Several ecologists have stressed the importance of insect-plant interactions to our understanding of ecosystem structure, function and management (Bormann and Likens, 1979; Odum et al., 1979; Schowalter, 1981).
Three general conclusions have emerged from studies of insect-plant interactions;
(1) the host plant’s defensive mechanisms inhibit insect feeding, development, or survival;
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(2) the insects alter the plant’s competitive relationships through selective heibivory, and this tends to speed up the rate of succession; and
(3) insects also regulate the rate and direction of nutrient cycling between plants and litter.
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The outcome of the former conditions is beneficial in the sense that both primary productivity and nutrient-cycling tend to increase; the consequence of the latter condition (stressed plants) is acceleration of plant decay and replacement.
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With the increase in biomass and competition during the course of succession, plants become stressed. Early successional plant species have a higher requirement of light and minerals and they tend to become stressed sooner than late successional species (Schowalter, 1981).
Abundance and stress make the early species more susceptible to attack by insects as compared to the late species. The effect of increasing population density of insect herbivores on stressed plants is to regulate long term nutrient cycling by accelerating changes in competitive relationships between plant species with different physiological requirements (Schowalter, 1981).
Some orchids have highly specialized and peculiar floral structure that can only be pollinated by a specific moth having corresponding mouth parts. In Madagascar, one such orchid was discovered in the last century by Alfred Russell Wallace (the contemporary of Charles Darwin).
This orchid could only be pollinated by a large moth with a very long tongue, but no such moth was known till then. Wallace confidently predicted the existence of such a moth and, true enough, it was later discovered and named Xanthopan morgani praedicta; it has a 25-cm long tongue that pollinates the orchid (see Owen, 1980).