ABSTRACT
Wild biodiversity is more threatened now than at any time since the extinction of the dinosaurs, with nearly 24% of all mammals, 12% of birds, and almost 14% of plants threatened with extinction.[2] If current trends continue, it is estimated that at least 25% of the Earth’s species could become extinct or drastically reduced by the middle of this century.[2] Conversion of natural ecosystems to agroecosystems is a primary cause of these alarming trends.[2,4,5] At least 28% of the Earth’s land area currently is devoted to agriculture to some degree.[1] Intensive agriculture dominates 10% of the earth’s total land area and is part of the landscape mosaic on another 17%, while extensive grazing covers an additional 10-20%.[2] Nearly half of the global temperate broadleaf and mixed-forest and tropical and subtropical dry and monsoon broadleaf forest ecosystems are converted to agricultural use (45.8% and 43.4%, respectively).[1,2] However, agriculture’s greatest impact has been on grassland ecosystems, including temperate grasslands, savannas, and shrublands (34.2%); fl ooded grasslands and savannas (20.2%); and montane grasslands and shrublands (9.8%).[1] Combined, 64.2% of the Earth’s grassland ecosystems have been converted to agriculture, primarily for production of cereal grasses-maize, rice and wheat.[1,2] In the past 20 years, net expansion of agricultural land has claimed approximately 130,000 km2/yr globally, mostly at the expense of forest and grassland ecosystems, but also from wetlands and deserts.[1]
The native ecosystems that agriculture has replaced typically had high biodiversity. A hectare of tropical rain forest may contain over 100 species of trees and at least 10-30 animal species for every plant species, leading to estimates of 200,000 or more total species.[6] In contrast, the world’s agroecosystems are dominated by some 12 species of grains, 23 vegetable crops, and about 35 fruit and nut crop species.[1] Furthermore, conversion of native ecosystems to agriculture causes dramatic shifts in ecosystem structure and function that affect ecosystem processes above and below ground including energy fl ow, nutrient cycling,
AgricultureBiodiversity
water cycling, food web dynamics, and biodiversity at all trophic levels.[7] The amount of wild biodiversity loss depends on the degree of fragmentation of the native landscape. Whereas some species require vast continuous areas of native habitat, many can survive as long as the appropriate size and number of patches with connecting corridors of native habitat are left intact and provided that barriers to species movement-such as road and irrigation networks-are limited. However, when conversion leads to critical levels of native landscape fragmentation, chain reactions of biodiversity loss have been observed as interdependent species lose the resources they need to survive.[2] Loss of wild biodiversity at this level leads to loss of numerous ecosystem benefi ts that are essential to agriculture, e.g., 1) drought and fl ood mitigation; 2) soil erosion control and soil quality regeneration; 3) pollination of crops and natural vegetation; 4) nutrient cycling; and 5) control of most agricultural pests.[5]
STRUCTURE AND FUNCTION OF AGROECOSYSTEMS AND BIODIVERISTY
The structure and function of agroecosystems are largely determined by local context, including interaction of ecological conditions (including bio-, geo-, and chemical) with social factors, including farmers’ economic needs, cultural and spiritual values, and social structure and technology. Two types of agrobiodiversity have been defi ned:[8] planned biodiversity is the specifi c crops and/or livestock that are planted and managed and associated biodiversity is nonagricultural species that fi nd the environment created by the production system compatible (e.g., weeds, insect and disease pests, predators and parasites of pest organisms, and symbiotic and mutualistic species).[8] Planned and associated biodiversity can enhance stability and predictability of agroecosystems.[5] Traditional forms of agriculture-such as home gardens and shade coffee farms in the New and Old World tropics[4,6,8] and traditional Amish dairy farms in North America[9]—have a complex and diverse spatial and vertical structure and high planned and associated biodiversity. For example, traditional neotropical agroforestry systems commonly contain over 100 annual and perennial plant species per fi eld.[4] Traditional agroecosystems create landscape patterns of smallscale diverse patches with many edges, habitat patches, and corridors for wild biodiversity.
