Many workers have studied the nature of plant defenses against herbivores. Various tropical shrubs have strong chemical defenses against herbivores. Early successional species seem to have fewer defenses than later successional and climax species. Tropical herbivorous insects are generally believed to be more host-specific than temperate species. The number of herbivorous insects on an individual tropical tree seems too small for interspecific competition, but the resource base by any single species may be very small. A plant’s evolutionary response to one species of herbivore sometime greatly reduces the suitability of the plant for another herbivore species.
The array and variation of plant defenses and the coevolutionary patterns among the plant species are closely associated with the numbers of herbivores interacting with the plants. Step-wise coevolution may be a suitable model for short-lived plants, but on perennial plants supporting a large array of herbivore species, coevolution is more likely to involve gradual and continuous adjustments rather than step-wise changes. During the course of evolution of land plants, diverse biochemical changes in their secondary compounds have taken place, often involving both increased structural complexity and diversity. Most of these secondary compounds play a vital role in plant defenses against herbivores as well as pathogens.
The evolutionary patterns of plant defenses are, however, also influenced and modified by diverse ecological, physiological and biological factors. Commonly, the effects of herbivores on plants are more chronic than acute. In conifers, terpenoid resins constitute a versatile defense potential against insect herbivory. There are certain plants that support only one or a few important taxa of herbivores. Thus Passijlora spp. mainly supports butterflies belonging to the genus Heliconius. These butterflies lay yellowish eggs on the plant’s leaves, but during the course of insect-plant coevolution, structures resembling the yellow eggs seem to have arisen independently in spp. of Passiflora.
These plant structures are thought to have evolved specifically to mimic the butterfly eggs (see Williams and Gilbert, 1981). These workers have shown that the female butterflies discriminate against plants with eggs (or rather the structures which look like eggs) and therefore tend to act as selective agents in the evolution of these egg-like structures. This response of Heliconius females to the presence of egg-like structures has mainly or wholly a visual basis. The Passiflora-Heliconius system constitutes a good example of a plant structural characteristic resulting from coevolution with a host restricted group of insect herbivores (Williams and Gilbert, 1981). Although many workers are now interested in studying the plant herbivore interactions, it would not be far wrong to state that till today there is no wild plant species anywhere in the tropics for which even an approximate herbivore load is known. Though the primary metabolites of plants are mostly the same, there is enormous diversity in the chemical nature, composition, and relative proportions of various secondary compounds synthesized by different plants. Many plant parts may contain just traces of a variety of secondary compounds, and a large amount of a few. Certain animals feed on many plant species differing among themselves in respect of secondary compounds.
Other animals are specialists in the sense that they will eat only one or a few plant species and no others. There are several promising areas of study in this field, e.g., the kinds of enzymes found in the guts and livers of the generalist herbivores, which enable them to move from one species of plant to another without any significant pause, and to digest the different kinds of secondary compounds. On the other hand, why is specialist animals specialized to only one kind of host? Does such specilization depend on any inter-system incompatibility of detoxification systems? Or is it like the canavanine system studied by Rosenthal et al.
(1977), where the beetle is eating something produced by the detoxification process, and where it seems that the animal avoids other hosts not only because they are toxic but also because they do not offer some special diet? However, it is not always that plants defend themselves against animals. In many cases, the interaction between a plant and its consumer is beneficial for the plant also. The best example of this kind of mutualism comes from grasses which survive best when grazed and are probably co- evolved with their grazers; one cannot exist without the other.
The grass- grazer mutualism is no less intimate than that between a flower and a bee (Owen, 1980). Many plants have evolved strategies that attract animals in a way beneficial to both the plants and the animals; a few such cases are described below. It has also been observed that the germination rate of seeds is often higher after they have passed through an animal gut. For instance, the deposition of sugary honeydew by aphids may increase the rate of nitrogen fixation beneath the plant by providing an energy source for free-living nitrogen-fixers (Owen, 1980).