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

Competition among distantly related plants for pollinators andamong distantly related animals for pollen and nectar playsa potentially important role in the organization of ecologicalcommunities and the coevolution of plant-pollinator relationships.Plants which rely on animals to disperse their pollen potentiallycompete for pollinators by processes similar to interferenceand exploitative competition. Coexisting plant species may evolveto avoid or reduce such competition by character displacementin floral morphology and/or phenology. One important differencebetween competition for pollinators and most other kinds ofcompetition is that pollinator resources are not used up andmade absolutely unavailable to competitors. Consequently, plantspecies can potentially overlap completely in their utilizationof pollinators. The disadvantages of competing apparently aresometimes outweighed by the advantages of sharing pollinators,because distantly related plant species frequently show evolutionaryconvergence in floral morphology, blooming time and nectar rewardsto utilize the same pollinators. Distantly related animal taxa may compete for floral nectarand pollen by both interference and exploitation. The mechanismsof such competition depend primarily on the energetic costsand benefits of foraging and aggression. Exploitative competitionis very important because nectar feeders of small body sizeand low energy requirements can forage economically and reducenectar availability to levels that will not support larger animals.Thus small nectarivores often can exclude larger competitorsfrom flowers to which both taxa have equal access. Plants mayevolve to influence the outcotre of competition among animalvisitors and favor species that provide the best pollinationservices. Thus flowers specialized for pollination by largeanimals often show morphological or phenological specializationswhich make rewards unava'lable to smaller animals. Interferenceis adaptive only when the benefits of exclusive use of a resourceoutweig.i the costs of defending it. Because distantly relatedkinds of flower visitors often differ in body size and energeticrequirements, interference competition among them is probablyrare although it is often important among closely related nectarivores. The community level consequences of competition in the ecologyand evolution of plant-pollinator associations are still poorlyunderstoood. Competition among distantly related pollinatorsfor plant floral rewards appears to play a major role, but competitionamong plants for pollinator services may be only a weak force.Although the basic interaction between plant and pollinatorusually is a mutualistic one, certain species of both plantsand animals parasitize this interaction and compete with themutualists for limited resources. Thus some animals rob nectarand pollen and compete with legitimate pollinators without providingpollination services. Similarly, some plants offer no floralrewards but obtain pollinator services by mimicing rewardingflowers of other species. The effects of these kinds of interactionson the organization of communities of plants and pollinatorsprovide a fertile area for future research.