Species invasions and the changing biogeography of Australian freshwater fishes

Aim By dissolving natural physical barriers to movement, human-mediated species introductions have dramatically reshuffled the present-day biogeography of freshwater fishes. The present study investigates whether the antiquity of Australia's freshwater ichthyofauna has been altered by the widespread invasion of non-indigenous fish species. Location Australia. Methods Using fish presence-absence data for historical and present-day species pools, we quantified changes in faunal similarity among major Australian drainage divisions and among river basins of north-eastern Australia according to the Sorensen index, and related these changes to major factors of catchment disturbance that significantly alter river processes. Results Human-mediated fish introductions have increased faunal similarity among primary drainages by an average of 3.0% (from 17.1% to 20.1% similarity). Over three-quarters of the pairwise changes in drainage similarity were positive, indicating a strong tendency for taxonomic homogenization caused primarily by the widespread introduction of Carassius auratus, Gambusia holbrooki, Oncorhynchus mykiss and Poecilia reticulata. Faunal homogenization was highest in drainages subjected to the greatest degree of disturbance associated with human settlement, infrastructure and change in land use. Scenarios of future species invasions and extinctions indicate the continued homogenization of Australian drainages. In contrast, highly idiosyncratic introductions of species in river basins of north-eastern Australia have decreased fish faunal similarity by an average of 1.4%. Main conclusions We found that invasive species have significantly changed the present-day biogeography of fish by homogenizing Australian drainages and differentiating north-eastern river basins. Decreased faunal similarity at smaller spatial scales is a result of high historical similarity in this region and reflects the dynamic nature of the homogenization process whereby sporadic introductions of new species initially decrease faunal similarity across basins. Our study points to the importance of understanding the role of invasive species in defining patterns of present-day biogeography and preserving the antiquity of Australia's freshwater biodiversity.