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:

MC_boook_cover.jpgHolyoak, M., M. A. Leibold, and R. D. Holt. 2005. Metacommunities: spatial dynamics and ecological communities. University of Chicago Press, Chicago, IL.

Published reviews of the book (links are to PDF files):

1. Michael Fuller, Ecology;

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.
Picture of the food web used to test how habitat patchiness altered food web structure (Holyoak 2000). The web consists of four species of protozoa and a mixed inoculum of bacteria. Click on the photo for  a more detailed view.

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