Disturbance in urban ecosystems: New insights from comparisons of Baltimore, Phoenix, and selected world cities
Disturbance has long been recognized as a crucial driver of ecological processes. For example, disturbance is one of the five core research areas of the US National Foundation’s Long-Term Ecological Research network. But how disturbance as a concept applies to the coupled social-ecological systems of urban areas is poorly understood. Indeed, the naïve ecological habit of taking urbanization per se as disturbance does not advance the understanding of spatially and organizationally complex urban systems. There is an urgent need to unpack the concept of disturbance in urban systems, and to advance the understanding of urban disturbance by comparing among urban social-ecological systems. We summarized insights emerging from comparison of disturbance in Baltimore and Phoenix to provide needed theoretical refinement and identify data gaps that must be filled. We considered several processes that might represent disturbances, using data in the form of maps of land cover and ecosystem structure over time, identification of effective historical events and policy shifts, examination of urban design interventions, and occurrence of extreme weather events.
The Phoenix and Baltimore LTERs provide the opportunity to explore the nature, limits, and insights from comparison of disturbances in and of urban systems. Drivers and effects of disturbance can be either biophysical or social in nature. In particular, suburban land conversion comprises discrete steps including soil disturbance, restructuring of the site, and prolonged maintenance of a new state with human intervention in Phoenix, whereas in Baltimore, revegetation following demolition of abandoned houses contrasts between remnant yards and the demolished building footprint. Extirpation of tree cover and burial of lateral canals in Phoenix accompanied management attempts to minimize water loss via evapotranspiration in the 1960’s, resulting in a water-impoverished living environment for residents. Catastrophic fire in Baltimore in the early 20th century profoundly restructured the city and prompted adoption of new building regulations. Urban flooding in both cities elicited responses that altered stream ecosystems, transforming them to new stormwater conveyance or retention structures. With these examples and selected results from other urban areas, we construct a new conceptual picture of the role of disturbance in these human-dominated ecosystems. The event of disturbance, its effects, and responses to it are shown to be integrated social and biophysical features of urban ecosystems.