Community structure refers to the organization of a biological assemblage based on numbers of individuals within different taxonomic groups and functional roles, and the underlying processes that maintain that organization. Numbers of species, their functional roles and relative abundances, and energy pathways through food webs are useful descriptors of community structure. The assembly and maintenance of a local community from a regional species pool results from the matching of individual species to available resources and habitats, influenced by disturbance and dispersal, and further modified by the internal dynamics of species interactions.

Local biological diversity is a subset of regional species richness, which in turn is a function of area, climate, and history. The number of species is greater in large river basins than smaller basins, and increases downstream partly due to the effect of increasing river size, and also because some species such as large river fishes are not adapted to smaller streams. Thus, species addition as well as replacement occurs along a river's length. Different geographic regions typically have their own distinctive biota, attesting to chance differences in the establishment and diversification of particular taxa and the local interplay of environmental and biological forces that direct evolutionary change. North America has nearly three times as many species of fishes as does Europe, and this is largely because the North American fauna had a much larger area free from glaciation and easier routes for recolo-nization, whereas mountain ranges running from east to west in Europe limited faunal dispersal routes. Although the historical biogeography of aquatic insects is less well known, diversity in major insect groups likewise shows continental-scale patterns such that certain groups are largely missing from some regional species pools. One of most general patterns in biogeography is for species richness to increase from high latitudes toward the tropics. Well documented for freshwater fishes, the existence of a latitudinal diversity gradient for aquatic insects is subject to debate.

The processes that influence the assembly and maintenance of local communities from their regional species pool include niche-based models that focus on the interplay between biotic interactions and abiotic forces, the habitat template model based on the association of species with habitat features, and disturbance models that emphasize the interplay between species interactions and variation in environmental factors that periodically reduce the abundance of some or all species in an assemblage. These are not mutually exclusive, and all can be incorporated into a framework that connects the regional to the local species pool through a hierarchical series of filters including landscape setting, resource and habitat conditions, environmental variability, and interactions with other species.

Disturbance, usually associated with hydrolog-ic variability as well as temperature extremes, pathogen outbreaks, sediment pulses, and species invasions, is an important force structuring stream communities. Streams experiencing environmental conditions that are persistently or very frequently harsh are likely to support fewer species than would be found in more benign environments. But because species vary in their resistance to disturbance and in their rates of recolonization and recovery, disturbance can ameliorate strong biological interactions and help to maintain populations of species that might otherwise be eliminated by their consumers or competitors. When floods or droughts occur with unpredictable regularity, characteristic flow regimes can be shown to determine community composition, the dominant interaction pathways, and organic matter dynamics.

The network of interactions portrayed in a food web provides the most complete yet succinct visual summary of a biological community. Identification of all links, although rarely achieved, serves as a useful reminder of the potential complexity within biological communities. Most energy flows through a subset of species, and typically it is the common species that dominate energy pathways. Food web studies reveal that exchanges of energy among ecosystems can have profound effects on both energy pathways and species interactions. These subsidies include allochthonous litter inputs, the exchange of invertebrates between the stream ecosystem and the terrestrial riparian zone, and upstream-downstream transfers due to water flows and the migrations of fishes. Because the magnitude and nature of subsidies varies with longitudinal position and the surrounding terrestrial ecosystem, the network of interacting species within stream ecosystems is strongly influenced by landscape setting. The loss of species due to human activities is an increasingly serious concern, and raises the possibility that simplified communities may become less productive and less resilient.

Chapter eleven

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