Predispersal seed predation is higher in a rare species than in its widespread sympatric congeners (Astragalus, Fabaceae).
Rare plants may be rare, in part, because they are more susceptible to damage by predispersal insect seed predators than widespread congeneric species; thus, seed predation may be an important determinant of plant rarity. Scant evidence exists to either support or reject this hypothesis, limiting our ability to predict herbivore effects on plant rarity. We used a comparative framework to test this hypothesis in rare-common, sympatric congeners. We compared seed consumption by insect type and seed production among a rare Astragalus (Fabaceae) species and two sympatric, widespread congeners. We measured plant traits and tested whether traits were correlated with seed damage within and among rare-common species. Seed predation was significantly higher in a rare species than in common congeners over 2 yr. Seed production per pod was significantly lower in the rare species. Seed weevils exclusively consumed seeds of the rare species. Higher seed predation in the rare species is related to a combination of factors: plant phenology, dispersal ability, and vigor; seed predator identity; and insect phenology. Our results support the hypothesis that a rare plant is more susceptible to seed predators than two common, sympatric congeners. Seed predator reduction is a promising management tool to increase seed-set, recruitment, and survival of the rare species. Further studies that compare rare-common, sympatric congeners are greatly needed to broadly evaluate the hypothesis concerning rarity and susceptibility to seed predators and to inform conservation plans for rare species.