Local vs. regional influences on local diversity in stream fish communities of Virginia
Local species richness is a function of many factors operating at multiple spatial and temporal scales. We examined stream fish communities from regions throughout Virginia to assess (1) the relative influence of local vs, regional factors on local species richness, (2) evidence for community saturation, and (3) scale dependency of regional influences. We defined regions at four spatial scales: major drainages, drainage-physiography units, hydrologic-physiography units, and sites. We used multiple regression to identify key correlates of local native and introduced diversity for each regional scale. Both local (e.g., microhabitat diversity) and regional (e.g., species richness) factors were correlated with local diversity; regional diversity was the most consistent correlate. Plots of local vs, regional native diversity were asymptotic for the three largest regional definitions, thereby suggesting community saturation. However, analogous plots for introduced species were not asymptotic; local introduced diversity was a linear function of regional introduced diversity. Introduced populations were pervasive, but less abundant locally than native populations, thereby suggesting that native species are better adapted. Overall, stream fish communities in Virginia appeared to be neither completely saturated nor freely invadable. The ability of regression models and particular independent variables to account for variation in local diversity changed considerably with regional scale. Most regional correlates of local diversity were scale dependent. The concept of hierarchical environmental filters provides a useful framework for integrating the multiple scales over which ecological processes organize communities. Retrospective analyses of the impacts of introduced species on native communities provide some insight regarding community saturation, but conclusive evidence must await studies that couple comparative and experimental approaches. Clear interpretation of regional influences on local diversity will require careful definition of regions. Comparative analyses at multiple regional scales may be the most insightful approach to understanding the complete array of processes that organize communities.