The wet tropics, with their almost continuous hot, moist conditions, are an ideal environment for growth of bacteria and fungi, many of which cause diseases, and the insects that carry these organisms. At high latitudes sub-freezing temperatures can reduce or eliminate populations of disease-causing organisms and herbivorous insects. At the beginning of each growing season, population growth for many insect and disease species must start again. This defense against population explosion is missing in continually moist tropical forests. In tropical areas where there is a distinct dry season, fire may play a role similar to that played by freezing temperatures at high latitudes. For example, in northern India there are pure stands of Shorea robusta (sal) due to periodic fires which favor the fire-resistant sal and eliminate competing trees (Goldhammer 1993). Such virtual monocultures are rare or non-existent in the moist tropics.
Because of the high herbivore pressure on many tropical plants, a variety of defense mechanisms have evolved. One that is common in tropical plants is the presence of secondary plant chemicals that make leaves unpalatable to many herbivores (Coley 1980). Another defense mechanism of tropical forests against pests and disease outbreaks is a characteristic of the ecosystem: high species diversity. High diversity of tree species in the tropics provides survival benefit for the forest. Many herbivores and diseases are specific to a particular species. If an insect or disease organism locates and attacks an individual of a certain species, the high diversity of the forest makes it difficult for the pest to locate other individuals of the same species. The greater the distance between individuals of a given species and the greater the number of other species between individuals of the same species, the lower the probability that an insect or disease organism will find the next individual, and the lower the rate of population growth of disease organisms and herbivores (Janzen 1970). That is why pure stands of native species are rare or non-existent in the wet tropics, at least in areas that are not stressed by factors such as soils that are saturated, salty, or extremely poor in available nutrients. Diversity is a naturally occurring defense against disease and insect attack (Box 2.2).
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