Influence of local density and sex ratio on pollination in an ambophilous flowering plant

PREMISE OF THE STUDY: Variation in local density and sex ratio in
dioecious plants can affect mating success through the actions of pollen
vectors, principally generalist insects or wind. Increased density and
male-biased sex ratios should promote pollen transfer and seed production,
but their combined effects have not been investigated for ambophilous
species, which exhibit both insect and wind pollination. METHODS: We
manipulated density (low vs. high) and sex ratio (1:1 vs. 3:1 male-biased)
in arrays of dioecious ambophilous Thalictrum pubescens. We quantified
visitation rates and foraging time to examine whether pollinators
exhibited sex-specific preferences and determined the seed set of
arrays.        KEY RESULTS: Pollinators visited more plants per foraging bout at high than low density. Visitation rates and foraging times of visitors were greater for male than female plants but did not depend on the density or sex ratio of arrays. However, whereas solitary bees displayed a strong preference for males, hover flies were indifferent to plant sex phenotype. Solitary bees also visited significantly more plants per foraging bout than hover flies. There was a significant interaction between density and sex ratio on seed set. At low density, seed set was greater for 1:1 than 3:1 arrays, but at high density the opposite pattern occurred.   CONCLUSIONS: The demographic factors we investigated had complex influences on pollinator foraging behavior and patterns of seed set. Several factors may explain our results including the influence of density and sex ratio on pollen export from arrays, grooming by pollinators and the contribution of wind pollination.