0213187 COVER SHEET FOR PROPOSAL TO THE NATIONAL SCIENCE FOUNDATION PD 98-1128 01/10/02

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COVER SHEET FOR PROPOSAL TO THE NATIONAL SCIENCE FOUNDATION
PROGRAM ANNOUNCEMENT/SOLICITATION NO./CLOSING DATE/if not in response to a program announcement/solicitation enter NSF 02-2
PD 98-1128
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01/10/02
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DEB - Ecology
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Patterns of community assembly in serpentine grasslands: the roles of
initial plant composition and functional complementarity
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Dept. of Biology
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1996
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David U Hooper
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Patterns of community assembly in serpentine grasslands: the roles of initial plant
composition and functional complementarity
Project Summary
The search for patterns of community assembly has occupied the last 25 years in
community ecology and remains controversial today. A commonly sought assembly rule is that
species coexistence in communities will be enhanced by differences in traits related to
competition for limiting resources (i.e., that niche overlap is minimized). Put another way, do
species from complementary guilds or functional groups have a better chance of persisting than
those with strong overlap in resource use characteristics? Evidence for the role of
complementarity among plant groups in structuring communities is sporadic. The balance
between rates of external stochastic processes (such as disturbance or priority effects of
colonization) compared to internal community dynamics (such as competition or facilitation)
may be critical in determining the degree to which internal assembly dynamics are manifested.
Gaining further insight into this question is critical for understanding how communities might be
structured, and for the implications of that structure for ecosystem dynamics.
We propose a five-year experiment to assess the roles of initial plant composition and
functional complementarity in structuring communities of serpentine annual grasslands. We
seek to test several hypotheses centered on the following questions, generally grouped into two
themes: 1) Does the initial composition of a community shape its subsequent development? If so,
for how long? Are there different stable endpoints? 2) To what extent does complementary
resource use (i.e., niche differentiation) versus dominance among plant species govern patterns
of community development? Are other factors, such as seed input or climatic conditions more
important in determining community composition? Our goal is not to prove or disprove the
predominance of assembly rules, but rather to better understand the conditions under which they
might strongly influence community composition versus be overwhelmed by other factors.
We will use as our experimental system a series of California serpentine grassland
communities that vary in functional group composition and richness. Functional composition
has been maintained since the plots were established in 1992. In fall 1999 and 2000, seeds of six
additional species (3 species each from two groups) were added to test the effects of functional
characteristics and diversity on invasibility of ecosystems. Thus we now have a system with
treatments that differ in initial functional composition, that have had common seed sources for a
number of additional species, and that will be subject to continuing seed rain from surrounding
grasslands. Our plan is to track changes in community composition over the next five years as
the communities develop from their initially different starting points.
The measurements we plan are fairly simple: 1) nondestructive measurements of species
abundances by the point quadrat method; 2) mapping of potential seed sources from the
surrounding community; and 3) measurement of weather variables (temperature, precipitation,
wind speed and direction). Patterns of community composition and change will be assessed by a
variety of statistical techniques including nonparametric indices of community concordance,
similarity indices, cluster analysis, detrended correspondence analysis (DCA), Analysis of
Variance, and null models. Monitoring of vegetation changes in these experimental communities
through the next several years provides a unique opportunity to gain insight into interactions
among a variety of processes that help shape ecological communities.
TABLE OF CONTENTS
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Project Summary
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Project Description (plus Results from Prior
NSF Support) (not to exceed 15 pages) (Exceed only if allowed by a
specific program announcement/solicitation or if approved in
advance by the appropriate NSF Assistant Director or designee)
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References Cited
5
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Biographical Sketches
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Budget
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(NSF Form 1030, plus up to 3 pages of budget justification)
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Current and Pending Support
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Appendix (List below. )
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NSF Form 1359 (10/99)
Community assembly in serpentine grassland
1
RESULTS FROM PRIOR NSF SUPPORT
NSF Pre-doctoral fellowship (1990-1993) and NSF Doctoral Dissertation Improvement
Grant (DEB-9212995) $15,000. (1992-1994). "Effects of Plant Species Diversity on Nutrient
Cycling in a Restored Serpentine Grassland in California", with Dr. P.M. Vitousek (P.I.).
I studied the effects of plant diversity on ecosystem processes using California serpentine
grassland as an experimental system. I established experimental plots with four plant functional
types: early season annual forbs (E), late seasonal annual forbs (L), perennial bunchgrasses (P),
and nitrogen-fixers (N). Two or three species of each group were planted in single functional
group treatments, and in 2-, 3-, and 4- way combinations of functional groups. This design
allowed me to assess effects of differences in functional group richness, as well as differences in
plant composition within levels of functional group richness. We were thereby able to
differentiate among mechanisms (e.g., complementarity vs. the sampling effect) by which plant
diversity might affect ecosystem productivity and resource dynamics. The study assessed how
plant composition and richness affected productivity, soil resource dynamics, and nitrogen
retention and distribution within the ecosystem. The study led to several novel findings, which
were published in a number of first-authored and co-authored articles (Vitousek and Hooper
1993, Hooper et al. 1995, Chapin et al. 1997a, Chapin et al. 1997b, Hooper and Vitousek 1997,
Hooper 1998, Hooper and Vitousek 1998, Chapin et al. 2000). The paper by Hooper and
Vitousek (1998) won the Mercer Award for an outstanding paper in the field of ecology given by
the Ecological Society of America in 2000. In addition, an undergraduate student at Stanford did
her Honors Thesis in collaboration with this project (Cynthia Benton, "Timing of release of labile
nutrients in decomposition of plant litter on the serpentine grasslands").
Effects of Plant Diversity and Functional Characteristics on Species Invasions
(DEB-9974159) In collaboration with Jeffrey Dukes and Julia Verville (University of Utah), I
have been using the experimental plots established in the previous study to investigate how plant
functional composition and diversity influence invasibility of ecosystems. We are testing the
hypothesis that greater plant functional diversity leads to decreased success of invading species.
