Pollen limitation in small populations of the self-incompatible plant, Hymenoxys herbacea
Fecundity of self-incompatible plants is expected to be limited by pollination (pollen limitation) in small populations due to stochastic loss of mating types (genetic drift) and insufficient pollinator visitation (Allee effect). I tested these predictions in the rare, insect-pollinated, self-incompatible plant, 'Hymenoxys herbacea'. First, I estimated the magnitude of genetic drift acting on populations by measuring effective size (N e). Second, I estimated the strength of pollen limitation, pollinator visitation rates and mate diversity in populations of different size. Effective size (Ne), estimated using a demographic model, ranged from 57 to 56,476,700, averaging 43% of the census population size. In most populations, drift was not sufficiently strong to affect the diversity of mating types. Percentage seed set varied widely among the 13 populations examined, from 27.5% to 66.2%: however, seed production was pollen limited in only one case. Mate diversity varied among populations and was positively correlated with population size, as theory predicts; however, it did not account for variation in pollen limitation. Contrary to expectations, pollinator visitation rates declined with population size and were not correlated with pollen limitation. Overall, these results suggest that population size impacts mate diversity and pollinator behavior but neither effect was strong enough to influence reproduction in 'H. berbacea'.