Agriculture damages biodiversity in a number of ways. It brings about habitat loss, depletes fresh water resources, contributes to climate change, and pollutes ecosystems.
Agriculture requires large tracts of land to be reappropriated for human use. Often this results in the displacement or destruction of wildlife populations. 85 percent of Canadian wetland losses are due to drainage for agricultural purposes, and agriculture in South and Central America results in deforestation (United States Fish and Wildlife Service, 2002).
Irrigation for agricultural systems consumes large quantities of fresh water. 60 percent of global water usage is devoted to irrigation (Dewar, 2007). This volume of water diversion changes ecosystems and causes pressures on wildlife. A notable example is the case of Keoladeo National Park in India. A dam and reservoir supplies water both to the park and local farmers (Chauhan, 2006). In times of drought, conflicts arise between the farmers and park advocates, and the biodiversity of the park hangs in the balance (Chauhan, 2006). This effect can be more pronounced in arid and semi-arid regions. Up to 80 percent of diverted water in these regions goes to agriculture (Fereres & Soriano 2007).
Fossil fuels burned in farm equipment, methane emissions from livestock, and land clearance for fields all contribute to climate change. Agriculture is responsible for 25 percent, 65 percent, and 90 percent respectively of anthropogenic CO2, CH4, and N2O emissions (Duxbury, 1994). Each of these gasses - particularly methane (CH4) - are greenhouse gasses.
Pollution due to agriculture is a significant threat to biodiversity in aquatic ecosystems. Chemical runoff from fields flows into streams and then accumulates in lakes and large ocean patches. Some of the most noticeable impacts of this aquatic pollution are algal blooms caused by nitrogen accumulation. These algal blooms deplete oxygen and cause dead zones. These dead zones cover thousands of square miles of open ocean (Dewar, 2007). Dewar (2007) notes that a study of the Mississippi River basin proved that 91 percent of the nitrogen content in the water could be traced back to agricultural sources.