Pollinator visitation, stigmatic pollen loads and among‐population variation in seed set in Lythrum salicaria

Abstract

Summary Small populations of reward‐producing plants are likely to be less attractive to animal pollinators than large populations. As a result, both the quantity and the proportion of compatible pollen deposited on receptive stigmas, and seed output per plant, may be lower in small than in large populations. We examined whether pollinator visitation, pollen deposition and seed set varied with population size in the self‐incompatible, tristylous herb Lythrum salicaria, in the Skeppsvik archipelago, northern Sweden. We documented both the number of compatible and incompatible conspecific and heterospecific pollen grains received per flower, seed set and degree of pollen limitation of long‐styled plants in 14 populations of different size in two consecutive years, and recorded the visitation rate to individual plants and the number of flowers visited per plant in eight of the populations. As predicted, the visitation rate tended to increase, while the number of flowers visited per plant tended to decrease with increasing population size. However, visitation rates were low overall and temporally highly variable, and these relationships only approached statistical significance. The proportion and absolute number of compatible pollen grains received increased with population size, while the total amounts of conspecific and heterospecific pollen grains received did not vary significantly with population size. The results of supplemental hand‐pollinations indicated that the among‐population variation in seed set was due to insufficient transfer of compatible pollen in small populations. Seed output increased with the receipt of compatible pollen grains up to about 200 compatible pollen grains received per flower. Between 73% and 98% of the L. salicaria pollen grains received were incompatible, and between 9% and 81% of the pollen grains deposited were heterospecific (population means). However, there was no evidence that the deposition of high numbers of incompatible conspecific and heterospecific pollen grains reduced seed set. In the study populations of L. salicaria, variation in seed output and pollen limitation are apparently governed primarily by factors influencing the transfer of compatible pollen. The results are consistent with the hypothesis that a reduction in the efficiency of pollen transfer among compatible mating types may markedly reduce the reproductive output in small populations of self‐incompatible plants. In light of the current rapid transformation and fragmentation of habitats, there is a pressing need both to clarify how the pollination success of plants with different pollination systems is affected by large‐scale changes in population size, density and isolation, and to determine the demographic consequences of differences in pollination intensity.