The Dependence of Bee-Mediated Pollen and Gene Dispersal Upon Plant Density

Abstract

Pollen dispersal and pollen-mediated gene dispersal in flowering plants are effected primarily by animals and air currents. It is generally accepted that the dispersal of pollen within a colony of a given species is under the control of these agents and beyond the control of the colony. This view may be valid in the case of anemophilous plants where alterations of colony parameters ostensibly would have no effect upon the characteristics of pollen flow, since these parameters would not affect the transferal agent. However, in entomophilous plants, pollen and associated gene movements are mediated by vectors which may be sensitive to stimuli emanating from the colony. If pollinators respond to such stimuli, permutations of which would alter foraging behavior, the colony, wittingly or unwittingly, would control the dispersal of its pollen and genes.A recent study on pollinator flight patterns in Liatris aspera (Levin and Kerster, 1969) has shown that plant density and spacing control the feeding-flight behavior of bees and the movement of pollen which they bear. The present investigation was undertaken to determine whether the dependence of bee-flight characteristics and bee-mediated pollen dispersal upon plant spacing is a general phenomenon. We will show that bees are highly responsive to plant spacing and that bee-mediated pollen dispersal and associated gene dispersal is strongly correlated with plant spacing, the plant species and floral mechanism notwithstanding.Bee pollination flights were observed in colonies of nine species representing eight genera and four families ( Table 1). The species were chosen because of their divergent reproductive characters and because they were particularly attractive to bees. Pollinator-plant relationships were studied in at least three colonies of each species, the colonies being designated as sparse, intermediate and dense for each species. A total of 28 colonies was considered. In each colony, distances between consecutively visited plants were scored for a minimum of 125 flights. In some dense colonies, the inflorescences of adjacent plants often were as close together as those within a plant. Under such circumstances all flights within a colony were scored with the inflorescence rather than the plant as the target.Some difficulty was experienced in following small bees over long distances, and in some populations we failed to observe the terminus of a small percentage of the flights. In order to avoid bias against long flights, "lost" flights were included in our calculations at an arbitrarily assigned distance of 15 m. The number of such flights per colony is noted in Table 2.The primary pollinators in all of the colonies were leaf-cutter bees (Megachilinae) , honeybees (Apinae), carpenter bees (Xylopinae), and bumblebees (Bombinae). Pollinators were observed in at least three colonies of a species, the colonies being widely disparate in density and mean spacing. In each instance, the three colonies were located in the same area, and observations were completed during a...

Mating patterns and spatial distribution of conspecific neighbours in the Mediterranean shrub Myrtus communis (Myrtaceae)

“…This means that isolated plants received a smaller amount of xenogamous pollen than aggregated ones. In self-compatible and animal-pollinated species, the levels of cross pollination, and consequently the outcrossing rates, depend on the frequency of pollinator visitation and foraging behaviour (Levin and Kerster 1969;Kunin 1997;Franceschinelli and Bawa 2000). During its maximum flowering peak, myrtle is the most abundant and almost the only flowering species in the study sites.…”

How do visitation patterns vary among pollinators in relation to floral display and floral design in a generalist pollination system?

“…Regression coefficients for long-tongued bees and bumble-bees are not shown due to lack of significance (P >0.05) of the regression model tors (Schmid-Hempel and Speiser 1988;Thomson 1988;Klinkhamer and de Jong 1990;Eckhart 1991), and several studies (e.g. Levin and Kerster 1969;Schall 1978;Schmitt 1983;Feinsinger et al 1991;Kunin 1993) have illustrated the importance of the surrounding density of flowers for insect visitation, although some authors have reported no such effect (Bell 1985;Jennersten and Nilsson 1993). My data illustrate the clear importance of flower density in patches of J. fruticans for pollinator visitation.…”

Experimental manipulation of plant density and its effect on pollination and reproduction of two confamilial montane herbs

“…Conspecific density exerts one obvious influence through its effect on the services of animal pollinators. For example, density can affect the faunal composition of pollinators (Kwak 1987), the overall rate with which flowers are visited (Dreisig 1995;Kunin 1997aKunin , 1997b, the number of flowers visited on each individual plant before a pollinator leaves (Bateman 1956;van Treuren et al 1994;Karron et al 1995), and the distance moved to the next plant (Levin and Kerster 1969;Schaal 1978). All of these aspects of pollination service can influence plant reproductive success.…”