Species Sorting

Species-sorting, is an approach that builds on theories of community change over environmental gradients and considers the effects of local abiotic features on population vital rates and species interactions. In this perspective, local patches are viewed as heterogeneous in some factors and the outcome of local species interactions depends on aspects of the abiotic environment. If different species can only inhabit exclusive habitat types, the resulting metacommunity can be broken down into two independent ones, but when individual species can inhabit multiple habitat types, there are a variety of outcomes that reflect how species interact at larger spatial scales. One way to model such dynamics is to extend assembly models to systems with multiple patch types. Like many patch dynamics models, this approach assumes that there is a separation of time scales between local population dynamics and colonization-extinction dynamics. Populations are assumed to go to their equilibrium behaviour (be it a stable point or a more complex oscillating or complex attractor) in between colonization events and before environmental perturbations that might cause extinctions to occur. This approach focuses on trade-offs among species that allow them to specialize on different patch types (local conditions) rather than on possible trade-offs between such traits and dispersal (as is found in the competition- colonization trade-off commonly found in patch dynamics models).

This species-sorting perspective has much in common with traditional theory about niche separation and coexistence. At larger spatial scales, however, metacommunity processes are important in allowing local community composition to track changes in the local environment (due to perturbations or gradual environmental change, for example) in ways that maintain the correspondence between local conditions and composition. Law & Leibold (In press) show how species-sorting models can have different dynamics in a metacommunity framework than in more conventional assembly models, one important difference is that metacommunity dynamics in cases with endpoint dynamics that consist of repeated cycles can be stabilized at the metacommunity scale. Shurin et al. (2003) show that alternate stable local communities are unlikely to occur in metacommunities unless they have sufficient environmental heterogeneity among patches. Metacommunity dynamics also constrain attributes of the regional biota in important ways that relate to ecological constraints at larger scales. The result is that species distributions are closely linked to local conditions and largely independent of unrelated purely spatial effects. Nevertheless, species sorting can still result in complex dynamics because of the possibility of cyclical assembly dynamics that are habitat-specific. In these situations communities go through assembly cycles that repeat themselves. One case that comes up in food web models is when a species from a low trophic level serves to assemble a food chain that is dependent on it and is excluded by competition with a competitor that has no resident consumers. The new basal species can then serve to assemble its own food chain that may be reciprocally invaded and excluded by the first species. Such food web assembly cycles involving species sorting (matching of prey to consumers and vice versa) appear in food web models of community assembly where their occurrence is enhanced by higher productivity. Pond plankton appear to be a good example of such metacommunities. In metacommunities consisting of ponds in a biogeographically constrained region local communities appear to be highly resistant to invasion by absent species from the region unless there are significant perturbations. This would indicate that local communities have reached endpoint assembly configurations. On the other hand, even under unusually high immigration, species from other patch types seem to have very little influence on these local communities, indicating that local population dynamics are not strongly influenced by such mass effects (see below). Consequently there is good correspondence between local composition and local abiotic conditions even after sudden environmental changes have occurred.

From Leibold et al. 2004. The metacommunity concept: a framework for multiscale community ecology. Ecology Letters 7: 601-613.