Herbivore pressure

Although herbivore pressure is more intense and is exerted by a more diverse assemblage of consumers in the tropics (Liming 1990), it is now known that herbivory fails to fully explain the production of secondary metabolites, and that these... 

There are many exceptions to these general observations. Small changes greatly affect the secondary compounds produced, if they occur early in the biosynthetic route. Further, it is easier to lose than to gain the ability to synthesize and accumulate a particular compound. Thus, it is not surprising to find many instances of apparent reversals in biosynthetic capacity. This should particularly be true in instances where the initial selection pressures that favored the presence of the compounds have disappeared. Species of plants that occur on islands (presumably with lower herbivore pressures) often have reduced secondary compound production and accumulation in comparison to mainland species of the same genus (Mabry, 1973 Seeligmann and Alston, 1967). 

Nutrition Cassava Cyanogenic glycosides Decrease Increased herbivore pressure... 

By necessity, herbivores have evolved GIT and systemic compensatory mechanisms that allow them to subsist on plant-based diets that have limited nutrient quality and include phytochemicals. Still, herbivores remain susceptible to some of the anti-nutrient and toxic phytochemicals. For example, several herbivores are sensitive to the phytotoxins associated with autumn crocus, which include colchicine (Yamada et al, 2000). As a consequence, herbivores tend to select species and portions of plants based on a combination of nutrient quality and concentrations of phytochemicals (Yeager et al, 1997), and this has an impact on habitat selection and plant ecology (Duncan and Gordon, 1999). Carnivorous species have not been under selective pressure to develop similar compensatory mechanisms, generally have only limited abilities to subsist on plant-based diets, and in many cases are less tolerant of phytochemicals. 

Coevolution is defined as reciprocal stepwise adaptations between at least two species (Ehrlich and Raven, 1964). Coevolution without the criterion of reciprocity is indistinguishable from evolution and hence a useless concept (Lindroth, 1988). Consider the following scenario. A plant develops effective antiherbivore defenses. In response, a herbivore counteradapts to circumvent these defenses and is at a competitive advantage over other herbivores. The plant, in turn, responds to this breach of its defenses. In insects, such pairwise reciprocal evolution can take the form of a chemical arms race (Dawkins and Krebs, 1979). Coevolution differs from evolution by being narrower, with fewer participants, perhaps even only two species or two populations. In reality, in most ecosystems, many species prey on many other species. Therefore, we can at best speak of diffuse coevolution, with a number of participants that exert diluted selection pressures. 

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