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A model for the functionings of an agricultural system, with all inputs and outputs. An ecosystem may be as small as a set of microbial interactions that take place on the surface of roots, or as large as the globe. An agroecosystem may be at the level of the individual plant-soil-microorganism system, at the level of crops or herds of domesticated animals, at the level of farms or agricultural landscapes, or at the level of entire agricultural economies.
Agroecosystems differ from natural ecosystems in several fundamental ways. First, the energy that drives all autotrophic ecosystems, including agroecosystems, is either directly or indirectly derived from solar energy. However, the energy input to agroecosystems includes not only natural energy (sunlight) but also processed energy (fossil fuels) as well as human and animal labor. Second, biodiversity in agroecosystems is generally reduced by human management in order to channel as much energy and nutrient flow as possible into a few domesticated species. Finally, evolution is largely, but not entirely, through artificial selection where commercially desirable phenotypic traits are increased through breeding programs and genetic engineering. Agroecosystems are usually examined from a range of perspectives including energy flux, exchange of materials, nutrient budgets, and population and community dynamics.
Solar energy influences agroecosystem productivity directly by providing the energy for photosynthesis and indirectly through heat energy that influences respiration, rates of water loss, and the heat balance of plants and animals. See Biological productivity, Ecological energetics, Photosynthesis
Nutrient uptake from soil by crop plants or weeds is primarily mediated by microbial processes. Some soil bacteria fix atmospheric nitrogen into forms that plants can assimilate. Other organisms influence soil structure and the exchange of nutrients, and still other microorganisms may excrete ammonia and other metabolic by-products that are useful plant nutrients. There are many complex ways that microorganisms influence nutrient cycling and uptake by plants. Some microorganisms are plant pathogens that reduce nutrient uptake in diseased plants. Larger organisms may influence nutrient uptake indirectly by modifying soil structure or directly by damaging plants. See Soil microbiology
Although agroecosystems may be greatly simplified compared to natural ecosystems, they can still foster a rich array of population and community processes such as herbivory, predation, parasitization, competition, and mutualism. Crop plants may compete among themselves or with weeds for sunlight, soil nutrients, or water. Cattle overstocked in a pasture may compete for forage and thereby change competitive interactions among pasture plants, resulting in selection for unpalatable or even toxic plants. Indeed, one important goal of farming is to find the optimal densities for crops and livestock. See Herbivory, Population ecology
Widespread use of synthetic chemical pesticides has bolstered farm production worldwide, primarily by reducing or eliminating herbivorous insect pests. Traditional broad-spectrum pesticides such as DDT, however, can have far-ranging impacts on agroecosystems. For instance, secondary pest outbreaks associated with the use of many traditional pesticides are not uncommon due to the elimination of natural enemies or resistance of pests to chemical control. Growers and pesticide developers in temperate regions have begun to focus on alternative means of control. Pesticide developers have begun producing selective pesticides, which are designed to target only pest species and to spare natural enemies, leaving the rest of the agroecosystem community intact. Many growers are now implementing integrated pest management programs that incorporate the new breed of biorational chemicals with cultural and other types of controls.
The last few decades have seen tremendous advances in molecular approaches to engineering desirable phenotypic traits in crop plants. Although artificially modifying crop plants is nothing new, the techniques used in genetic engineering allow developers to generate new varieties an order-of-magnitude faster than traditional plant breeding. In addition, genetic engineering differs from traditional breeding in that the transfer of traits is no longer limited to same-species organisms. Scientists are still assessing the effects that the widespread deployment of these traits may have on agroecosystems and natural ecosystems. There is some concern, for instance, that engineered traits may escape, via genes in pollen transferred by pollinators, and become established in weedy populations of plants in natural ecosystems, in some cases creating conservation management problems and new breeds of superweeds. As with pesticides, there is evidence that insects are already becoming resistant to some more widespread traits used in transgenic plants, such as the antiherbivore toxin produced by the bacterium Bacillus thuringiensis. See Biotechnology, Genetic engineering