The factorial design of the study also allows us to test how the functional characteristics of the
invaders interact with the functional characteristics of species already in the community to
determine invasion success. Invaders were seeded in known amounts into all treatments in the
experimental diversity gradient (described above). We selected three species from each of two
functional groups (E and L; 6 species total) to use as invaders. Each of these species was seeded
into a separate subplot of each community to avoid confounding the success or failure of an
invader with the establishment of other invaders. This project just finished its second field
season. The data show strong effects of both functional group traits and functional group
diversity on invasibility. For early season annual (E) invaders, the presence of other species from
the same functional group in the original community is the strongest determinant of invasion
success. For late season annual (L) invaders, both the presence of other L's in the original
community, as well as the functional diversity of the original community influenced invasion
success. Finally, more diverse plots had lower average success for all invaders considered
together.
Results of this work have been presented at several meetings (Hooper et al. 2000, Dukes
et al. 2001a, b, Hooper and Dukes 2001). One manuscript is in preparation (Hooper et al., in
prep) and at least two others are planned. I have also been extensively involved in efforts to
synthesize and come to consensus on the state of research on questions of biodiversity and
ecosystem function (Hughes and Petchey 2001). Training has included three Research
2
Experience for Undergraduate (REU) students during summer 2000 and 2001. Results from one
student’s undergraduate thesis based on this work will be submitted to Soil Biology and
Biochemistry (Chiarelli et al., in prep). Two other undergraduate students have done independent
study projects based at least in part on this research.
PROJECT DESCRIPTION
INTRODUCTION
A fundamental question in ecology is why different species assemblages occur next to
one another under what appear to be identical environmental conditions (e.g., McCune and Allen
1985, Drake 1991, Petraitis and Latham 1999). Theoretically, communities of different
composition might arise from 1) differences in starting conditions, or from 2) disturbances that
alter species densities sufficiently to push the community across a threshold and towards a new
equilibrium state (Gilpin and Case 1976, Hughes 1989, Laycock 1991, Knowlton 1992). In what
cases do different starting points lead to different stable states? If communities converge from
different starting points, how long does it take them to converge, and what determines the
trajectory? How do external factors (climate, disturbance) influence the outcome? Similar
questions have been addressed in some models (e.g., Law and Morton 1996) and microcosm
studies (e.g., Robinson and Edgemon 1988), but the identification of basic rules governing the
development of complex communities remains on the frontier of ecological research (Thompson
et al. 2001).
The study of assembly rules for ecological communities seeks to understand the processes
that select for membership in a given community from a regional pool of species (Belyea and
Lancaster 1999, Keddy and Weiher 1999). Assembly rules may be studied across a range of
different scales and settings, from the regional scale of variation in species composition across
islands to local variation in community composition within a habitat type (Cody 1999). Our
study proposes to investigate plant community composition at this smallest scale. Given the
same habitat type (climate, soil type, soil depth), what factors determine plant community
composition? We ask this question in the context of trying to understand patterns of species
invasion as a function of initial community composition.
In the proposed experiment, we focus on two primary groups of questions. First, does the
initial composition of a community shape its subsequent development? If so, for how long? Do
different initial starting points lead to different stable endpoints? Second, to what extent does
complementary resource use (i.e., niche differentiation) among plant species influence
community composition? Is plant community assembly dependent on different functional groups
of species, or is dominance by one or a few species the rule? Or, are stochastic factors, such as
seed input or climatic conditions, more important in determining community composition? Our
goal is not to prove or disprove the generality of an assembly rule based on complementarity, but
rather to better understand the conditions, if any, under which such a process might strongly
influence community composition.
While of theoretical interest to basic ecology, these questions are relevant to many
applied issues as well. First, ecologists and land managers have a major challenge in trying to
understand patterns of spread in exotic species (Luken and Thieret 1997, Mack et al. 2000).
Many questions in this field are directly relevant to issues of community assembly, including:
Which types of species are most likely to invade? Which types of ecosystems are likely to be
invaded? How do traits of invading species interact with those in the extant community? A
second applied issue is trying to understand how community composition will change in response
to global environmental changes, and the resulting consequences for ecosystem processes and
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services (Dukes and Mooney 1999, Walker et al. 1999b). Finally, improvement of restoration
ecology techniques depends on prediction of the trajectory of community composition based on
initial composition (Lockwood and Pimm 1999). If restoration ecologists can determine which
initial community compositions will lead to a desired endpoint, this might simplify initial
restoration efforts and reduce the management necessary to guide the community towards the
target outcome.
BACKGROUND
Assembly rules?
The term "assembly rules" has been controversial in ecology. In a nutshell, ecologists
want to know whether particular processes govern the inclusion or exclusion of species from
communities and if such processes lead to any predictability in the composition of those
communities. A primary issue in the debate over assembly rules in ecology focused on the most
appropriate null models with which to compare the composition of pre-existing communities (for
example, Diamond 1975, Connor and Simberloff 1979, Colwell and Winkler 1984, Gotelli and
Graves 1996). When selecting a null model, it can be difficult to determine which species should
be included in the potential species pool and how to deal with non-overlapping or partially
overlapping geographic ranges, differences in relative abundance, differences in habitat
preferences, or any other factors that might influence the probability of species being found
together (Fox 1999, Simberloff et al. 1999). Much of this difficulty disappears, however, when
sampling occurs only on a local level under common conditions, such that all species considered
share ranges and habitat preferences (Simberloff et al. 1999).
Potential rules involving functional groups
To test for the possibility of complementarity in community structure, traits of species,
rather than species themselves, need to be the focus of investigation. One of the first assembly
rules proposed (Diamond 1975), and one that is still commonly analyzed and debated, is that
competition limits the similarity of species able to persist in communities. A common test for
this involves whether certain species are more or less likely to be found together in natural
communities than at random. However, many researchers have argued that a more general
approach would look at species traits rather than taxonomic affiliations (Fox and Brown 1993,
Weiher and Keddy 1998, Belyea and Lancaster 1999). Stated another way, are coexisting species
more likely to be from different guilds or functional groups (presumably based on suites of traits
related to resource utilization) than would be expected from random communities? Many studies
- particularly with mammals and birds - have found evidence for such community structure,
while others have either not found patterns or have questioned the validity of those that have (see
references in Weiher et al. 1995, plus the ongoing debate: Kelt and Brown 1999, Fox 1999,
Simberloff et al. 1999). Much of this debate turns on the questions of appropriate null models
and test statistics outlined above. Importantly, the models of Fox, Kelt, and coworkers
specifically attempt to test for complementary guild or functional group structure. However, they
have not been applied to plant communities to our knowledge.
