Flowering phenology, pollinator competition, and species coexistence in a flowering plant community

Theory predicts that the division of resources facilitates species coexistence. Recent meta-analyses indicate that pollen limitation is a common phenomenon in angiosperms. Furthermore, empirical studies have demonstrated that plant species that co-occur in space and time are likely to compete for pollinators and experience pollen limitation. In addition, competition for pollinators should be most pronounced among close relatives. Phenological shifts and pollinator specialization are two means by which species can avoid the negative consequences of pollinator competition. California vernal pool communities frequently contain diverse assemblages of endemic flowering plants, including some close relatives. Species in the pools are often spatially restricted to a certain pool depth (elevation). A high incidence of self-incompatibility may enhance pollen limitation and pollinator competition among vernal pool plants. Here we use data gathered from flowering plant species occupying vernal pools in the Jepson Prairie Reserve to determine the degree to which species coexistence in space and time is facilitated by differences in flowering time, pool depth, and phylogenetic relatedness. Our results confirm the prediction that species occupying the same pool are likely to be more distantly related than by chance, and that species that occupy the same flowering time niche are less likely to occur together in a pool. Furthermore, species that occupy a similar pool depth tend not to flower at the same time, suggesting that competition may either prevent the establishment of co-flowering species, or drive character displacement.