Arthropod diversity has been correlated with aspects of plant diversity in agro-ecosystems. A greater variety of plants conforming to a particular crop pattern should lead to a greater variety of herbivorous insect species, and this in turn should determine a greater diversity of predators and parasites (Figure 5). A greater total biodiversity can then play a key role in optimizing agroecosystem processes and function (Altieri, 1984).
Several hypotheses can be offered to support the idea that diversified cropping systems encourage higher arthropod biodiversity (Altieri and Letourneau, 1982):
1. Heterogeneity hypothesis. Complex crop habitats support more species than simple crop habitats; architecturally more complex species of plants and heterogeneous plant associations, with greater biomass, food resources, variety and temporal persistence, have more associated species of insects than do architecturally simple crop plants or crop monocultures on an area-for-area basis. Apparently both species diversity and plant structural diversity are important in determining insect species diversity.
2. Predation hypothesis. The increased abundance of predators and parasites in rich plant associations (Root, 1973) reduce prey densities, at times to such low levels that competition among herbivores should be reduced. This reduced competition should allow the addition of more prey species, which in turn support new natural enemies.
3. Productivity hypothesis. Research has shown that in some situations crop polycultures yield more than monocultures (Francis, 1986; Vandermeer, 1989). This greater
productivity can result in greater insect diversity as the number of food resources available for herbivores and natural enemies increases.
4. Stability and temporal resource-partitioning hypothesis. This hypothesis assumes that primary production is more stable and predictable in polycultures than in monocultures. This stability of production, coupled with the spatial heterogeneity of complex crop fields, should allow insect species to partition the environment temporally as well as spatially, thereby permitting the coexistence of more insect species.
Further research is needed to clarify whether insect species diversity parallels diversity of vegetation and the productivity of the plant community or simply reflects the spatial heterogeneity arising from the mixing of plants of different structures.
There are several environmental factors that influence the diversity, abundance, and activity of parasitoids and predators in agroecosystems: microclimatic conditions, availability of food (water, hosts, prey, pollen, and nectar), habitat requirements (refuges, nesting and reproduction sites, etc.), intra- and interspecific competition and other organisms (hyperparasites, predators, humans, etc.). The effect of each of these factors will vary according to the spatial and temporary arrangement of crops
and the intensity of crop management, as these features affect the environmental heterogeneity of agroecosystems in several ways (van den Bosch and Telford, 1964).
Although natural enemies seem to vary widely in their response to crop distribution, density, and dispersion, experimental evidence suggests that structural (i.e., crop diversity, input levels, etc.) attributes of agroecosystems influence parasitoid and predator diversity and dynamics. Several of these attributes are related to biodiversity and most are amenable to management (i.e., crop sequences and associations, weed diversity, genetic diversity, etc.). Based on the available information, natural enemy biodiversity can be enhanced and effectiveness improved in the following ways (van den Bosch and Telford, 1964; Rabb et al., 1976; Altieri and Whitcomb, 1979):
• Multiple introductions of parasitoids and predators through augmentative releases for biological control;
• Reducing direct mortality by eliminating pesticide use;
• Provision of supplementary resources other than hosts/prey;
• Increasing adjacent and within-field vegetational diversity;
• Manipulating architectural, genetic, and chemical attributes of host plants;
• Use of semiochemicals (behavioral chemicals such as kairomones) to stimulate host/prey searching behavior and natural enemy retention in the field.
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