Do Grazers Maximize Individual Plant Fitness?

Abstract

The hypothesis of Owen and Wiegert stating that grazers may increase individual plant fitness has been found plausible from a theoretical point of view by analyzing two mathematical models. The technique adapted is to search for evolutionarily stable strategies (or ESS) in a system where the plant's palatability is the only evolutionary variable considered. It is concluded that the fitness of particularly shortlived plant species is enhanced when grazed by animals wasting extensive amounts of clipped vegetation and/or defecating close to the eaten plant. Based on this, several generalizations are stated: The hypothesized form of mutualism is (1) more likely to apply for plants which are r-strategists than K-strategists; (2) more likely to apply in early successional stages than close to climax; and (3) more likely in terrestrial than in aquatic systems. Furthermore (4), a gradient with respect to herbivore defense features as a function of plant life histories is likely to exist only in systems with extensive grazing pressure. From these discussions it is concluded that theories for herbivore population regulation relying on mechanisms such as toxic substances in plants are mainly likely to apply for herbivores feeding on long lived plant species. Lastly it is concluded that further theoretical studies on herbivory might prove rewarding by supplementing such studies on predator-prey interactions.