Agricultural Ecosystems

An agricultural ecosystem, which is also known as an agroecosystem, is a place where agricultural production—a farm, for example—is understood as an ecosystem. When something like a farm field is examined from an ecosystem viewpoint, food production can be understood as part of a whole, including the complex kinds of materials entering the system, or inputs, and the materials leaving the system, or outputs. At the same time, the ways that all of the parts of the system are interconnected and interact are of great importance.

Humans alter and manipulate ecosystems for the purpose of establishing agricultural production, and in the process, can make the resulting agroecosystem very different from a natural ecosystem. At the same time, however, by understanding how ecosystem processes, structures, and characteristics are modified, management of an agricultural system can become more stable, less dependent on inputs brought in from outside the system, and more protective of the natural resources with which it may interact.

Scientists who study agricultural systems as ecosystems are known as agroecologists, and the field they work in is known as agroecology. An agroecologist applies the concepts and principles of ecology to the design and management of sustainable agroecosystems. Sustainability refers to the ability to preserve the productivity of agricultural land over the long term, protect the natural resources upon which that productivity depends, provide farming communities with a fair and prosperous way of life, and produce a secure and healthy food supply for people who do not live on the farms. The challenge these scientists face is developing agroecosystems that achieve natural ecosystem-like characteristics while maintaining a harvest output. With a goal of sustainability, a farm manager strives as much as possible to use the ecosystem concept in designing and managing the agroecosystem. In doing so, the following four key traits of ecosystems are included.

Energy Flow

Energy flows into an ecosystem as a result of the capture of solar energy from the Sun by plants, and most of this energy is stored as biomass or used to maintain the internal processes of the system. But removing energy-rich biomass from the system causes changes. Human energy (considered renewable) as labor, and industrial energy (considered nonrenewable) from fossil fuels, become necessary. Agroecologists look for ways to increase the efficiency of the capture of energy from the Sun and increase the use of renewable energy, achieving a better balance between the energy needed to maintain internal processes and that which is needed for harvest export.

Nutrient Cycling

Many nutrients are cycled through ecosystems. Biomass is made up of organic compounds manufactured from these nutrients, and as organisms die and decompose, the nutrients return to the soil or the atmosphere to be recycled and reused again. Agricultural ecosystems lose nutrients with harvest removal, and because of their more simplified ecological structure, lose a greater proportion of nutrients to the air or by leaching in rain and irrigation water. Humans must return these nutrients in some form. In a well-designed agroecosystem, the farmer strives to keep nutrient cycles as closed as possible, reducing nutrient losses while searching for sustainable ways of returning exported nutrients to the farm.

Regulation of Populations

Complex interactions between organisms regulate their numbers in natural ecosystems. Competition, mutualisms, and other types of interactions are promoted by the organization and structure of the system. Growing one or very few crops in modern agriculture eliminates many of these interactions, often removing natural control mechanisms and allowing pest outbreaks. An agroecological alternative seeks to reintroduce more complex structures and species arrangements, often including both crop and noncrop species, in order to reduce the use of pesticides and enhance natural controls.

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 sustainability 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.

Stephen R. Gliessman

Bibliography

Altieri, Miguel A. Agroecology: The Science of Sustainable Agriculture. Boulder, CO: Westview Press, 1995.

Gliessman, Stephen R. Agroecology: Ecological Processes in Sustainable Agriculture. Chelsea, MI: Ann Arbor Press, 1998.

Lowrance, Richard, Ben R. Stinner, and Gar J. House. Agricultural Ecosystems: Unifying Concepts. New York: John Wiley & Sons, 1984.

National Research Council. Alternative Agriculture. Washington, DC: National Academy Press, 1989.

Odum, Eugene P. Ecology: A Bridge Between Science and Society. Sunderland, MA: Sinauer Associates.

Swezey, Sean L., Jim Rider, Matthew Werner, Marc Buchanan, Jan Allison, and Stephen R. Gliessman. "Granny Smith Conversions to Organic Show Early Success." California Agriculture 48 (1994): 36-44.

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