Diversity as a defense against plant disease and pests

The mahogany shoot borer, Hypsipyla grandella, is a particularly noxious pest in forest plantations throughout the American tropics. Larvae of this insect bore into the stems and terminal shoots of young plants, particularly genera such as Cedrela (Spanish cedar) and Swietenia (mahogany). As a result, seedlings in monoculture plantations can be killed or severely distorted. A number of approaches to control the pest have been tried, including biological control, genetic engineering of host plants, and sustained release of systemic chemicals (Orians et al. 1974; Navarro et al. 2004). However, planting seedlings at wide spacing within native forests perhaps is most effective because it mimics the protection that the trees have under natural conditions. Planting Cedrela trees in agroforestry systems with coffee has been shown to be an effective way to avoid insect at tack. In experimental research in Costa Rica the attack of the shoot borer insect was lower when Cedrela trees were planted in adult coffee plantations rather than in recently planted or pruned coffee (Navarro et al. 2004). Mixed plantations and agroforestry systems are discussed in more detail in Chapter 6.

There could be objections to the idea that the high plant diversity in moist tropical forests is an adaptation to herbivore pressure. Adaptation takes place at the species level and diversity is an adaptation at the community level. However, adaptation here is not used in the classical sense. It simply means that low-diversity forests are rare in the wet, fertile tropics because it is difficult for them to become established. The proximity of individuals of the same species makes them easy prey for outbreaks of disease. Diverse communities are less susceptible to sustained attack and destruction by pests and diseases.

The diversity of species in tropical forests not only helps to ensure the survival of each species, it also helps to ensure the continued existence of tropical forest ecosystems. While individual populations within a forest peak and crash (May 1972), the flow of energy and cycling of nutrients within a forest remain relatively stable. At each trophic level, there is a continual dynamic flux, as the population of one species that is rising replaces the population of another that is falling (McCann 2000; Tilman 2000).

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