The Theory of Ecology

Human infl uences on ecological drivers are increasingly recognized as dominant processes across a range of spatial and temporal scales. Regional to global-scale changes in drivers and important resources, such as atmospheric carbon dioxide concentrations, climate, and nitrogen deposition, are known to alter biotic structure, ecosystem function, and biogeochemical processes with feedbacks to human activities and the atmosphere (Petit et al. 1999; Grimm et al. 2000; Fenn et al. 2003; IPCC 2007). Human activities also directly affect ecosystems at fi ner scales through urbanization, species movement and extinction, and changes in land use that, in aggregate, have global impacts as human populations continue to increase and migrate (Alig et al. 2004; Th eobold 2005; Grimm et al. 2008b). Although large volumes of data on global change drivers and ecological responses to them have been synthesized (e.g., Heinz Center 2002; Millennium Ecosystem Assessment 2005; Canadell et al. 2007; IPCC 2007), a coherent body of ecological theory focused on global change is lacking (Peters et al. 2008). Th e U.S. Global Change Research Act of 1990 defi ned global change as: “Changes in the global environment—including alterations in climate, land productivity, oceans or other water resources, atmospheric chemistry, and ecological systems—that may alter the capacity of the Earth to sustain life.” It is now clear that direct and indirect human actions are responsible for most of the more dramatic change occurring today and forecast for the future (Vitousek et al. 1997b). Global change is an aggregate of diff erent forces that op-