EFFECTS OF POSTDISPERSAL SEED PREDATION ON SPATIAL INEQUALITY AND SIZE VARIABILITY IN AN ANNUAL PLANT,ERODIUM CICUTARIUM (GERANIACEAE)

By decreasing seed density, ants introduced into flats of uniformly sown seeds of Erodium cicutarium (Geraniaceae) created differences in the neighbor-free area available to individual plants. The changes in spatial patterns brought about by the ants were greater when a higher proportion of seeds was removed but were independent of initial seed density. These spatial changes and differences in seed density were examined for their effects on plant size and reproduction. Gini values were calculated to determine inequalities. As the inequality in space among individual plants increased, the variation in final biomass increased. The number of individuals reproducing was constant among treatments, and yet seed production per plant was significantly greater for populations in which the spatial pattern was influenced by seed predation. The decrease in density and changed spatial pattern, due to previous seed predation, resulted in a few individuals having much more space than others and consequently producing many more seeds. The increase in reproductive effort per flat was much greater than could be explained by the changing density alone. Our experiment demonstrates that spatial inequality, such as that generated by seed predators, can be more important than density in generating size inequalities in plant populations. This result can profoundly alter the competitive interactions between plants and determine which plants produce seed for the next generation.