Human activity has dramatically accelerated both species extinctions and introductions, and the balance of these two processes is generally expected to reduce biodiversity and increase taxonomic homogenization. However, few tests of this hypothesis have been made. We tested whether macroinvertebrate invaders in North American freshwaters can replace the recent loss of biodiversity, focusing on molluscs. We determined the number of native species of freshwater macroinvertebrates using published authoritative books and papers on North American invertebrate diversity. Species at risk were identified through web-based databases (e.g., U.S. Fish and Wildlife Service Environmental Conservation Online System; NatureServe, 2015; IUCN Red List). Invaders were identified through web-based databases, primarily USGS Nonindigenous Aquatic Species Database. We also searched the primary literature, and books and reports based on data. For each species we identified geographic origin and habitat types used. We then tested whether taxa the number species within a family were over or underrepresented among invaders or species at risk, and whether molluscan species at risk and invaders use similar or different habitat types.
Results/Conclusions
We found that molluscs are overrepresented among endangered and recently extinct species, as well as among invaders relative to other freshwater macroinvertebrates. For molluscs, the number of recently extinct species (79 species) was more than twice that for exotic species (38 species). In addition, molluscan invaders are from different taxonomic families than recently extinct or endangered species. While most extinct and endangered molluscs are from streams and rivers, invaders preferentially colonize lakes and reservoirs. The impact of humans on species introductions and extinctions increases with spatial scale (from local to continental scales), resulting in the increased phylogenetic dissimilarity between introduced species and native communities. Construction of dams and alteration of the flow regimes of lotic systems will continue to lead to the extinction of native species, and promote the spread of invaders, resulting in a loss of biodiversity and taxonomic homogenization.