Consequences of pollination neighborhood composition on mating success
Individuals of many species – including most animals, many fungi, and more than half of angiosperm plants – must find mates of the opposite sex, mating type, or self-incompatibility type to successfully produce offspring. The absolute number and relative frequencies of each type within a mating neighborhood may therefore alter the rate of mating success with important consequences for the population dynamics of these species, but how mating neighborhood composition affects mating success has gone largely untested especially in natural populations. We sought to determine the effect of mating neighborhood composition on seed set in the dioecious plant Valeriana edulis, and to understand to what degree these effects depend on sex ratio-dependent versus sex ratio-independent factors. We determined the size of the mating neighborhood by following a pollen analogue dispersed by insect pollinators. Within this mating neighborhood, we tagged all open flowers on focal females and measured the distance from the focal female to each open conspecific flower. With these data we compared candidate models to assess the ability of sex-explicit and sex-blind indices of mating neighborhood composition to predict seed set on focal females.
Our results indicate that the mating success of Valeriana edulis is limited by both sex ratio-dependent and sex ratio-independent factors. The pollen analogue was most effectively transferred to the closest plants by pollinators and decayed sharply with distance. Fitting a conservative fat-tailed distribution to the data revealed that approximately 50% of pollen transfer occurs within 2 m, with <10% of pollen traveling more than 12 m. The best model explaining focal female seed set included both an index of neighborhood pollen availability (sex ratio-dependent) and total conspecific plant density (sex ratio-independent), outcompeting all other neighborhood composition measures singly and in combination. Increasing pollen availability led to increased seed set, and density had a negative effect on the number of seeds produced. The positive effect of pollen availability was stronger than that of density across the range of variation of these measures we observed (12- and 13-fold, respectively) and both effects are consistent with the only previous study relating mating environment to reproductive success. Our study indicates a need for more robust measures of mating environment than sex-dependent factors alone to accurately describe mating potential in dioecious species.