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| Description: A Global Perspective Jonathan Foley, of the University of Wisconsin, along with other colleagues from the United States, Canada, and the United Kingdom, has summarized the global impact of land use (Foley et al. 2005). Humans occupy a substantial portion of the land area of the plant, and we directly utilize between ? and ½ of global productivity (Foley et al. 2005). To date, changes in land-use due to human activity have arguably had the single greatest impact on the distribution and functioning of natural communities. The most dominant human land uses are cropland and pastures, which together cover nearly 40% of the land surface of the planet (fig. 23.15). Associated with conversion of native grasslands and forests to croplands and rangelands has been a 700% increase in fertilizer use, and a 70% increase in irrigated croplands in the last 40 years. These two activities associated with modern agricultural practices place enormous pressures on freshwater resources and global nitrogen cycles (as described above). It is estimated that 85% of the freshwater used by humans goes to agriculture, and this represents 10% of the global reserves. In addition to more nitrogen being deposited from the atmosphere, fertilizers routinely leach into streams and groundwater, decreasing the quality of the water and changing aquatic ecosystems that surround agricultural fields.
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