The model of Wilson (1989), on the other hand, looks for constant proportionality of
different guilds or functional groups across different areas of a sampled community. It asks
whether certain functional groups are found in more constant proportions than would be expected
at random across many samples of a community or across communities (Wilson 1999). If such a
pattern holds, as it does for at least some groups in several plant communities (Wilson and
Roxburgh 1994, Wilson et al. 1995, Wilson and Whittaker 1995), then an assembly rule may
bring about that pattern. This approach does not test directly for complementarity, although
4
competition or complementarity interactions could be the underlying mechanism that leads to a
guild proportionality pattern.
A third trait-based approach looks at the distribution of various morphological or
functional traits, and asks whether the distribution of those traits is broader (overdispersed) or
narrower (underdispersed) than would be expected at random (Weiher and Keddy 1998). To the
extent that the traits analyzed are relevant to resource acquisition or utilization, overdispersed
traits would indicate that competition for a limiting resource leads to selection for
complementary traits at the community level. Underdispersed traits are thought to indicate
environmental constraints or filters allowing access to a given type of habitat (Weiher et al. 1995,
1999). The latter approach is consistent with classifications of plant functional groups across
environmental gradients (c.f. Grime 1979, Chapin et al. 1996, Díaz et al. 1999). Because both of
these processes could operate simultaneously, the logical question follows: To what extent does
complementarity among plants actually influence community structure in natural plant
assemblages?
The role of complementarity in structuring communities is of great interest to ecological
disciplines outside of community ecology, particularly to the debate on diversity effects on
ecosystem function (Kaiser 2000). Some groups argue that complementarity among species or
functional groups has a strong effect on ecosystem processes (Tilman et al. 1996, Tilman et al.
1997, Hector et al. 1999, Loreau and Hector 2001, Tilman et al. 2001), while others argue that
the traits of dominant species are the primary biotic determinant of process patterns and rates
(Huston 1997, Wardle et al. 1997a, Wardle et al. 1997b). The latter group has also criticized the
experimental design of the former, saying that because ecological communities follow distinct
(but unspecified) assembly rules, randomized experiments cannot provide insight into the effects
of diversity on processes (Wardle 1999). If complementarity among functional groups is one
such strong community assembly rule, it has immediate implications for this debate.
While there is some evidence for plant community structure responding to guild or
functional group complementarity, the evidence so far is mixed. Positive associations among
plant species of different functional groups at individual sampling points suggests that
complementary plant associations affect community composition and structure at relatively small
neighborhood scales (Wilson and Roxburgh 1994). Weiher and Keddy (1995) found qualitative
evidence for complementary distribution of dominant species among functional groups of
different plant growth forms in experimental wetland communities. In grazed grasslands in
Australia, those species with the highest rank abundances were from different functional groups
and additional species from the same groups had relatively low rank abundances (Walker et al.
1999a). This pattern suggests complementarity among dominant species, and dominance
patterns among species of the same functional group. Our own studies have found evidence of
complementarity among plant functional groups (Hooper 1998, Dukes 2001b), as have many
others in both experimental ecosystems and intercropping (e.g., Trenbath 1974, Vandermeer
1990, Haggar and Ewel 1997). However, demonstrating that complementary interactions
influence primary production in manipulated communities does not necessarily demonstrate that
they are a driving force in community assembly in natural systems. Studies of community
development under primary succession point to simultaneous effects of facilitation, competition,
and tolerance among species in structuring communities (Chapin et al. 1994). It remains to be
seen how guild or functional group complementarity influences community composition and how
such internal community dynamics interact with stochastic forces from outside the community.
Interaction of assembly rules with stochastic forces
5
There is abundant evidence in both model (e.g. Lockwood et al. 1997) and microcosm
communities (e.g. Robinson and Dickerson 1987, Robinson and Edgemon 1988, Samuels and
Drake 1997) that colonization precedence can lead to alternate stable states. Weiher and Keddy
(1995) focused on different environmental conditions and how they affect community structure
and convergence in wetland communities. Some differences in environmental conditions still
allowed for community convergence (differences in fertility), but others did not (differences in
water level). There was a stochastic component of community composition in terms of relative
abundances of both core species (those that were in all replicates of a given environmental
treatment) and occasional species. Questions remain about the relationship between species and
functional composition, however. For example, are species compositions relatively labile and
subject to priority effects within the constraints of assembly guided by functional traits, or do the
predominant functional traits vary as well?
Other stochastic processes such as disturbance and climate variability can also have
profound and persistent effects on community structure and composition. In aquatic microcosms,
Fukami (2001) found that different sequences in disturbance led to different final community
composition. In California serpentine grasslands, rainfall variability, timing of soil disturbance
by gophers, and harvester ant activity all influence local plant species composition (Hobbs 1985).
A critical question is whether species composition within functional groups might vary, even as
relative proportions of functional groups stay comparatively constant.
In addition, structuring of community composition by assembly rules may depend on
rates of colonization relative to rates at which the community comes to equilibrium. Modeling
studies suggest that if rates of colonization exceed the rate at which species are selected by
assembly rules, then assembly rules may be disrupted (Belyea and Lancaster 1999). Modeling
studies also suggest that simultaneous species additions will confound any structured assembly
process (Belyea and Lancaster 1999, and references therein). While these reviewers express
concern that testing these hypotheses in the real world will be difficult or impossible, we suggest
that our experimental system is in a good position to do just that.
