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Plant defense against herbivorous insects
Kessler, André Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York.
- How do plants defend themselves?
- Defensive secondary metabolites
- Costs of plant defenses
- Induced direct and indirect defenses
- Elicitation of plant responses
- Further ramifications
- Related Primary Literature
- Additional Reading
Plants and their insect herbivores comprise a major proportion of the Earth's biodiversity, with roughly 22% of all macroscopic described species being plants and 26% being herbivorous insects. In the process of finding causal links between these interacting groups of organisms, ecologists and evolutionary biologists were driven by two major questions: Why are there so many species of plants and insects? Why is the world still green, given the diversity and abundance of plant-eating organisms? Research on these questions quickly led scientists to a third question: Why do plants produce so many chemical compounds that are seemingly not important for life-sustaining processes? Many of these chemicals, called secondary metabolites to differentiate them from primary metabolites necessary to sustain fundamental vital functions (for example, proteins, carbohydrates, and lipids), have long been used as active ingredients in medicinal and spiritual tinctures, but their ecological functions were not immediately obvious to scientists. In an influential paper in 1964, biologists P. R. Ehrlich and P. H. Raven proposed that secondary metabolites function as toxic plant defenses. They argued that insects are under strong natural selection to cope with toxic plant secondary metabolites and that this pressure would favor insects that are resistant to chemical defenses. The resistant insects would in turn put plants under natural selection to evolve new defensive metabolites in order to keep pace with the adapting herbivores. This continuing cycle of coevolving plant defenses and insect counterdefenses could thus contribute to the formation of new plant and animal species.
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