Competition Between Distantly Related Taxa In the Coevolution of Plants and Pollinators

Abstract

Competition among distantly related plants for pollinators and among distantly related animals for pollen and nectar plays a potentially important role in the organization of ecological communities and the coevolution of plant-pollinator relationships. Plants which rely on animals to disperse their pollen potentially compete for pollinators by processes similar to interference and exploitative competition. Coexisting plant species may evolve to avoid or reduce such competition by character displacement in floral morphology and/or phenology. One important difference between competition for pollinators and most other kinds of competition is that pollinator resources are not used up and made absolutely unavailable to competitors. Consequently, plant species can potentially overlap completely in their utilization of pollinators. The disadvantages of competing apparently are sometimes outweighed by the advantages of sharing pollinators, because distantly related plant species frequently show evolutionary convergence in floral morphology, blooming time and nectar rewards to utilize the same pollinators. Distantly related animal taxa may compete for floral nectar and pollen by both interference and exploitation. The mechanisms of such competition depend primarily on the energetic costs and benefits of foraging and aggression. Exploitative competition is very important because nectar feeders of small body size and low energy requirements can forage economically and reduce nectar availability to levels that will not support larger animals. Thus small nectarivores often can exclude larger competitors from flowers to which both taxa have equal access. Plants may evolve to influence the outcotre of competition among animal visitors and favor species that provide the best pollination services. Thus flowers specialized for pollination by large animals often show morphological or phenological specializations which make rewards unava'lable to smaller animals. Interference is adaptive only when the benefits of exclusive use of a resource outweig.i the costs of defending it. Because distantly related kinds of flower visitors often differ in body size and energetic requirements, interference competition among them is probably rare although it is often important among closely related nectarivores. The community level consequences of competition in the ecology and evolution of plant-pollinator associations are still poorly understoood. Competition among distantly related pollinators for plant floral rewards appears to play a major role, but competition among plants for pollinator services may be only a weak force. Although the basic interaction between plant and pollinator usually is a mutualistic one, certain species of both plants and animals parasitize this interaction and compete with the mutualists for limited resources. Thus some animals rob nectar and pollen and compete with legitimate pollinators without providing pollination services. Similarly, some plants offer no floral rewards but obtain pollinator services by mimicing rewarding flowers of other species. The effects of these kinds of interactions on the organization of communities of plants and pollinators provide a fertile area for future research.