Diversified cropping systems, such as those based on intercropping and agrofor-estry or cover cropping of orchards, have been the target of much research recently. This interest is largely based on the new emerging evidence that these systems are more sustainable and more resource-conserving (Vandermeer, 1995). Much of these attributes are connected to the higher levels of functional biodiversity associated with complex farming systems. In fact, an increasing amount of data reported in the literature documents the effects that plant diversity have on the regulation of insect herbivore populations by favoring the abundance and efficacy of associated natural enemies (Altieri, 1994). Several hypotheses are emerging postulating the mechanisms explaining the relationships between plant species number and the stabilization of agroecosystem processes including the buffering of populations (Tilman et al., 1996). One aspect that is clear is that species composition is more important than species numbers per se. The challenge is to identify the correct assemblages of species that will provide through their biological synergisms key ecological services such as nutrient cycling, biological pest control, and water and soil conservation.
The exploitation of these synergisms in real situations involves agroecosystem design and management and requires an understanding of the numerous relationships among plants, herbivores, and natural enemies. Clearly, the emphasis of this approach is to help to restore natural control mechanisms through the addition of selective biodiversity within and outside the crop field, through a whole array of possible crop arrangements in time and space.
Data and practical experience indicate that it is possible to stabilize the insect communities of agroecosystems by designing and constructing vegetational architectures which support populations of natural enemies or that have direct deterrent effects on pest herbivores (Altieri, 1991). What is difficult is that each agricultural situation must be assessed separately, since herbivore-enemy interactions will vary significantly depending on insect species, location and size of the field, plant composition, the surrounding vegetation, and cultural management. One can only hope to elucidate the ecological principles governing arthropod dynamics in complex systems, but the biodiversity designs necessary to achieve herbivore regulation will depend on the agroecological conditions and socioeconomic restrictions of each area.
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