GENERALIZATION IN POLLINATION SYSTEMS, AND WHY IT MATTERS

One view of pollination systems is that they tend toward specializatio n. This view is implicit in many discussions of angiosperm evolution a nd plant-pollinator coevolution and in the long-standing concept of '' pollination syndromes.'' But actual pollination systems often are more generalized and dynamic than these traditions might suggest, To illus trate the range of specialization and generalization in pollinators' u se of plants and vice versa, we draw on studies of two floras in the U nited States, and of members of several plant families and solitary be e genera, We also summarize a recent study of one local flora which su ggests that, although the colors of flowers are aggregated in ''phenot ype space,'' there is no strong association with pollinator types as p ollination syndromes would predict. That moderate to substantial gener alization often occurs is not surprising on theoretical grounds. Plant generalization is predicted by a simple model as long as temporal and spatial variance in pollinator quality is appreciable, different poll inator species do not fluctuate in unison, and they are similar in the ir pollination effectiveness. Pollinator generalization is predicted w hen floral rewards are similar across plant species, travel is costly, constraints of behavior acid morphology are minor, and/or pollinator lifespan is long relative to flowering of individual plant species. Re cognizing that pollination systems often are generalized has important implications. In ecological predictions of plant reproductive success and population dynamics it is useful to widen the focus beyond flower visitors within the ''correct'' pollination syndrome, and to recogniz e temporal and spatial fluidity of interactions. Behavioral studies of pollinator foraging choices and information-processing abilities will benefit from understanding the selective advantages of generalization . In studies of floral adaptation, microevolution, and plant speciatio n one should recognize that selection and gene flow vary in time and s pace and that the contribution of pollinators to reproductive isolatio n of plant species may be overstated. In conservation biology, general ized pollination systems imply resilience to linked extinctions, but a lso the possibility for introduced generalists to displace natives wit h a net loss of diversity.