The number of species naturally inhabiting a location (native species richness [NSR]) is ultimately driven by the combined processes of speciation, extinction, and immigration, and proximately by the suite of environmental, ecological, historical, and evolutionary factors that determine the interplay of these processes [1]. An important feature of the Anthropocene is the extent to which human activities have enhanced immigration [2], such that species are being intentionally or accidentally transported and introduced to areas well beyond the biogeographic barriers that normally prevent their spread, and at unprecedentedly high rates.