Serpentine grassland as an experimental system
California serpentine grassland is a promising system in which to address questions of
community assembly. This ecosystem is characterized by small-statured, predominantly annual
species with a variety of growth strategies. Several plant functional types based on phenology,
growth form, and nutritional dynamics (e.g., N-fixation) have been identified by previous
researchers. For example, in this Mediterranean-type climate, annual species germinate after the
first significant rains in the fall. Early season annuals set seed and die by early in the dry season
(~May), whereas late season annuals remain in rosette form through the wet season, then flower
from June - October (Gulmon et al. 1983, Mooney et al. 1986, Chiariello 1989). Perennial
bunchgrasses set seed in late May, then senesce aboveground during the dry season (Jackson and
Roy 1986, Jackson and Roy 1989). Other functional groups that have been identified include
perennial forbs (both mono- and dicotyledonous, including geophytes) and annual grasses (both
native and exotic). In addition to differing in phenology and morphology, these groups differ in
several traits relevant to resource acquisition and nutrient dynamics, including rooting depth,
root:shoot ratio, size, and leaf C:N content (Gulmon et al. 1983, Hobbs 1985, Mooney et al.
1986, Chiariello 1989, Armstrong 1991). These characteristics allow easy experimental
manipulations using species with well-defined suites of functional traits.
In a previous experiment, we established plots differing in both functional group
composition and richness (Hooper and Vitousek 1997, Hooper 1998, Hooper and Vitousek
6
1998). We chose species from four groups: 1) early season annual forbs, 2) late season annual
forbs, 3) perennial bunchgrasses, and 4) nitrogen-fixers (hereafter abbreviated as E, L, P and N,
respectively). In that experiment, we focused on the effects of functional group composition and
richness on plant productivity and nutrient cycling (see Results of Prior NSF Support). Initial
studies in these plots (Hooper 1998, Hooper and Vitousek 1998), subsequent measurements, and
other projects (Dukes 2001b) have shown strong evidence for complementarity among at least
some functional groups. These functional groups had different seasonal patterns of resource use,
particularly of nitrogen and water. Other studies have found similar patterns in this system
(Gulmon et al. 1983). If complementarity has a strong role in structuring natural serpentine
communities, then we predict that species from complementary functional groups will be
community dominants.
Furthermore, functional group composition, richness, and complementarity can all
influence the success of subsequent invasions. In a microcosm experiment, Dukes (2001c,
2001a) found that mixtures of species from complementary functional groups decreased invasion
success of the aggressive exotic weed Centaurea solstitialis (yellow star-thistle). In a follow-up
experiment to Hooper and Vitousek (1998), we used the original experimental gradient of
functional composition and diversity to evaluate the dynamics of species invasion. We
established six subplots within each plot, and introduced one invading species into each of the
six subplots. This design allowed us to study which factors are most important in determining
the success of an invader: the characteristics of the invading species, the characteristics of the
community being invaded, or the interaction of these two factors. We have found interactions
between invader and community functional groups, as well as effects of overall community
diversity (See Results from Prior NSF Support). We are currently working to understand how
patterns of resource availability, as influenced by the different species, influenced these results.
These plots, and species from the surrounding natural serpentine grassland, are now an
excellent resource to investigate patterns and mechanisms of community assembly, including the
extent to which initial composition determines future composition, the importance of
complementarity in structuring community composition, and the rate at which communities
converge. We have variable and known initial composition, known seed sources, and some
species common to all experimental treatments (the seeded invaders). This arrangement will
allow us to test a number of hypotheses related to complementarity among functional groups and
effects of initial composition on subsequent community trajectories (see below). In addition,
substantial areas of intact serpentine grassland outside of the experimental plots are available for
simultaneous monitoring of natural communities for comparison with our plots.
We strongly expect that community composition in all experimental treatments, even the
most initially diverse, will change through time. What is not clear, however, is how these
communities might change. These plots do not currently represent natural serpentine grassland,
particularly because of our strict control of functional composition. Given the patterns of
weeding over the last several years, it is apparent that none of the experimental communities are
"saturated" with respect to species richness. Clearly, the bare plots most closely resemble a
recently disturbed site. Because of colonization of additional species, we anticipate that species
and functional composition will more closely mimic unmanipulated grassland over the next
several years. But we could be wrong – and that would be interesting. Will the roughly
complementary structure of the most diverse communities stand up to invasion by new species,
or will they become dominated by particularly aggressive competitors? Will the seeded invaders
dominate the bare plots indefinitely, or will there be a succession of species? Will initially bare
or depauperate communities eventually resemble the more diverse complementary communities,
7
or will the original species continue to dominate? Will deterministic forces (e.g.,
complementarity), govern species additions to the communities, or will more random forces
(proximity of seed sources, disturbance history) predominate? This experimental system is
poised to address these questions.
HYPOTHESES
We propose to test several hypotheses related to community assembly in these
experimental plots. We describe the more prominent ones here.
Patterns of dominance and composition
1. Patterns of composition across treatments
H1. The same species will dominate all experimental plots regardless of initial composition.
(null hypothesis.)
H1a. Differences in community composition among treatments prior to this experiment
(2000-2001 growing season) will persist through time.
H1b. Differences in community composition prior to this experiment will lead to new but
different community compositions.
Explanation. We will be able to test these hypotheses by comparing the eventual
compositions of replicate plots from across the initial gradient of diversity and community
composition. If community dynamics occur quickly in this annual grassland and community
assembly is relatively deterministic, then it is possible that species composition in the different
treatments will converge within the time of study. This would allow us to rule out H1a and H1b.
On the other hand, if the initial composition strongly governs community dynamics over the first
several years and communities do not converge by the end of the funding period, we will not be
able to rule out H1 - it may be that more time is required for communities to converge. However,
we will be able to observe rates of community convergence among treatments of different initial
composition to see which ones become most similar most quickly over the duration of the
experiment. We can test for similarity in patterns of dominance by using nonparametric tests
[e.g., Kendall's coefficient of concordance (Zar 1996)] and for overall similarity in community
composition by using Sorenson’s similarity index, cluster analysis (Magurran 1988), and
ordination techniques such as detrended correspondence analysis (DCA) (e.g., Weiher and Keddy
1995, Fukami 2001) (see Statistics, below).
