Relative roles of natural and anthropogenic drivers of watershed invasibility in riverine ecosystems

Observational studies of patterns of species invasions provide important insights into invasion processes at large spatial scales, leading to improved and focused efforts to prevent invasions. Nonnative species richness (NNSR), a common measure of ecosystem invasibility, varies widely among systems and is regulated by human activity and ecosystem characteristics such as habitat heterogeneity and climate. Patterns of NNSR may be confounded by variation in research effort, defined as the effort made to identify and catalogue invasions, and colonization pressure, defined as the number of species introduced. To explain spatial variation in ecosystem invasibility by fishes in the Mid-Atlantic region of the United States, we explored relationships among watershed characteristics, colonization pressure, research effort, and NNSR. Colonization pressure, research effort, and range in elevation were most predictive of differences in NNSR among river basins. Montane watersheds had greater NNSR, probably because of greater habitat heterogeneity due in part to human activity; widespread impoundments here created novel lentic habitats, while lotic environments were maintained elsewhere in these watersheds. Our results illustrate the importance of examining invasion patterns of specific taxa, within regions delineated by ecologically relevant criteria. Fish invasions in the Mid-Atlantic region seem to be regulated by different factors than in other regions of North America, demonstrating that invasive species management plans need to account for geographic variation among ecosystems. In the Mid-Atlantic region, reduction of novel, artificial environmental conditions (e.g., impoundments) in montane watersheds may reduce the invasibility of these ecosystems by generalist fish invaders.