Collaborative Research: The role of network topology and environmental filtering in shaping the ecology of spatially structured communities

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Why are species found in certain combinations and not others? Why are some species not found in habitats that seem suitable for their needs? A fundamental goal of ecology is to understand the processes that drive patterns in species occurrence. An organism's "local" environment can exert influence on where it occurs, e.g., climate, competition for resources, and disturbance. However, "regional" factors - such as the ability of organisms to colonize available habitat - are also important. This project addresses two fundamental questions: 1) What is the relative importance of local versus regional factors in controlling species composition? 2) How does that relative importance shift with elements of landscapes, and the ability of organisms to move across those landscapes? The project will address these two questions using communities of aquatic invertebrates in river networks. These organisms include the larval forms of many insects, such as mayflies and caddisflies, and animals like snails and crayfish. These communities are ideal for addressing the questions because rivers have a very distinct, branching configuration. Furthermore, rivers endure intense human degradation, and aquatic invertebrates are often used as indicators of water quality. The project takes advantage of stream restoration activity, which almost always includes large-scale habitat modification: the ultimate manipulation of "local" factors. Three types of approaches will be employed to address the research questions: surveys of restoration sites, experimental manipulations of local factors - specifically stream-bottom habitat features - in river systems, and experiments using artificial streams to control for both local and regional processes. The project will have important implications for applied ecology because stream assessment often involves characterizations of aquatic communities under the assumption that local habitat alone determines their composition. Stream restoration efforts (estimated at $1 billion annually in the U.S.) are often evaluated based on such bioassessment techniques. Previous findings suggest that this assumption may apply only to specific portions of a stream network. This research will be the first to begin to explore spatial dependence of habitat factors in detail, not only advancing the field of ecology, but providing information about the importance of river network position in developing expectations for restoration.