are defined by relatively abrupt changes in the abundance of common species (Paine and S.A. Levin, 1981), or by the physical phenomena contributing to patch formation and maintenance (Fig. 1). This is the scale over which individual organisms interact with each other and with their immediate environment. Patches in the pelagic environment are three-dimensional and mobile, whether they consist of actively moving nekton (Hamner et al., 1983; Boudreau, 1992) or passively advected plankton (Haury et al., 1978; Fasham, 1978; Yoder et al., 1981). In pelagic systems, patches are found on almost every scale of observation (Owen, 1989; Powell and Okubo, 1994), and measures of patchiness seem to change continuously across scales (e.g., S.A. Levin et al., 1989). Defining and understanding characteristic patch sizes for plankton thus remain a focus of considerable interest, and although patchiness in the pelagic environment is less well-understood than in some benthic systems, new acoustic and imaging technologies are increasing the ability to measure patchiness (see Box 12). For intertidal and sublittoral benthic communities, distinct patches are much more evident and may impose a hierarchical organization to the system (Paine and S.A. Levin, 1981). In these systems, interindividual and interspecific interactions occur to a larger extent within patches, and processes such as dispersal, disturbance, and predation impose interpatch correlations (Sousa, 1985).

The collection of patches that interact or are otherwise connected constitute a "metapopulation" (Gilpin and Hanski, 1991). Note that this includes empty patches of suitable habitat, for example, in a mussel bed or on a coral reef, where a group of mussels or corals has been recently consumed by sea stars or destroyed by moving debris during violent storms (Connell, 1978; Paine and S.A. Levin, 1981). The subsequent successional events within each patch, and the movements of organisms among patches, can be integrated to characterize the demography of patches at the site. Physical processes operating over a wide range of time and space scales may affect metapopulation dynamics (Fig. 1). The theory of metapopulations is still in its infancy, and few marine populations have been studied at this scale. Communities are the sum total of metapopulations in a given habitat at a site.

A "region" comprises all the sites within a biogeographic province whose limits are defined by the relative homogeneity of the biota, as well as by unifying geographic features and oceanographic processes (Fig. 2). Ecological connections between sites within a region depend on fundamentally different processes from those between patches within sites (Butman, 1987; Roughgarden et al., 1988; Underwood and Petraitis, 1993). For example, the exchange of nutrients, food, and colonists between one site and another all depend on patterns of coastal and ocean currents that are largely independent of local events (e.g., see Box 5). Such processes can be modeled as a metapopulation of sites or metacommunities (Gilpin and Hanski, 1991; D.S. Wilson, 1992). There are, however, almost no data showing how changes in biodiversity at one site within a region affect the diversity at another.

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