
be of interest for comparison with available
data on Texas coastal populations,
we offer
some caution regarding the comparison of sur
vey data across studies. Eye-shine night-counts
and population
estimates of alligators can be
influenced
by survey methods
(Chabreck,
et al., 1978; Woodward
and
1967; Magnusson
Marion, 1978; Taylor and Neal, 1984; Wood et
al., 1985) and differences among habitat types.
it might be difficult to draw com
Therefore,
parisons between a coastal habitat and a small
inland lake.
Distribution and Habitat Use-The
distribution
of alligators in Lake Raven also was investigat
ed by analyzing their occurrence
in 6 regions
of the lake (i.e., Prairie Branch Creek and Al
ligator Branch Creek arms, north main, central
main, south main, Big Chinquapin Creek arm,
and Little Chinquapin
Creek arm). A Kolmo
gorov-Smirnov
goodness of fit (Zar, 1999) in
dicated that distribution
of alligator observa
tions was random (Z = 0.770, df = 5, P =
each monthly census was 5.0 m (SE = 0.64, n
= 12). Most alligators were inactive; only 3%
(6 of 195) of alligators observed were either
cruising the shoreline or crossing the lake.
This study provides information
on an in
land and coastally isolated (but see Ryberg et
al., 2002) alligator population
from a highly
disturbed
public site. Although
the relative
abundance
of this population
is lower than
less-disturbed Texas populations
(e.g., Welder
Wildlife Refuge) and we observed few large al
ligators in this population, we observed a re
productive event in which hatchling alligators
were sampled. What might be of greatest in
terest is that this population
seems to be sus
tainable despite being subjected to high urban
disturbance. The idea that alligator occurrence
might be restricted to those localities that are
more
or less inaccessible
to man
(Brown,
1950) has been reevaluated and deemed
inac
curate. Alligators are now commonly found on
both private and public lands, and state agen
cies (e.g., Texas Parks and Wildlife)
have had
to educate the general public about laws and
regulations regarding the protection, manage
ment, and conservation
of this species. Much
effort is dedicated also to teaching the public
to respect the potential
of this animal as a
predator while sharing recreational
sites such
as Lake Raven. More importantly, inland pop
ulations of alligators in Texas have received less
attention
than larger coastal and reservoir
populations. Additional monitoring
of alligator
populations
in all habitat types might provide
comparative data for the sustainability of these
in areas of human disturbance.
populations
0.593). Thus, alligators did not avoid or use
particular areas of the lake (Fig. 1). Many of
the repeated localities might be observations
of the same alligator using the same area of
the lake each month. Visual inspection of the
shoreline
indicated that general habitat types
were similar around the lake except for the
shoreline near the swimming area.
Observations
of general habitat use indicat
ed a tendency for alligators to be in the water
at the time of surveying. The mean estimated
distance of alligators from the shoreline
for
Wildlife
thank
We
Smith
late H.
the
(Assistant
staff at the Huntsville
for
use
of
of
and
with
thanks
to G. Creasy
GIS
and Wildlife),
D. Millican,
for
improve
Spanish
the Health
of
reviews
and
translation
and
Cen
extended
Research
for
(TRIES)
We
give
as
special
Texas
Parks
D. Hill-Hollyday,
and
J. Williams
L. Thies
comments
of our
and
We
for
Biologist,
C. Wall,
this manuscript,
Parks
survey.
the Texas
J. Harrison,
thank M.
other
and Kinesiology
Studies
(Wildlife
R. Puckett,
We
this
and mapping.
data
and
and Texas
University
R. Rush
L. Dent,
assistance.
K. Jones,
Park
for Environmental
D.
(Park Manager),
to conduct
State
sistance
das
us
Houston
a canoe
Institute
field
State
allowing
R. Duncan
thank
ter at Sam
Elahi
Park Manager),
and
that
have
G.
Quiroz
abstract.
for
X. Glau
helped
to
for
the
September
Lutterschmidt
2006
and Wasko-American
Cited
Literature
(Alligator
mississippiensis)
Texas. Texas Journal
Refuge,
146.
Brandt,
L.
1991. Long-term
the Welder
alligator
Wildlife
51:139
of Science
in a population
Carolina.
Jour
changes
in South
of Alligator mississippiensis
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