H2. New species establishment in plots will reflect proximity to seed sources, independent
of initial community composition (i.e., propagule pressure is more important than initial
community functional composition).
H2a. New species establishment will depend on initial composition but not proximity to
seed sources.
H2b. Neither initial composition nor seed source proximity matters.
H2c. Both matter.
Explanation. This hypothesis and H6 (below) make interesting tests of the generality of
the results we are currently getting with our invasibility experiment (see Results of Prior NSF
Support), as well as other such experiments (e.g., Levine 2000). We will be able to test directly
for effects of propagule pressure on success of individual species (focusing on those that were
not part of the original factorial design of functional groups). By mapping potential seed sources
and keeping track of wind speed and direction during seed dispersal periods for the various
species, we can weight distance to nearest seed source by these variables. We can then include
8
this estimate of propagule pressure as a covariate in an Analysis of Covariance of individual
species’ abundances in response to initial functional group composition (see Statistics, below).
2. Within treatment comparisons
Comparison of replicates within a treatment will give a better understanding of stochastic
components of community change as well as effects of dispersal and invasion loci. Do replicates
of the same initial composition diverge or stay the same? If replicates diverge initially, do they
converge again later? If replicates diverge, does this depend on proximity of sources for new
colonists?
H3. Species composition is similar among all replicates of a given treatment.
H4. Functional group composition is similar among all replicates of a given treatment.
H5. Divergence among replicates depends on proximity to invasion loci for new species.
Explanation. Hypotheses 3 and 4 can be tested in a couple of different ways. First, do
replicates group together in a cluster analysis based on the relative abundances of either species
or functional groups when compared to other treatments. Is this consistent through time?
Second, do similarity indices among replicates of a treatment, based on either species or
functional characteristics, show significant linear or nonlinear trends over time? Hypothesis 5
can be tested by regression of abundances of new species (not those in the original community)
against estimates of propagule pressure for replicate plots within treatments.
3. Tests of focal species: the seeded invaders.
H6. Abundance of seeded invaders will be equal throughout all plots (null hypothesis).
Explanation. Because the six species introduced in our previous invasion study were all
sown in constant, known amounts in all treatments, they provide excellent test species to follow.
While we state the null hypothesis, we predict that those species we seeded in as invaders in
1999 and 2000 will have greatest dominance in experimental plots that were initially less diverse.
So far, seed input of these experimental invaders has largely been restricted to what we have
added ourselves. Under these conditions, initial community composition has strongly affected
the success of these invaders (Fig. 1). However, population dynamics of the original invaders
and colonization of new species could alter the interactions that determine final community
composition. Testing these hypotheses can be done with ANOVA of invader abundances in
response to the initial functional group treatments (see Statistics, below).
Patterns of diversity
H7. After 5 years, there will be similar species diversity in all plots, regardless of initial
composition. (null hypothesis.)
H7a. Final species diversity will be a function of initial composition, independent of initial
diversity.
H7b. Final species diversity will be correlated with initial diversity.
Explanation. Hypothesis 7 and its alternates are somewhat similar to H1- H1b, but are
worth testing in their own right, because the overall diversity of subordinate species may differ
despite similarities in dominants, or vice versa. Testing this hypothesis can be done with
ANOVA of plot species richness in response to the initial functional group composition (see
Statistics, below). In addition, we can compare variation in diversity with variation in different
micro-environments in the natural grassland to get a better understanding of how it fits with
natural variation (see Measurements, below).
Role of functional composition of structuring communities
9
We predict that if complementarity is a strong community assembly rule, then in a given
location the species that are most abundant will be from different functional groups (c.f. Walker
et al. 1999a). We propose to test the following hypotheses in intact grassland, as well as in the
experimental plots.
H8. The most abundant species will be random with respect to functional groups. (null
hypothesis)
H8a. The proportion of the most abundant species that belong to complementary
functional groups will be greater than that expected at random.
H9. The degree of functional complementarity among abundant species is independent of
micro-environment in natural grasslands. (null hypothesis)
Explanation. We have information about the functional characteristics and groupings of
many serpentine species - both those already in the plots and those likely to invade. The simplest
test for H8 involves contingency tests (Chi-square or G-test) of co-occurrence of species from
similar or contrasting functional groups (Bowers and Brown 1982, Kelt and Brown 1999). This
approach could also be used for tests of natural communities in different microenvironments (i.e.,
slope and aspect, which vary significantly in species composition; see Measurements, below).
Tests in different micro-environments in natural communities will help shed light on factors that
may control the degree of complementarity vs. dominance, such as soil fertility. Null models can
also be used to test hypotheses 8 and 9. Douglas A. Kelt, University of California, Davis, has
developed a suite of null models (Kelt et al. 1995, Kelt and Brown 1999) that are applicable to
our system, and has agreed to help us with these calculations (see Statistics, below, and attached
letter). Because performing the necessary measurements in the intact grassland will require
substantial additional work, we request funding for two sequential masters student research
assistantships to carry out the aspect of the project involving the intact grassland.
METHODS
Approach
Our overall approach is to monitor changes in species composition over time in
experimental plots that initially differ in plant functional group composition and diversity. The
measurements we propose are simple non-destructive estimates of plant community composition.
The original experimental treatments were actively weeded to maintain control of species
composition. The current invasion experiment added seeds in controlled amounts and restricted
invading individuals to separate subplots. However, we plan no further manipulations or control
of species composition, and no further control of seed inputs. We will compare species and
functional composition in these plots with that of the surrounding natural grassland.
Experimental design
Study site
Hooper (1998) established experimental plots at Kirby Canyon in south San Jose,
California (37o15’ N, 121o45’ W) near a landfill operated by Waste Management, Inc. This
region experiences a Mediterranean-type climate, with cool wet winters and a dry season
extending from approximately May to October. Average rainfall is ~370 mm/yr (Huenneke et al.
