POLLINATOR FORAGING BEHAVIOR AND GENE DISPERSAL IN SENECIO (COMPOSITAE).

Wright (1943, 1946) showed that in a continuous population of organisms, the pattern of genetic differentiation is largely determined by the number of individuals in a local random breeding unit, or neighborhood. When neighborhoods are small, populations are subject to greater differentiation, both randomly and in response to natural selection. Neighborhood size is a function of population density and gene dispersal. The breeding systems and stationary spatial distributions of animal-pollinated flowering plants are consistent with the assumptions of Wright's models; such systems have proved excellent for the study of neighborhood size, because gene dispersal can be estimated directly by measuring pollinator movements and seed dispersal distances (Kerster and Levin, 1968; Levin and Kerster, 1968, 1969a, 1969b, 1974; Schaal and Levin, 1978; Beattie, 1979). The foraging behavior of pollinators has major importance for patterns of gene dispersal in plant populations (Levin, 1979a, 1979b). Foraging behavior in turn may be affected by the quality and distribution of the nectar sugar rewards offered by flowers (Heinrich and Raven, 1972; Heinrich, 1975). Pollinators can utilize only flower resources which provide sufficient caloric reward to make foraging energetically profitable (Heinrich and Raven, 1972; Heinrich, 1975). Types of pollinators may differ by several orders of magnitude in the metabolic energy costs they incur during foraging and thermoregulation (Heinrich and Raven, 1972; Heinrich, 1975). For example, butterflies, which thermoregulate by basking (Watt, 1968) and have relatively low foraging costs, can profitably utilize flowers with relatively small nectar rewards (Watt et al., 1974), while bumblebees and hawkmoths, which thermoregulate metabolically and expend more energy in foraging (Heinrich, 1975), would operate at a loss on the same resource. Many flower species have specialized features of nectar presentation adapting them to a particular pollinator type (Heinrich and Raven, 1972; Faegri and van der Pijl, 1979). For example, plant species specialized for bumblebee or hawkmoth pollinators often provide rich nectar rewards in deep-spurred nectaries that are inaccessible to low-energy pollinators. On the other hand, many plants provide minute quantities of nectar that are profitable only to animals with low metabolic costs. If pollinator types with different energy requirements differ in their foraging behavior, then the neighborhood structures of plants specialized for those pollinators can be expected to differ also. Linhart (1973) has shown that pollen dispersal patterns in tropical Heliconia differ dramatically depending on whether territorial or traplining hummingbirds are the pollinators. Moreover, for plants which are generalists, pollinated by several types of animals, neighborhood structure may be significantly affected by the proportion of pollen transferred by different pollinator types. Two aspects of pollinator foraging behavior have particular importance for patterns of plant gene dispersal. First, flight distances between plants will determine the distance over which pollen is transferred. Second, in self-compatible plants the number of flowers visited per plant will determine the proportion of seeds set that are selfed or outcrossed, and thus will affect levels of inbreeding. Moreover, if pollen from a given flower is carried over to more than one of the flowers subse-