ABSTRACT
There is considerable concern that human activities are having detrimental impacts on Earth’s biological diversity and the function of biophysical systems. These human processes may influence, interact with, alter, and even control the operations of biophysical functions at many levels of biological organization. Studies have shown that within many ecosystems, human activities have roles directly controlling entire biophysical systems (such as land-use practices that alter native habitat and expatriate species) (Kaiser and Gallagher, 1997; Tilman et al., 1997; Vitousek et al., 1997). Examples of direct human alteration of ecosystems include desertification of marginal lands (Schlesinger et al., 1990; Mainguet, 1994), eutrophication of lakes (Schindler et al., 1990) and coastal areas (Lapointe and O’Connell, 1989), loss of coral reefs in the Caribbean (Hughes, 1994), loss of soil organic matter due to agriculture in the Great Plains (Seastedt, 1995) and collapse of marine fisheries (Botsford et al., 1997). Many times, however, impacts are indirect, such as changes in regional species composition and the addition of pollutants to the soil, surface and groundwater, and atmosphere. Indirect effects are a secondary result of direct species interactions and may alter species composition and community structure and as such may have strong effects through natural selection (Miller and Travis, 1996; Rosemond, 1996). Recent studies have shown that both direct and indirect effects have large impacts on the functioning of ecosystems, thus affecting human health (Botsford et al., 1997; McMichael, 1997).