1990), though both the timing and amount are highly variable from year to year (Armstrong
1991, Hobbs and Mooney 1995). Species composition in the native grassland at this site depends
on a variety of factors, but is strongly influenced by local topography (Huenneke 1990). A
previously denuded area of grassland was used for the experimental plots. Topsoil stockpiled
from the landfill was placed over serpentinitic subsoil (C horizon) to a depth of approximately 30
cm., covering an area of approximately 0.25 ha. This gave a fairly homogeneous substrate on
10
which to plant the experimental treatments (Hooper and Vitousek 1998). The area was fenced to
exclude mammalian herbivores (e.g., gophers), and has so far been successful at doing so.
Experimental plots
For manipulations of functional group diversity in the original experimental communities,
species from four distinct functional categories were used, all of which are major components of
the surrounding grasslands (Gulmon et al. 1983, Hobbs and Mooney 1985, Mooney et al. 1986,
Chiariello 1989, Hobbie et al. 1992). The groups are early season annuals (E), late season
annuals (L), perennial bunchgrasses (P), and nitrogen-fixers (N). Each functional group was
planted alone and in various combinations in experimental plots measuring 1.5 by 1.5 meters
separated by 0.5-1 m buffer strips, using two or three of the most common species from each
group (Hooper 1998). Because of the small stature of serpentine vegetation, these plots are
large enough for self-supporting populations of all plants: hundreds (late season annuals) or
thousands (early season annuals) of individuals are encompassed in a 1 m2 area in native
serpentine grassland (McNaughton 1968, Gulmon et al. 1983). The experimental design is a
randomized complete block with six replicate blocks and a total of 10 treatments in each block.
Treatments include a full factorial cross of E’s, L’s and P’s: bare plots (B), each group alone (E,
L, P), all 2-way combinations (EL, EP, LP), and the 3-way combination (ELP). In addition, there
were two treatments including N-fixers: a single functional group treatment (N) and a 4-way
combination (ELPN). This provided a range of functional group richness from 0 (bare plots) to 4
(0-9 species). Treatments were established in Jan. 1992 at total densities and biomass that
approximated the natural community (~200 g/m2). Species not originally seeded in have been
continually weeded out to maintain the functional group treatments. Weed biomass has been less
than 5% of total biomass in most treatments (except bare and N-fixer) and years. No effort has
been made to control densities of individuals of the originally planted species.
For the initial invasion experiment, 6 species (3 each from 2 functional groups – E’s and
L’s) were seeded into separate subplots (30 cm x 30 cm each) in each experimental plot, as
described above. Equal amounts of seed were added to all treatments in Fall 1999 and again in
Fall 2000. Seeds germinating outside of the invading species’ subplot were counted to estimate
total germination success and then weeded out to avoid interactions among invaders. Seed
germinating in the 2000/2001 growing season is a combination of seeds we added and seeds
deposited from the previous years’ growth. We plan no further control of seed input or weeding.
Natural grassland
In addition to measurements in our experimental plots, we propose to measure species
composition, diversity, and relative abundances in natural serpentine grasslands adjacent to our
experimental plots. These measurements will give us a comparison of expected levels of species
diversity, and year-to-year and plot-to-plot (within treatment) variability in species composition.
They will also allow testing of effects of complementarity in community structure in natural
communities (via null models, as described below) for comparison with results in the
experimental plots. We will use 24 replicate plots (1.5 m x 1.5 m) randomly stratified across
four different micro-environments (6 plots per micro-environment) to capture the range of
natural variability in species composition and densities of individuals most relevant to what we
anticipate in our experimental plots. The four micro-environments are swales, level hilltops,
moderately sloping north aspects, and moderately sloping south aspects. Permanent plots will be
established and measurements will be by the point quadrat method in years 2-5, as described
below.
11
Suitability of the experimental design
Previous reviewers questioned if the experimental plots were appropriate for examining
the hypotheses of community assembly outlined above. We strongly feel that they are, for the
following reasons:
1. Plot size. The plots are large enough relative to the size of individuals that they can
support self-sustaining populations of all species. At the same time, they are small enough and
close enough together that seed dispersal should differ minimally among the individual plots by
the end of the experiment (see 4, below for more discussion on this issue).
2. The different initial community compositions allow us to directly test for priority
effects among treatments. Previous reviewers commented critically on the fact that these plots
were weeded for previous experiments but will not be weeded for this one, and that there are
likely to be differences in bare area among treatments. Contrary to being a weakness in the
experiment, we feel that this is an important component of the experiment. The very question we
are trying to test is how community composition changes when more species are allowed to
establish. The initial communities had been in place for eight years before the invasion
experiment. Because we weeded constantly throughout each year, weed biomass was always a
very small percentage (<5%) of total community biomass, and therefore should have had only
minor effects on plant densities. Therefore, differences in the amount of bare ground among
treatments are not an experimental artifact, but are a reflection of the maximum natural density of
individuals in any particular species composition treatment. Difference in available space is one
potential reason that different initial communities might diverge or take longer to converge. Our
point framing measures have quantified the amount of bare area in all plots prior to cessation of
weeding, so we can statistically test for such effects. Finally, disturbance by gopher mounds is a
common generator of bare ground in serpentine grassland (Hobbs and Mooney 1991), so
understanding community dynamics in such situations is directly applicable to surrounding
natural systems.
3. Similar species were seeded in during the invasion study. Three species from each of
two functional groups (early and late season annuals) were added to the plots as part of the
invasion study. Total experimental seed rain for these six species was identical across all
treatments. In the first two years of that study, however, strong differences developed in total
biomass of those invaders, but again, this is not an artifact. It is a result of the species
interactions in those different treatments arising from the differences in initial community
composition. This serves as the beginning step of our experiment on community assembly.
These initial six invaders can now serve as focal species. If the differences in invader success
persist throughout the proposed study, then that provides evidence for strong priority effects. If,
on the other hand, the initial differences decrease with subsequent invasion of new species from
both adjacent plots and from outside the experimental area, then depending on the patterns, that
could provide evidence for either random assembly or species interactions as strong factors in
community assembly.
