The trend of agriculture through most of the twenthieth century was toward greater uniformity of agricultural production, achieved by means of standardization, mechanization, and monoculture. In many cases, that trend violated nature's own inherent biodiversity and its checks-and-balances synergisms. Monocultures are especially vulnerable to pests, diseases, and climate anomalies. New approaches to sustainable agriculture are impelled by a growing recognition of the indispensable importance of biodiversity, which in the long run promotes agro-ecosystem stability, immunity, and resilience in the face of climate variability and pests. Agriculture is dependent on biodiversity through the genetic spectrum of the wild ancestors of domesticated plants and animals, pollinators, soil microbia, and natural biological control agents and mechanisms that can avoid the use of toxic chemicals and the deleterious effects of their residues. In particular, soil microbial diversity contributes to soil fertility through decomposition of organic matter and the cycling of nitrogen and carbon; maintenance of soil structure and the hydrological cycle through aggregation of soil particles and aeration; the greenhouse effect through influence on soil carbon sequestration and trace gas fluxes; and air and water purification by degradation of pollutants. Agricultural biodiversity both affects, and will be affected by, the magnitude and rate of climate change, and its geographical and seasonal patterns. New agro-ecosystem approaches that integrate farming and ecology may better promote nutrient recycling, biological pest and disease control, pollination, soil quality maintenance, water-use efficiency, and carbon sequestration, as well as responses to weather extremes, such as droughts and floods.