POPULATION-SIZE, POLLINATOR LIMITATION, AND SEED SET IN THE SELF-INCOMPATIBLE HERB LYTHRUM-SALICARIA

In reward-producing animal-pollinated plants, small populations are li kely to be less attractive to pollinators than large populations. The likelihood of pollinator limitation of seed production should therefor e increase with decreasing population size. I documented the number of flowering plants and plant fecundity in 18 populations of the self-in compatible, 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 popul ations, I performed supplemental hand-pollinations in eight (1993) and 13 (1994) of the study populations. To test the hypothesis that commo n mating types are more likely than rare types to experience inadequat e pollination, I compared the natural level of seed production and the effect of supplemental pollination in different style morphs in the f ive populations in which greater than or equal to 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 sh oots or number of flower-producing leaf nodes per shoot. However, ther e was a positive relationship between population size and seed product ion per flower and between population size and total seed number per p lant. In contrast, there was no significant correlation between popula tion size and seed production of flowers that had received supplementa l pollination. In both years, the difference in mean seed production p er 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 pro duction was more likely to be pollinator limited in long-styled than i n short-styled plants, but this difference could not be attributed to a preponderance of the long-styled morph in the studied populations. R esults of the supplemental hand-pollinations indicate that the positiv e correlation between population size and seed production is a functio n of insufficient pollen transfer in small populations. Additional dem ographic studies are needed to determine to what extent the reduced le vel 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.