Phylogenetic Patterns of Fish Community Assembly in the American Southwest: The Influence of Non-Native Species and the Environmental Template

The spatial hierarchy of systems greatly influences biogeographical patterns and the composition of communities.  Further, environment conditions can act as a filter, determining the presence of species in a location.  Fish are constrained by barriers to dispersal, and their distribution should therefore reflect the geomorphological features of the landscape, e.g., fish generally cannot disperse across drainage boundaries.  However, the introduction of non-native species and alteration to the prevailing environmental drivers erodes dispersal barriers and alters the hierarchical influence of the landscape.  Using a large database on fish occurrence from the Lower Colorado River Basin, we examined phylogenetic patterns and beta diversity in native and non-native fish communities, measures of differences in species composition across space.  Historically, fish communities in the arid American Southwest were shaped by harsh environmental conditions, including droughts and floods, leading to the evolution of a highly endemic and specialized fauna.  However, dam construction and flow regulation have significantly altered the environmental conditions in the region, enabling non-native species that are not adapted to harsh conditions to survive and thrive, displacing native species.  Therefore, we hypothesize that fish assemblages dominated by native species are phylogenetically clustered (consisting of closely related members), but that assemblages dominated by non-native species are overdispersed (consisting of distantly related members), reflecting competitive influences generated by altered flow conditions favouring non-natives.  We constructed a phylogeny for native and non-native fish species in the Lower Colorado River Basin, and calculated measures of phylogenetic diversity, relatedness, and beta diversity for both native and non-native communities at a hierarchy of spatial scales to account for the influence of hydrological and geomorphological features on the landscape.  We observed that native fish communities were generally more closely related than random expectations, likely reflecting the historical influence of harsh environmental conditions associated with arid-land river systems.  Understanding patterns and drivers of species compositional change is imperative to predict responses to future environmental change, particularly in light of the rapid spread of non-native species across historic biogeographical barriers.