Do Consumers Maximize Plant Fitness?

Abstract

The hypothesis is developed that consumers, like pollinators, have a mutualistic relationship with plants. Three possible examples of this are presented. 1) The supply of nitrogen is often the main limiting factor to a plant's productivity and reproductive success. The large quantity of sugary honeydew excreted by aphids may stimulate free nitrogen fixation beneath the plant from which the sugar is extracted. Hence provided a plant can produce surplus sugar, and provided this can be fed to the soil through the action of aphids, both plants and aphids could benefit. 2) Certain cercopids release large quantities of water from trees, possibly increasing the uptake of nutrients by nearsurface roots during otherwise dry conditions. 3) Grazing can result in premature leaf-fall and extend the normal season and cycle of decomposition beneath a plant, thus increasing the conservation of nutrients. In most terrestrial ecosystems less than 10% of the living material goes to consumers. We suggest plants regulate and even encourage a spectrum of consumers, whose overall effect is to maximize plant fitness. The common idea that all plants fight an evolutionary battle in which they defend themselves against the detrimental effects of consumers requires re-examination and modification.