Humans profoundly re-make biotic communities in cities, from highly visible changes in species composition, to less noticeable shifts in behaviour and the coupling of population dynamics. Initial explanations for urban biodiversity patterns focused on the application of island biogeography theory. 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. Building on this foundation, researchers from the Baltimore and Phoenix Long-Term Ecological Research sites have advanced alternative theoretical and mechanistic approaches to explaining patterns of biodiversity. One body of work draws on metacommunity theory to recognize the explicit role of space and dispersal relative to local environmental conditions and interspecific interactions. An alternative, though not mutually exclusive view is that species interactions, such as competition between colonizing and native species, play a key role in shaping community composition. Altered basal resources from anthropogenic inputs are implicated as having pervasive impacts, both directly on population sizes or indirectly mediating top-down control. Interdisciplinary approaches that incorporate the social sciences yields greater insights into all these questions compared with purely disciplinary approaches.
A decade of observational and experimental studies in Phoenix and Baltimore has yielded several important findings. Native bird species are strongly associated with the native vegetation, though in different ways in the two cities. Over time, however, bird, plant, and arthropod communities appear to be shifting toward a ‘domesticated’ biota. Species losses are occurring, even for some common and broadly distributed species, lending support for the idea that patterns of biodiversity in younger cities may reflect an ‘extinction debt.’ The mechanisms underlying these shifts are still under examination, but support for extinction-colonization dynamics, species interactions, and dispersal limitation can all be found to varying degrees. Signatures of both natural and human-derived perturbances like drought and economic crises can be seen in Phoenix for multiple taxa. Human inputs, such as water or anthropogenic food resources, appear to be altering community structure for both birds and herpetofauna in Phoenix. A key outcome is that human exposure to biodiversity is becoming increasingly limited, particularly in neighborhoods already experiencing greater poverty and reduced access to green spaces in both cities. Continued long-term studies may reveal options for retaining some native species, and the uniqueness they contribute to each region’s fauna.