Population Size, Pollinator Limitation, and Seed Set in the Self‐ Incompatible Herb Lythrum Salicaria

In reward—producing animal—pollinated plants, small populations are likely to be less attractive to pollinators than large populations. The likelihood of pollinator limitation of seed production should therefore increase with decreasing population size. I documented the number of flowering plants and plant fecundity in 18 populations of the self—incompatible, tristylous herb Lythrum salicaria within an archipelago in northern Sweden in two consecutive years. To test the hypothesis that a positive correlation between population size and seed set is due to a higher degree of pollinator limitation in small than in large populations, I performed supplemental hand—pollinations in eight (1993) and 13 (1994) of the study populations. To test the hypothesis that common mating types are more likely than rare types to experience inadequate pollination, I compared the natural level of seed production and the effect of supplemental pollination in different style morphs in the five populations in which $\geq 10$ plants per morph were included in the experiment. There was no significant correlation between population size and plant size in terms of number of floral shoots or number of flower—producing leaf nodes per shoot. However, there was a positive relationship between population size and seed production per flower and between population size and total seed number per plant. In contrast, there was no significant correlation between population size and seed production of flowers that had received supplemental pollination. In both years, the difference in mean seed production per flower between hand—pollinated flowers and controls decreased with increasing population size. In two of five populations, the effect of supplemental pollination differed significantly among morphs. Seed production was more likely to be pollinator limited in long—styled than in short—styled plants, but this difference could not be attributed to a preponderance of the long—styled morph in the studied populations. Results of the supplemental hand—pollinations indicate that the positive correlation between population size and seed production is a function of insufficient pollen transfer in small populations. Additional demographic studies are needed to determine to what extent the reduced level of seed production in small populations limits the growth of young populations, and to what extent it may threaten the local persistence of L. salicaria.