4. Because we seek to test ideas related to species interactions as forces structuring
communities, we ideally would have identical seed rain for all species in this experiment so we
could control for dispersal limitations. We have that case for the six initial invaders. Seed rain
from species colonizing from the surrounding grassland will not necessarily be identical initially
(the experimental optimum). Some original invasion loci will undoubtedly be closer to some
plots than others. This is why we intend to continue mapping species composition in the vicinity
of the plots. In the long run, however, dispersal limitations to colonization should be minimal
because of the small plot size relative to seed dispersal distance of these species, because most
12
species are annuals, and because there are untended aisles between all plots where invading
species can establish and from which they can disperse. Eventually, all species should have
relatively similar potential to reach all plots. If early differences in community composition that
might result from early differences in dispersal, or from initial differences in invader success (3,
above) persist throughout, then that will be an indication that priority effects are more important
than competitive or complementary interactions in structuring these communities.
5. Replicates. We can thoroughly investigate questions of dispersal vs. priority vs.
complementarity among treatments, even with the complications discussed in 4, because we have
multiple replicates of all treatments. We can observe whether initial similarities in composition
among replicates diverge early in the experiment because of differences in seed input resulting
from different proximities to invasion loci (see Hypotheses 3-5, above). We can observe if any
early differences among replicates within a treatment diminish, persist, or intensify, in addition to
testing across treatments for convergence or divergence. The replicates will also allow us to test
if convergence or divergence among treatments is a general phenomenon or if there is variability
in that response. Such replication is exceptionally rare in semi-natural field experiments that
examine these questions.
Measurements
Species composition
Species composition and relative abundance will be assessed by using the point quadrat
method (Goodall 1952) with 90 points in a 10 x 9 point grid. A sharpened metal rod is passed
through a point frame fitted with levels to ensure that it is vertical. We currently have such a
frame and it is sufficient to fit over both bunchgrasses and late season annuals. To obtain an
estimate of relative leaf area of the different species, only contacts with the tip of the rod will be
recorded as the rod is passed down through the canopy (Goodall 1952). We are successfully
using this technique in our current study to assess species composition in the plots. High wind
can present difficulties in accuracy with some of the taller individuals, but we will attempt to
minimize such problems by avoiding measuring during excessively windy periods.
Unfortunately, we know of no other technique that would enable us to nondestructively measure
composition in as efficient and accurate a manner. At current rates, we estimate that with 90
points/plot, two people will take five days to measure all plots. This intensity of sampling
balances workload with accuracy, giving us a 60% chance of hitting species with 1% cover, an
84% chance of hitting species with 3 % cover, and a 99% chance of hitting species with 5%
cover (Goodall 1952). To reliably determine total species richness, we will note species that are
missed by point framing and visually estimate whether their abundance is greater or less than 1%.
The same methods will be used in plots in the intact grassland.
Because serpentine grasslands go through progressive flowering throughout the growing
season from March through late summer, we propose to assess species composition and cover at
three points in the season: 1) mid-April at peak biomass of the early season annuals, 2) early
June, when many geophytes and perennial bunchgrasses are flowering; and 3) mid-August, at
peak biomass of late season annuals. This intensity of measurement should capture most species
at their peak abundances, and will minimize the number of monitoring trips necessary. Finally,
we will continue to map out locations of important seed sources close to the plots to test
Hypotheses 2 and 5, above. We are requesting travel funds for these sampling trips for Dukes
(now based in Palo Alto, CA) and Hooper (based in Bellingham, WA). In addition we request
travel funds for 3 undergraduates to act as field assistants for these measurements (two recording
13
for Hooper and Dukes in the experimental plots, and one assisting the grad student in the natural
grassland).
Weather variables
Because interannual variability in weather patterns contributes significantly to patterns of
species abundance in serpentine grasslands (Hobbs and Mooney 1991, 1995), and because we
expect seed dispersal of several species to depend on wind patterns, we plan to install a
minimally-outfitted weather station on-site. This station will be instrumented to measure
precipitation, air temperature, and wind speed and direction. Because we expect to visit the site
only three times per year, we will download data from the station remotely, using an on-site
phone modem (telephone and power lines are available at the site). Data will be downloaded
directly to computers in Hooper's lab at Western Washington University. Funds are being
requested for this weather station and communication equipment.
Complications
While this is an excellent system for examining community assembly, several
complications require attention:
1) Time. Whether the species that initially invade plots are able to persist over several
years is an open question. While this is a predominantly annual grassland, with relatively rapid
population dynamics, we do not expect to see stable community composition after only three
years, especially with variable climate and disturbance regimes (see 3 and 4, below). We
therefore propose to track community dynamics over five years, and request funding for this
duration. Even then, we recognize that our observations may only capture the initial trajectories
of species change in some communities, but we have to start somewhere. If initial results are
promising, we can then apply for a grant extension.
2) Seed sources. Seeds of potential invaders from both outside and within the plots are
not equidistant from all plots. We propose two solutions to this problem. First, we are mapping
major potential seed sources in relation to plots to be able to investigate effects of seed proximity
on composition (which turns this complication into a strength). Because the experimental area is
separated by asphalt from the surrounding grassland, only a relatively limited area needs to be
assessed. In addition, we need to observe patterns of wind direction and speed during seed
dispersal to appropriately weight distances from different seed sources. For these measurements,
we will need a weather station (see 3, below). Second, we need enough time for seed sources to
spread throughout plots - another reason for asking for five years of funding rather than three.
3) Weather variability. As described above, rainfall variability is known to influence
serpentine community composition. Furthermore, local geography around the San Francisco Bay
area has a large effect on temperature and rainfall, both of which can vary greatly over the space
of several kilometers, so we cannot necessarily use measurements from local airports. We
therefore request funds to install a weather station at the site to enable precise measurements of
local temperature, rainfall, and wind direction and speed. This information will help to
understand fluctuations of populations present in the plots, as well as patterns of seed availability
(see 2).
4) Disturbance. Soil disturbance by gophers can greatly affect community composition in
serpentine grassland (Hobbs and Mooney 1995). The relative isolation of our experimental plots
from the native grassland has kept gophers out, however, we will trap to remove them should
they become a problem.
