Altered resources, disturbance, and heterogeneity: A framework for comparing urban and non-urban soils

We propose a framework of key concepts useful in understanding how urban soils can contribute to general ecological theory. The major factors that can cause urban soils to be different from soils in non-urban ecosystems are identified and related to the familiar state factor approach. We evaluate directly altered resource availability, and the role of stress in mediating resource availability in urban ecosystems. Modified groundwater and stream flow, and atmospheric deposition of nitrogen and base cations are particularly important resource fluxes to soils in urban ecosystems. Disturbance can be conceptualized in the same way in urban as in non-urban ecosystems. However, in addition to biophysical disturbances familiar to ecologists studying wild lands, demographically and socially mediated changes in ecosystem structure must also be considered. These changes include human migration and population structure, institutional shifts, and the effects of human health. Finally, spatial heterogeneity, including fragmentation and differential connectivity, integrates the effects of resources and disturbance, and has an effect on subsequent resource availability and susceptibility to disturbance. Layers of heterogeneity include not only the geomorphic template, but urban climate, biotic composition, buildings and infrastructure, and demographic-social patterns. The complex layering of natural and social factors that constitute urban heterogeneity permit the continuation of important ecological processes, as well as modify ecological fluxes involving soils. We present a modification of the state factor approach as an expanded framework for the study of urban soils. The understanding of urban soils can contribute to general ecological theory by testing the generality of important ecosystem drivers and their linkage with social processes in an under investigated ecosystem type that is increasing in extent and impact worldwide.