Fish Invasions in California Watersheds: Testing Hypotheses Using Landscape Patterns

An important goal of invasion biology is to identify physical and environmental characteristics that may make a region particularly receptive to invasions. The inland waters of California (USA) are highly invaded, particularly by fishes, although there is wide variation in numbers of nonnative fishes across the state's watersheds. Here we examine patterns of fish invasions in California watersheds and their associations with natural environmental characteristics, native fish diversity, and various measures of human habitat disturbance. Our analysis is based on an extensive data set on the distribution of freshwater fishes across California's watersheds and on GIS land-use coverages for the entire state. We used canonical correspondence analysis to examine associations between environmental characteristics and the distributions of both native and nonnative fish species. We then employed an information-theoretic model-selection approach to rank hypothesized models derived from the fish invasion literature with regard to how well they predicted numbers of nonnative fishes in California watersheds. Our results indicate that pervasive, anthropogenic, landscape-level changes (particularly the extent of urban development, small-scale water diversions, aqueducts, and agriculture) influenced spatial patterns of invasion. In addition, we find that deliberately stocked fishes have different habitat associations, including a strong association with the presence of dams, than other introduced fishes. In our analysis, watersheds with the most native species also contain the most nonnative species. We find no evidence that existing watershed protection helps to prevent fish invasions in California, but we suggest that restoration of natural hydrologic processes may reduce invasion impacts.