Today, scientists worldwide are increasingly starting to recognize the role and significance of biodiversity in the functioning of agricultural systems (Swift et al., 1996). Research suggests that, whereas in natural ecosystems the internal regulation of function is substantially a product of plant biodiversity through flows of energy and nutrients and through biological synergisms, this form of control is progressively lost under agricultural intensification and simplification, so that monocultures in order to function must be predominantly subsidized by chemical inputs (Swift and Anderson, 1993). Commercial seedbed preparation and mechanized planting replace natural methods of seed dispersal; chemical pesticides replace natural controls on populations of weeds, insects, and pathogens; and genetic manipulation replaces natural processes of plant evolution and selection. Even decomposition is altered since plant growth is harvested and soil fertility maintained, not through nutrient recycling, but with fertilizers.
One of the most important reasons for maintaining and/or encouraging natural biodiversity is that it performs a variety of ecological services (Altieri, 1991). In natural ecosystems, the vegetative cover of a forest or grassland prevents soil erosion, replenishes groundwater, and controls flooding by enhancing infiltration and reducing water runoff. In agricultural systems, biodiversity performs ecosystem services beyond production of food, fiber, fuel, and income. Examples include recycling of nutrients, control of local microclimate, regulation of local hydrological processes, regulation of the abundance of undesirable organisms, and detoxification of noxious chemicals. These renewal processes and ecosystem services are largely biological; therefore, their persistence depends upon maintenance of biological diversity. When these natural services are lost as a result of biological simplification, the economic and environmental costs can be quite significant. Economically, in agriculture the burdens include the need to supply crops with costly external inputs, since agroec-osystems deprived of basic regulating functional components lack the capacity to sponsor their own soil fertility and pest regulation. Often the costs involve a reduction in the quality of the food produced and of rural life in general due to decreased soil, water, and food quality when erosion and pesticide and/or nitrate contamination occurs (Altieri, 1995).
Nowhere are the consequences of biodiversity reduction more evident than in the realm of agricultural pest management. The instability of agroecosystems becomes manifest as the worsening of most insect pest problems is increasingly linked to the expansion of crop monocultures at the expense of the natural vegetation, thereby decreasing local habitat diversity (Altieri and Letourneau, 1982; Flint and Roberts, 1988). Plant communities that are modified to meet the special needs of humans become subject to heavy pest damage, and generally the more intensely such communities are modified, the more abundant and serious the pests. The effects of the reduction of plant diversity on outbreaks of herbivore pests and microbial pathogens are well documented in the agricultural literature (Andow, 1991; Altieri, 1994). Such drastic reductions in plant biodiversity and the resulting epidemic effects can adversely affect ecosystem function with further consequences on agricultural productivity and sustainability (Figure 1).
In modern agroecosystems, the experimental evidence suggests that biodiversity can be used for improved pest management (Altieri and Letourneau, 1984; Andow, 1991). Several studies have shown that it is possible to stabilize the insect communities of agroecosystems by designing and constructing vegetational architectures that support populations of natural enemies or that have direct deterrent effects on pest herbivores (Perrin, 1980; Risch et al., 1983). This chapter analyzes the various options of agroecosystem design which, based on current agroecological theory, should provide for the optimal use and enhancement of functional biodiversity in crop fields.
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