Human land-use activities, while extracting natural resources such as food, fresh water, and fiber, have transformed the face of the planet. Such large-scale changes in global land use and land cover can have significant consequences for food production, freshwater supply, forest resources, biodiversity, regional and global climates, the cycling of carbon, nitrogen, phosphorus, etc. In particular, land management and changes in land use can affect fluxes of greenhouse gases including carbon dioxide, nitrous oxide, and methane.
Recently, Hannah et al. (1994) estimated that roughly 50% of the planet's land surface (75% of the habitable area) has been either moderately or severely disturbed; only the core of the tropical rainforests and boreal forests, deserts, and ice-covered surfaces are still relatively untouched by humans. Moreover, Vitousek et al. (1997) estimated that around 40% of the global net primary productivity is being co-opted by humans, while Postel et al. (1996) estimated that over 50% of the available renewable freshwater supply is being co-opted.
The major mode of human land transformation has been through agriculture. Since the invention of agriculture, ~10 000 years ago, humans have modified or transformed the land surface; today, roughly a third of the planet's land surface is being used for growing crops or grazing animals (National Geographic Maps, 2002; Foley et al., 2003).