Homogenization, Differentiation, and the Widespread Alteration of Fish Faunas

—Widespread introduction of common species coupled with extirpation of endemic species can cause fish assemblages to lose much of their regional uniqueness. This process of biotic homogenization contrasts with biotic differentiation, whereby initially similar fish faunas diverge due to introductions of different species. The relative importance of homogenization and differentiation in altering fish faunas has been examined across the world. Synthesis of these studies indicates that homogenization of fish faunas has been widespread and that introductions, especially of sport fishes, have played a bigger role in altering fish faunas than extirpations. In the United States, pairs of states now average 15.4 more species in common than before European settlement. Additionally, the 89 pairs of states that formerly had no fish species in common now share an average of 25.2 species. While homogenization is prevalent at large spatial scales, differentiation of fish faunas is evident at intermediate spatial scales such as among watersheds within an ecoregion. This differentiation is largely the result of the idiosyncratic nature of fish introductions among individual lakes and streams. In general, translocated species (i.e., species that are native somewhere in the region but that have been moved to new locations) cause homogenization, whereas exotic species (species not native to the region) cause differentiation. Habitat and flow homogenization are major drivers of biotic homogenization because altered habitats create conditions that favor a few generalist species at the expense of more-specialized endemic species. * Corresponding author: [email protected] Introduction Homogenization and differentiation are two processes by which humans have altered spatial patterns in the similarity of biotic assemblages. Homogenization refers to an increase in the similarity of biotas and differentiation to a decrease in the similarity of biotas over time. Natural processes such as glaciation, stream capture, the formation of land bridges, and speciation can alter spatial patterns in biotic similarity, but these operate over relatively long periods. By contrast, humans have greatly accelerated changes in biotic similarity by increasing colonization and extinction rates, by simplifying and degrading habitats, and by enabling species to bypass historic biogeographic barriers (Olden 2006; Rahel 2007). Whether homogenization or differentiation occurs depends on the relative frequency of introductions versus extirpations, the spatial scale of