Statistical analyses
14
The statistical analyses necessary to test the hypotheses outlined above fall into three
categories: 1) tests of similarity among community composition and dominance using both
nonparametric statistics, similarity indices, clustering and multivariate data reduction (DCA); 2)
tests of similarity among initial functional group treatments for continuous variables (species
diversity, abundances of particular species, DCA scores) using ANOVA and appropriate a priori
and a posteriori tests; and 3) tests of nonrandomness of functional group composition using null
models. We briefly outline each of these approaches below, but note that there will be
substantial opportunity to test different approaches to the data analysis.
Community composition and dominance (H1, H3, H4)
Patterns of dominance can be tested by using nonparametric tests such as Kendall's
coefficient of concordance (Weiher and Keddy 1995), or a measure of top-down concordance,
with appropriate weightings (e.g., Savage scores) for the most abundant species (Zar 1996). In
addition, we can use Sorenson's similarity index and clustering (Magurran 1988) to investigate
patterns of species or functional composition within communities. Finally, overall patterns of
community composition can be compared using ordination techniques such as detrended
correspondence analysis (DCA) (Weiher and Keddy 1995, Fukami 2001). While DCA is
commonly used for nonlinear data such as species distributions across gradients (Pielou 1984,
Wartenberg et al. 1987), it can also be used to simplify multivariate abundance data into a few
primary axes. The DCA scores of plots from the initial functional group treatments can then be
subjected to our ANOVA design (described below) to test whether initial differences in species
composition persist throughout the experiment.
Analysis of variance for continuous variables (H2, H6, H7)
Hypotheses centered on continuous univariate data can be tested with a 4-way ANOVA,
with interactions and block effects, using initial functional group composition as treatments
(Hooper and Vitousek 1998). Appropriate a priori and a posteriori tests can then be applied to
differentiate among treatments (Hooper and Vitousek 1998). We have information on current
species composition in each plot, as well as information on location of prominent sources of seed
from new species. Distance to a seed source can be included either as a covariate in the analysis
or as the independent variable in a regression (H5). Significant effects of this covariate,
particularly if they strongly diminish any effects of initial functional composition, will provide
evidence in support of dispersal influence on community composition. While we expect the
influence of initial composition to decline with time, using the original planted species
composition as a treatment variable will allow us to statistically test if that is the case.
Nonrandomness of functional composition and null models (H8, H9)
Most of our hypotheses can be tested without null models. Hypotheses 8 and 9, however,
have precedents in the literature, particularly with respect to testing for functional
complementarity among guilds of desert rodents (Fox and Brown 1993, Kelt et al. 1995, but see
also Wilson 1995, Stone et al. 1996, Kelt and Brown 1999, Simberloff et al. 1999) and for
constant proportionality of plant guilds across sites (Wilson 1989, Weiher and Keddy 1998).
Wilson’s (1989) test for constant guild proportionality does not specifically examine
complementarity among groups, however, so we will not pursue those models. On the other
hand, Fox and coworkers (Fox and Brown 1993, 1995, Fox 1999, Kelt and Brown 1999)
explicitly look for potential complementarity among animal guilds by focusing on favored states
- i.e., combinations of species in which the number of species in each guild is roughly similar. In
particular, Kelt's model (Kelt et al. 1995, Kelt and Brown 1999) allows calculation of an estimate
15
of the strength of interaction among species and therefore gives an estimate of the power of the
test for nonrandom distributions guilds. Doug Kelt has agreed to work with us to use his model
with our data set (see attached letter of support).
Some complications arise in this application. First, the favored states approach assumes
similar numbers of species in all functional groups, an assumption that may not hold in more
diverse plant communities (Wilson 1995, Fox 1999). Our own hypothesis would be that
complementarity among plant functional groups would lead to relative abundances of different
groups that are more evenly distributed than at random. However, null models in general utilize
only presence/absence data, not abundances. For these reasons, we suggest examining the
rank/abundance curves for each plot and testing for complementarity among groups using only
those species that make up the majority of community biomass. There is precedent for this
approach in both plant (Walker et al. 1999a) and animal (Bowers and Brown 1982) studies. In
diverse communities, species in the tail of the rank abundance curve are likely to be those whose
presence/absence is more sporadic, potentially reflecting chance events rather than sorting by
community interactions (Weiher and Keddy 1995; Grime 1998). We can vary the proportion of
biomass and total numbers of species included to investigate if potentially complementary
patterns disappear when including species in low abundance.
A previous reviewer suggested a second complication - that null models require the
assumption of equal probabilities of dispersal to all sites or plots. While the model we propose
to use can correct for biased distributions (Kelt and Brown 1999), we will focus on data from the
last two years of our study, by which time equal probabilities of dispersal should occur. This is
also important for avoiding potentially confounding effects of initial species composition from
the original treatments (for which we will be able to test, as described above). Third,
determining the most appropriate statistic against which to test actual co-occurrences for nonrandomness is a complicated process that depends on a variety of assumptions (Gotelli and
Graves 1996, Kelt and Brown 1999, Simberloff et al. 1999). We will necessarily rely on Kelt's
guidance in these issues. Because of the controversial history of null models, we emphasize that
this component only comprises two of our nine proposed hypotheses. Substantial fundamental
information on community assembly can be gleaned from these experiments even without using
null models.
PERSONNEL AND FEASIBILITY
I will work closely with Jeff Dukes for the sampling, data analysis, and writing for this
project. Funds are requested for two months of salary per year for each of us (field time to work
in the experimental plots, plus time for data analysis and writing). To carry out the additional
work to sample in the intact serpentine grassland, I request research assistantships for two
masters students (positions to be filled sequentially to cover the duration of the experiment). In
addition, I request one quarter (spring) of release time from teaching in years 2 and 4. This will
enable me to train the new graduate students during our spring field season, focus on
measurements of the experimental plots, and work with Doug Kelt on null modeling. Our
proposed start date is 7/1/02 so we can continue sampling for the entire 2002 field season (my
current grant ends on 6/30/02).
1
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