Introduction

A metacommunity is defined as a group of communities that are connected by the dispersal of one or more interacting species. The term was first used in 1991 and since then has grown into an important concept for studies of species diversity and community structure. The concept enlarges the scale at which community dynamics are considered. Metacommunities have several distinctions that make them a valuable unit for consideration in both theoretical and applied ecology, such as in considering the effects of habitat fragmentation on biodiversity and ecological communities.

First, traditional community ecology often relies on the assumption that communities are closed, isolated entities. This assumption arises from consideration of mathematical models such as the Lotka and Voterra competition equations, which are simpler if the community is assumed to be closed to movement. By contrast, theories like source and sink dynamics propose that immigration allows some species to be present even in sink habitats where there are inadequate resources to support viable populations, or where species would be excluded by predators or competitors in the absence of immigration. Therefore opening communities to immigration may change the species present in local communities, such that the conditions assumed in closed models do not apply to open communities. Movement of individuals might also modify species interactions and the local dynamics of individual species. It is widely acknowledged that numbers of individuals moving and the distances they move are some of the most difficult parameters to measure in ecology, and therefore our knowledge of the degree to which real communities are open or closed is limited.

Second, the responses of species diversity to habitat change, including partial destruction and fragmentation, may arise either because of the responses of individual species or because species influence one another. Hence mechanisms may arise at levels of populations, metapo-pulations, communities, and metacommunities. If we consider only some of these levels we will have at best an incomplete idea of the effects of habitat change.

Third, it is useful to recognize that regional metacommunity-level species diversity is determined by the sum of both local and regional processes. The local and regional parts merit elaboration. The primary local process influencing species diversity is niche partitioning an idea that is derived from traditional community ecology. A variety of regional processes are possible: (1) There may be a balance between the extinction of species from local communities and their (re)colonization, creating a balance that allows species to persist regionally even if they do not have a predictable place in any given local community. Such dynamics may be rendered more likely if there are tradeoffs in the abilities of species to colonize new or vacant habitat and their ability to compete locally. (2) Immigration may forestall extinction from local communities, as in source and sink models and through so-called rescue effects where immigration raises population size and rescues local populations from extinction. (3) Differences in habitat type among local communities might create different niches for different species. (4) The potential for (1)—(3) may depend on the degree to which dynamics are independent (or asynchronous) in different local communities. There is also the possibility that immigration and emigration modify either the structure of the community (species abundance and composition) or the interactions of species within that community, which represent an interaction between local and regional processes. Although two spatial scales are recognized (local and regional) this is arbitrary and more spatial scales are often represented in metacommunity models. The feedback between local and regional scales differs sharply from the equilibrium theory of island biogeography where species diversity is viewed as being fixed by a permanent mainland pool of species, and individual habitat islands contain arbitrary subsets of this species pool.

Fourth, interaction among species means that the metacommunity is not simply a collection of metapopu-lations where species are largely independent from one another. From the 1970s onwards, ecologists tended to view pairwise spatial interactions involving repeated local extinction and colonization as the domain of meta-populations, whereas larger numbers of species are more the domain of metacommunity theory. Many-species spatial competition models were not originally termed metacommunity models but best fit this categorization.

Empirical work on species diversity has long recognized a spatial component to species diversity, through the division of regional (gamma) diversity into local (alpha) diversity and the turnover of species among sites (beta diversity). The existence of differences in species composition among sites (beta diversity) creates the potential for movement to alter local community composition if species are able to move among local communities. The concept of alpha, beta, and gamma diversity also recognizes that regional diversity is made up of local diversity and the differences among local communities. Such diversity patterns do not, however, distinguish what is creating the diversity - in other words, whether local diversity contributes to regional diversity or whether it is a subset of regional diversity.

In the following sections we describe models of species diversity, and then describe some of the factors other than species diversity that are influenced by a metacommunity structure.

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