## Introduction

Community ecology as a field is concerned with explaining the patterns of distribution, abundance and interaction of species. Such patterns occur at different spatial scales and can vary with the scale of observation, suggesting that different principles might apply at different scales (e.g. Levin 1992; Rosenzweig 1995; Maurer 1999; Chase & Leibold 2002). Remarkably, however, much of formal community theory is focused on a single scale, assuming that local communities are closed and isolated. Within these local communities, populations are assumed to interact directly by affecting each other's birth and death rates, as modelled by population dynamic models such as the classic Lotka-Volterra equations and their extensions (e.g. May 1973; Pimm & Lawton 1978; McCann *et al.* 1998). It has been recognized, however, that other ecological processes involving species interactions occur at other scales (Wiens 1989; Levin 1992; Holt 1993; Maurer 1999; Hubbell 2001). For example, species interactions can occur in a network of local communities where they affect colonization probabilities and extinction patterns at a larger scale than those typically addressed by population dynamic equations (e.g. Levins & Culver 1971; Vandermeer 1973; Crowley 1981; Holt 1997; Mouquet & Loreau 2002, 2003). The interactions and demography of local communities could also be influenced by other kinds of spatial dynamics, such as the flow of individuals that create mass effects (Shmida & Wilson 1985) and source–sink dynamics (Holt 1985; Pulliam 1988). These dynamics involve interactions among local communities at larger scales that we refer to as metacommunities.

We define a *metacommunity* as a set of local communities that are linked by dispersal of multiple potentially interacting species (Gilpin & Hanski 1991; Wilson 1992). Metacommunity theory describes processes that occur at the metacommunity scale and suggests novel ways of thinking about species interactions. Here we evaluate current knowledge about such metacommunity theory, and we discuss how it can contribute to explanations of the patterns of distribution, abundance and interaction of organisms at local as well as regional (metacommunity) scales that are larger than those addressed by more conventional community theory.

In the following synthesis, we review some simple aspects of metacommunity theory that ask how the fact that local communities are embedded in a larger regional biota affects local phenomena and patterns of variation among local communities. Embedding local communities within a metacommunity is likely to result in various spatial dynamics, which can alter local species diversity both directly and indirectly by altering local community processes that feed back to alter features of the regional biota. Most previous theoretical investigations ignored how this larger regional biota might be constrained (e.g. the fixed mainland species pool in the equilibrium theory of island biogeography, MacArthur & Wilson 1967). Therefore we also ask how metacommunity dynamics affect the attributes of these larger regional biotas, and how this effect feeds back to patterns of local variation. It is clear from the little work done to date that answers to this second question are likely to greatly alter how we interpret many ecological patterns and phenomena.