III.8 Spatial and Metacommunity Dynamics in Biodiversity
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III.8 Spatial and Metacommunity Dynamics in Biodiversity M. A. Leibold OUTLINE 1. Two important consequences of dispersal in metacommunities 2. The four paradigms of metacommunity ecology 3. Synthetic efforts 4. Application of metacommunity thinking to food webs and ecosystems 5. A critique of metacommunity thinking Spatial dynamics presents some of the biggest challenges in modern ecology. These occur when the movement of organisms in space affects their populations and consequently affects how they interact with other species. It has long been known that spatial dynamics can be very important in regulating species interactions. For example, Huffaker (1958) found that spatial structure in the form of patchy resources with limited dispersal was important in allowing coexistence of the predatory mite Tylodromus occidentalis with its prey, the six-spotted mite Eotetranychus sexmaculatus. In a different context, Watt (1947) recognized that a spatial ‘‘mosaic’’ of patches was key in regulating the process of succession in communities because patches at different stages of succession were key sources of colonists during the process as patches underwent successional cycles. Despite this long recognition that spatial effects were important in community ecology, however, a satisfying conceptual, theoretical, and experimental understanding of spatial dynamics is still in development (Tilman and Kareiva, 1997; Hanski, 1999; Chesson et al., 2005). GLOSSARY mass effects. Variation in community composition determined by source–sink relations among patches metacommunity. A set of local communities connected by the dispersal of at least one component species neutral dynamic. Variation in community composition determined by stochastic effects of dispersal and demography among species with equivalent niches patch dynamics. Variation in community composition determined by extinctions of species in patches and colonization among patches species sorting. Variation in community composition determined by the optimization of fitness among species across patches Spatial dynamics is intimately linked with the principle of dispersal. Much of the work has examined passive dispersal in which organisms do not have much control over where they go (cases of dispersal where there is such control are mostly studied in behavioral ecology, where they often involve habitat selection behavior). There are numerous approaches to understanding how dispersal affects community interactions, and some of these are outlined in table 1. These approaches vary (Durrett and Levin, 1994; Bolker and Pacala, 1997) in whether they view space as consisting of discrete patches or a continuous landscape, whether they view dispersal as a local process or a global one, and whether they account for space explicitly (having a ‘‘map’’ of locations) or implicitly (just taking into account that there are distinct areas but not keeping track of where they are) as well as whether they account for the discrete nature of individuals. Generally, the simpler approaches are easier to understand but are more likely to oversimplify the situations than the more complex ones. These approaches also differ in their goals, with some of them focused on accounting for how population density varies in space and time, some focused on understanding coexistence, and some focused on understanding diversity or other questions. Although different approaches often give somewhat different answers, there are many common insights that can result (Durrett and Levin, 1994). A useful and simple organizing framework for thinking about the some of the basic elements of these approaches is the metacommunity. A metacommunity is defined as a set of local communities that are linked by dispersal of at least one component species (plate 5). It thus views spatial structure in a simple hierarchical way with local communities existing at a distinct and lower level than the metacommunity itself. The advantage of ‘‘metacommunity thinking’’ is that it captures many of the salient features of spatial ecology in a way that is reasonably accessible for verbal modeling, for guiding our intuition, and for generating more precise theoretical models. And although there are a number of important challenges for future work (some of these discussed below) and limitations, it also serves as a useful way to explore more complex spatial dynamics that does not match the strict hierarchy of spatial organization assumed in the metacommunity concept. 1. TWO IMPORTANT CONSEQUENCES OF DISPERSAL IN METACOMMUNITIES Current work on metacommunity thinking has focused on two effects that dispersal plays in such a simple hierarchy. First, dispersal is key in allowing new species to colonize local communities from which they were previously absent (Hanski, 1999). Thus, in a closed community (no dispersal), changes in community composition are limited to extinction (and possibly sympatric speciation), but this will be very different in communities that...