Trait-based approaches for understanding plant species coexistence, community assembly, and responses to global change are complicated by the existence of intraspecific trait variation. The same coexistence mechanisms that operate among species may impact community dynamics by operating within species, but knowledge of the strength and scope of these impacts is currently limited. Which coexistence mechanisms are affected by intraspecific trait variation, and how big are those effects? We present the results of experiments and observations exploring the links between intraspecific variation in a key functional trait—seed size—and aspects of plant performance linked to coexistence: seed dispersal, fecundity, and seedling performance (competition-colonization and tolerance-fecundity tradeoffs), as well as susceptibility to density-responsive natural enemies (Janzen-Connell effects). For two bird-dispersed trees on Saipan in the Mariana Island chain, we used field seed rain collection to characterize size-dependent seed dispersal and we sowed seeds of known size in soil collected near or far from conspecifics to characterize size-dependent susceptibility to natural enemies.
Intraspecific variation in seed size caused large variation in different aspects of plant performance across the two species tested. Psycotria mariana seeds exhibited a size-mediated tradeoff between tolerance of natural enemies and dispersal to areas of low conspecific density, whereas Premna serratifolia exhibited a tradeoff between fecundity and seedling performance. Our results demonstrate that two performance tradeoffs that are thought to underlie coexistence among species occur within species as well. This work documents that density-responsive natural enemies—typically thought to be specialized on plant species—can be sensitive to plant traits, and that a single trait can mediate distinct coexistence mechanisms in co-occurring species. These results enable trait-based predictions for the impact of defaunation, suggesting negative fitness consequences for small seeds following seed disperser extirpation.