Environmental Impacts of Agriculture

The Agricultural Revolution

Crop plants were first domesticated around 10,000 yrs. (years) ago, when centers of crop domestication emerged independently in the Eastern Mediterranean, Asia, Africa, and the Americas. The three staple grains that today account for more than 60% of all caloric intake—wheat, rice, and corn—were all domesticated by ca. 7,000 yrs. BP (before present). However, the domestication events themselves were inconsequential in terms of immediate environmental change. The pervasive and lasting imprints of the agricultural revolution on the global environmental commons did not begin to develop until ca. 4,000 to 5,000 yrs. BP, when agriculture was rapidly emerging across the globe as the primary means of food procurement.

Forest Clearance: Ecosystem and Atmospheric Impacts

Between 5,000 and 3,000 yrs. BP, large cities (10,000 to 100,000-plus inhabitants) were established on several continents, and this growing urban trend was fueled by the conversion of the wooded hinterlands into agricultural fields. Fire was used extensively as a means of clearing both dead and living biomass in preparation for the planting of agricultural plots; stone and metal tools (i.e., axes, metal plows) were also adopted for agricultural purposes, particularly in the Near East, Europe, and Asia. The oft-cited clearing of oak woodlands throughout the Mediterranean region during classical times led to increased soil erosion, which was made worse by the proliferation of domesticated grazing animals such as goats and sheep. The process of stocking domesticated animals on areas that were formerly forested has been repeated on every agricultural continent, often with the same end result: retardation of vegetative succession and the maintenance of the treeless landscape.

The decreases in natural vegetation cover lost to agricultural conversion have resulted in extensive native habitat loss and forest fragmentation and have contributed significantly to modern extinction rates that are several orders of magnitude higher than those experienced during pre-agricultural times. Since agricultural landscape patchworks have been carved out of formerly contiguous tracts of native vegetation, large migratory and wideranging animal species have often been relegated to habitat patches too small to satisfy their dietary and breeding requirements. Farmers have also historically viewed native animal species, both herbivores and carnivores, as competitors or threats to successful food production. These “agricultural pests” have long been targets of local extermination efforts and include animals in the United States such as prairie dogs, gophers, wolves, and foxes. In summary, the net effect of agriculture on global ecosystems has been one of biological simplification or decreasing biodiversity.

The rise of agriculture has also precipitated a marked increase in anthropogenic greenhouse gas emissions. The conversion of native ecosystems—forests, shrublands, and grasslands—to agricultural fields has resulted in increased carbon dioxide (CO2) fluxes from the biosphere to the atmosphere. Some scientists argue that this process of agricultural conversion led to climatically effective anthropogenic greenhouse gas emissions by the Middle Holocene (ca. 5,000 yrs. BP). Proponents of this theory contend that farming activities, increasing throughout the Holocene, led not only to elevated levels of atmospheric CO2 but also to rapidly rising levels of atmospheric methane (CH4) because of the pervasive adoption of rice paddy farming in Asia. Modern livestock production has further contributed to anthropogenic methane emissions via cattle and swine production, CO2 emissions via fossil fuel consumption, and N2O emissions via nitrogen-based fertilizer use. While the timing of agriculturally driven increases in greenhouse gas emissions is still a matter of some debate, agriculture today is clearly a major contributor to the anthropogenically enhanced greenhouse effect.

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