Does spatial population structure affect seed set in pollen-limited Thymus capitatus?

Plant population size has been shown to affect insect visitation and reproductive success. Small populations are at risk because individuals are more likely to be affected by stochastic processes and inbreeding depression (Allee effect). Additionally, several studies have found that plants in small populations also experience lower pollinator visitation rates, which may further decrease reproduction. In this study, seed set, pollinator visitation and pollen limitation of Thymus capitatus (L.) was assessed in 32 patches in eight populations of various sizes on the island of Lesvos, Greece. All populations except one were significantly pollen-limited. We found that although free-pollinated flowers produced more seeds in larger populations this was not due to higher pollinator visitation rates as flowers which received pollen supplements also produced more seeds in larger populations. We hypothesize that the higher seed set is due to a generally greater genetic variability or better habitat quality. We show that honeybee visitation alone significantly decreases pollen limitation.