As the pattern of reduced diversity and increased abundance associated with urbanization has crystallized in recent years, competing explanations for the patterns have emerged. Application of island biogeography theory finds, in general, that species-area relationships are preserved in urban patches. Some studies have directly quantified the extinction and colonization processes that the theory proposes as underlying patterns of diversity. According to this view, the species present in a patch are the combination of the species colonizing the novel habitats formed with urbanization and those remaining after local extinctions are caused by isolation or habitat alteration from urbanization. An alternative, though not mutually exclusive view, is that species interactions, such as competition, play an important role in shaping community composition in urban habitats. For example, altered habitat structure and resource availability may favor colonizing species over native ones in competition for resources, leading to local extinctions of native species and elevated densities of a few strong competitors. In addition, there is a pressing need to address the evolutionary implications of urbanization, from effects on trait evolution to speciation. We argue that using interdisciplinary approaches that incorporate the social sciences will yield greater insights into all these questions than solely disciplinary approaches. We present examples of altered species interactions and evidence of underlying human drivers from five cities: Baltimore, Boston, Fresno, Phoenix, and Raleigh-Durham
Results/Conclusions: Documented effects of urbanization on species interactions from our studies include: reduction in perceived predation risk by passerine birds, elevated apparent competition between synanthropic species and specialists in birds, and elevated floral antagonisms for a forest plant. While we have not yet documented these changes in interactions in all five cities, we do find some consistent effects of human activities and common human drivers. Basal resources such as food and water appear to be elevated in the city relative to surrounding wildlands, but specialized resources, such as nest sites for cavity nesting birds, may be relatively depauperate. Socioeconomic gradients are associated with key resources such as water (Fresno), specialized food types (Phoenix), and floral resources (Raleigh-Durham) as well as with habitat features such as plant species richness (Phoenix), forest cover (Baltimore), and preserved open space (Boston). Humans, therefore, are a major force in the spatial structuring of resources in metropolitan regions across the United States. The consequences of this spatial structuring may be profound, with implications ranging from foraging behavior to animal communication to natural selection on native plant traits.