Background/Question/Methods   The fundamental scope of community ecology is to understand the mechanisms that explain species coexistence. Our knowledge of ecological communities is limited, however, to relatively pristine systems. This is despite the fact that humans and their influence are a major presence on the landscape, fundamentally changing the structure of the environment, the flux of material and organisms among habitats, and disturbance regimes. But do we need to revise ecological theory to explain patterns in species coexistence in human-dominated ecosystems? We present here a conceptual overview of the problem, and bring to bear an emerging theme in community ecology, the concept of the metacommunity, as an instrument to integrate these processes. In developing this concept for urban ecosystems, we contend that human valuation of species and human behavior at the local scale has the potential to strongly influence species sorting patterns. At larger scales, human modification of spatial features, especially those related to connectivity between local communities, mediates dispersal patterns and distance-decay relationships.

Results/Conclusions   In applying metacommunity theory to urban ecosystems, we distinguish between “facilitated communities”, where human behavior drives the environmental conditions supporting a specific species composition, and “self-assembled communities”, where environmental conditions are free of direct human influence. If local factors predominate, then community similarity should increase with environmental similarity in both facilitated and self-assembled communities. If regional factors predominate, then community similarity should either decline with distance or be neutral relative to distance. Community similarity should decline with distance in facilitated communities when human activities are spatially correlated. We contend that basic ecological theory cannot completely explain patterns in biodiversity in urban ecosystems, and suggest that true integration of an ecological and socioeconomic perspective is necessary. We offer a new conceptual model of urban community ecology, including specific, testable hypotheses that will aid in understanding the general mechanisms by which species assemble in urban places, thus explaining patterns in alpha, beta and gamma diversity.