The forces that shape community structure are those that determine which and how many species occur together, which species are common and which are rare, and the interactions amongst them. Thus, the topic of community structure involves a synthesis of all the environmental factors and ecological interactions influencing an assemblage of co-occurring species. For communities to exhibit predictable structure requires that their assembly is the outcome of nonran-dom processes that result in repeatable patterns, rather than chance and dispersal. This leads us to expect that the same species, in roughly the same abundances, will be found in the same locale as long as environmental conditions do not change greatly, and that similar communities should occur wherever environmental circumstances are comparable.

Explanations for patterns in species diversity and community structure frequently are based on niche-based models (MacArthur 1972, Chase and Leibold 2003), in which the presence and abundance of individual species is a reflection of their fit to habitat conditions and success in interspecific interactions, the subjects of Chapters 5 and 9, respectively. In stable or moderate environments, biological interactions are considered to be particularly influential in the assembly and maintenance of communities. However, many environments experience periodic disturbances, and stream ecosystems are no exception. Environmental disturbances such as floods and droughts, when sufficiently extreme or frequent, are likely to prevent biotic interactions from acting with the strength and regularity required to result in consistent community patterns. Very harsh environments or frequent disturbances may severely restrict the number of species that can survive those conditions and thus reduce diversity, whereas a moderate level of disturbance may enhance diversity by counteracting the tendency of a few superior species to win out. As a counterpoint to niche models, neutral models (Hubbell 2001) treat species as ecologically equivalent and substitutable, to be replaced from a regional species pool whenever a chance local extinction depletes site diversity. The recol-onization of a lost population requires dispersal, and so distance, life-history traits, and other factors such as terrain can determine whether dispersal limits the opportunities for a particular species to reestablish.

Studies of local assemblages often assume that communities are determined solely by environmental conditions and species interactions at the local scale, without regard for such larger scale processes as dispersal, speciation, and historical biogeography. However, regional species pools and factors that influence dispersal at large spatial scales influence local diversity and assemblage structure by determining the pool of species that are available to colonize a location (Ricklefs and Schluter 1993). Thus, regional and historical factors determine diversity at large spatial scale, which in turn has an influence upon local diversity through the action of environmental factors acting at progressively smaller scales, often visualized as a hierarchical series of filters. Consideration of regional diversity also reminds us that the long-term persistence of a species usually does not depend solely upon its survival in any one local community. Separate populations of a species may exhibit different trends in different locales, with the consequence that dispersal permits a long-term regularity on a larger scale that is not apparent by detailed investigation on a finer scale. Such a perspective lessens the need for equilibrium-enhancing interactions, because regional processes of immigration and emigration may contribute some of the buffering against extinction that otherwise must be attributed to biotic factors.

Food webs depict the network of vertical and horizontal linkages extending from basal resources to top consumers in a single, integrated visualization of a biological community. Although the number of connecting links can be very large, a modest number of species often contributes the majority of the biomass and are responsible for most energy flow. Some species have complementary or overlapping roles, but at least in some cases certain species appear to be functionally irreplaceable. Thus, the potential loss of a species from intact, functioning communities due to overharvest, habitat degradation or other human actions raises the specter that simplified biological communities will be less efficient or productive than is observed in their unaltered state.

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