Urbanization is a major driver of biodiversity loss, often restructuring biological communities by favoring only a few native species and excluding others. Several studies have identified biological traits that enable species to persist in urban environments, but mechanisms of this biotic filtering remain unclear. We present a comparative approach to this question by examining the local species pools and urban bird communities of Fresno, California, Tucson, Arizona, and Phoenix, Arizona, USA. We ask if biotic filtering occurs similarly in these three cities. By compiling data from three point count projects in these cities, we have both species checklists, as well as site-specific bird counts. Preliminary analyses have focused on the species lists, using Jaccard’s index to assess similarity, and Chi-square tests for shifts in the distribution of nominal traits, such as dietary guild and migratory status.
Results/Conclusions
Jaccard’s similarity indices show that the regional species pools of these cities are more similar than their urban bird communities. Indices comparing urban avifaunas (Fresno-Phoenix: 0.376, Fresno-Tucson: 0.293, Tucson-Phoenix: 0.581) are lower than indices comparing species pools (F-P: 0.595, F-T: 0.532, T-P: 0.733) in every pairwise comparison. Jaccard’s indices between cities and their respective species pools follow a water use gradient; cities with lower water use levels harbor larger urban avifaunas that are more similar to their species pool. Fresno harbors the smallest suite of native species of all the three cities, and also exhibits the strongest trait-based filtering. The urban bird community differs significantly in dietary traits from the local species pool in Fresno (Chi-square p=0.002, df =8), but not in Phoenix (p=0.65, df=8), or Tucson (p=0.98, df=8). Fresno exhibits an increase in the proportion of granivores and omnivores, and a decrease in the proportion of insectivores, a pattern found in many previous studies of avian traits in urban areas. A similar significance pattern emerges in migratory status. We propose that this is due to differences in urban habitat structure, i.e., more mesic vegetation in Fresno contrasting with arid native habitats, than in Phoenix or Tucson. Our results confirm recent findings that cities may not homogenize avifaunas as suspected, and point to the potential for even large, dense cities to retain native species diversity through more careful design of urban habitats and water use.