Overall seed dispersal effectiveness is lower in endemic Trillium species than in their widespread congeners

Comparing ecological attributes of endemic species with related, widespread species can reveal differences accounting for rarity. Forests of the southeastern United States are home to many range-restricted endemic and widespread species of Trillium, a genus of ant-dispersed herbs. Evidence suggests that aspects of seed-related life history stages are often correlated with plant rarity, but few studies have tested whether the process of seed dispersal differs for endemic and widespread species. To address this question, we compared aspects of seed dispersal effectiveness (SDE) for three sympatric, widespread endemic Trillium species pairs.

We observed seed dispersal for Trillium species pairs by ants at eight sites, recorded numbers of seeds dispersed and dispersal distances, and described disperser interactions. To test disperser preference, we presented seeds of each pair to captive colonies of Aphaenogaster picea, a keystone disperser. Seeds were assigned scores based on worker behavior, and we recorded proportions of seeds dispersed after 1 h and 24 h.

Field observations yielded some significant within-pair differences. Ants dispersed more seeds of widespread species for all pairs, although dispersal distances did not differ. In laboratory experiments, after 24 h, ants dispersed more seeds of widespread species into nests.

Endemic Trillium species had lower overall SDE than did their widespread congeners. These findings add to the list of ecological and demographic challenges that face endemic plants when compared to common congeners. Lower SDE may negatively impact reproductive rates and the colonization of new habitats, which may contribute to patterns of endemism.