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Metacommunities
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This emerging subfield of ecology
encapsulates the idea that ecological communities do not exist in isolation.
Inter-community boundaries are porous and movement of organisms between local
communities can alter local species diversity, community structure and
ecosystem processes. The metacommunity concept combines spatial dynamics with
community ecology to increase our understanding these processes. A
metacommunity is defined as a series of local communities that are connected
by dispersal. I was co-editor of a book on the subject with Mathew
Leibold and Bob
Holt: Published
reviews of the book (links are to PDF files): 2. Mark
Urban, Conservation Biology; 3. Don
Driscoll, Austral Ecology; 4. Bob
Paine, Integrative and Comparative
Biology. 5. Jan
Pergl, Folia Geobotanica 6. Kevin
Gaston, Environmental Conservation The cover illustration shows islands in the
Great Barrier Reef linked by a hypothetical food web with points representing
species and the circles representing the area covered by a population of a
species on an island. One of the biggest challenges is to
understand how and why food web structure changes during habitat
fragmentation and as a result of spatial structure of the environment. To this end I conducted a very simple
experiment to test how spatial dynamics (habitat patchiness) altered the
structure of a simple food web (Holyoak 2000). In a laboratory community consisting of
four species, the top predator showed decreased persistence with
fragmentation. However, prey species were more persistent with fragmentation,
possibly because predator populations declined. The result that top predators
are more sensitive to fragmentation is commonly observed in field systems,
and further work on this system could help elucidate mechanisms for this
pattern. Thinking about metacommunities often starts
with four conceptual models,
termed Patch Dynamics, Neutral Community Dynamics, Species Sorting and Mass
Effects (Leibold
et al. 2004). These models build on existing work on spatial community
dynamics and are a starting point for considering the possibilities for how
spatial dynamics might function in metacommunities. Patch Dynamics build on metapopulation dynamics and multispecies
competition models in homogeneous environments with competition-colonization
tradeoffs (e.g., Amaresekare
et al. 2004). Neutral Community
Dynamics gained notoriety with the publication of Hubbell's (2001) book
"The Unified Neutral Theory of Biodiversity" and assume both a
uniform environment and that all individuals (and therefore species) have
identical fitness. In neutral models diversity is maintained through
dispersal limitation, which limits competitive exclusion by keeping species
somewhat separate in space, and by speciation which replaces species that are
lost because of competition. Conversely, Species
Sorting assumes that species sort into different habitats, creating
separate spatial niches through habitat heterogeneity, and the only role of
dispersal is to occasionally replace species that go extinct. The final
model, Mass Effects, also assumes
habitat heterogeneity but that species disperse frequently enough that they
spill over from good (source) habitats into poorer (sink) habitats. These
models differ in the roles of habitat heterogeneity, dispersal, and species'
traits (and trade-offs in traits). These factors are therefore critical to
understanding and testing the role of spatial structure and dynamics in
creating biodiversity. Work with Kendi
Davies, Valerie Offeman, Kim Preston, and Quenby Lum conducted the first
community experiment in which both habitat heterogeneity and dispersal were
manipulated together. The experiment explored the effects on species
diversity and tested the relevance of the four conceptual models described
above. The experiment was conducted using communities of over 20 species of
algae, protozoa and rotifers, which were loosely based on a New Jersey pond
community, following work by Peter
Morin's lab. When heterogeneity was present in which resources were
present in different patches more species were maintained in interconnected
microcosms than in similar microcosms with the same resources everywhere.
Differences in abundance between connected and unconnected patches containing
different resources showed that source-sink dynamics were involved.
Comparisons with homogeneous systems showed that species diversity was
elevated by mass effects in heterogeneous systems. No role of patch or
neutral dynamics was found. We extended this work to explore the effects of
patch heterogeneity on ecosystem functioning and invasion by exotic species. Work with Alicia
Ellis and Phil
Lounibos examined a small metacommunity of tree hole-dwelling mosquitoes
in Florida for which an unusual dataset of 26 years and samples every 2 weeks
existed. Ellis
et al. (2006) found that dynamics showed strong elements of species
sorting, but with considerable temporal turnover of species, as predicted by
the patch dynamics model. Consistent with patch dynamics, there was
substantial asynchrony in dynamics for different tree holes, substantial
species turnover in space and time, and an occupancy-colonization trade-off.
Substantial correlations of density and occupancy with tree hole volume were
consistent with the species-sorting model, but unlike this model, species did
not have permanent refuges. The results did not match a single model and
therefore caution against overly simplifying metacommunity dynamics by using
one dynamical characteristic to select a particular metacommunity
perspective. Another recent research direction with Richard Law, Kendi
Davies and Valerie Offeman looks at the effects of movement of species
between patches on the assembly of ecological communities. This work is being
conducted using patch occupancy models developed by Richard Law and a simple
three species food web module consisting of protozoa. Experiments
investigated the frequency of community states (different combinations of
species) present with different rates of dispersal. Like many of the projects
conducted in the lab it combines experiments in simple microcosm systems that
form an intermediate step between predictions from mathematical models and
complex field systems in which it would be difficult to thoroughly test the
theoretical predictions. Work with Matt Schlesinger and Pat
Manley investigated metacommunity structure in land birds and how it was
modified by urbanization in Lake Tahoe basin, in the Sierra Nevada of
California and Nevada. This project funded by US Forest Service (link to project
website) and other agencies investigated how bird species diversity and
composition changed during urbanization. I am also interested in neutral community
models, such as those of Steve
Hubbell, that assume that all species are equal competitors or equal in
fitness. These models are provocative because of there simplicity and the
fact that they can produce passable patterns of things like the rank
abundance distribution of species within communities. I'm interested in the
models because they include localized dispersal and emphasize stochasticity
and transient dynamics. I edited a special feature in Ecology together with Michel
Loreau and Don
Strong, which discusses how to test neutral community models. The paper
by Holyoak
and Loreau is available here. I would like to find ways of investigating
the role of spatial dynamics in modifying species interactions in
metacommunities, and to investigate changes in food web structure that result
from metacommunity structure. I am also interested in extending work to new
field systems, and to explore the implications of spatial community structure
for ecosystem functioning, invasive species and conservation. |
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The microcosms used to create
patchy interconnected habitats. The 30-ml bottles were interconnected using silicon
rubber tubes. All of the protozoa used disperse through these tubes, but this
is very infrequent for Amoeba. Other microcosms contained large undivided
volumes or isolated 30-ml volumes. |
Last updated 12 May 2007