System Stability and Change

Ecosystems maintain themselves over time and have the ability to recover from natural disturbances such as a fire or a hurricane. In agricultural ecosystems, disturbance from cultivation, weeding, harvest, and other agricultural activities is much more intense and frequent. It is difficult to maintain any equilibrium in the system with this disturbance, requiring constant outside interference in the form of human labor and external human inputs. By incorporating ecosystem qualities such as diversity, stability, recovery, and balance, the maintenance of an ecological foundation for long-term sus-tainability can be established.

Agroecologists use the idea of an agricultural ecosystem as a focus for the study of farming systems that are converting from single crops and synthetic inputs to ecologically based design and management. Ecological concepts and principles are applied for the development of alternative practices and inputs. A good example is research done by Sean Swezey and his colleagues on apples in California. After three years of using organic farming techniques, an apple orchard had begun to show a reduction in the use of fossil fuel energy. Nutrients were supplied from compost and annual cover crops planted in the rows between the trees during the winter season. Nutrient recycling and storage in leaves and branches within the apple agro-ecosystem improved soil conditions, reduced the need for fertilizer, and even led to increased yields. Insect pests normally controlled by synthetic pesticides were reduced instead by beneficial predatory insects that were attracted to the organic orchard by mustard and fava-bean flowers in the rows between apple trees. Cover crop species smothered weeds so that herbicides were not needed. In the spring when the cover crop was mowed and cultivated into the soil, microorganism abundance and diversity increased, acting as a biological barrier to the outbreak of diseases in the soil. As the use of external human inputs for the control of the ecological processes in the apple system was reduced, a shift to the use of natural ecosystem processes and interactions and locally derived materials took place. Such an ecological foundation is an important way of determining the sustainability of the agricultural ecosystems of the future. see also Agriculture, Modern; Agriculture, Organic; Ecosystem.

Stephen R. Gliessman

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