Variation in seed dispersal along an elevational gradient in Great Smoky Mountains National Park

Abstract

Seed dispersal by ants is ecologically important and geographically widespread as 20–50% of all herbaceous species in eastern deciduous forests are dispersed by ants, but we know little about how or why such interactions vary geographically. In this paper, we examined variation in seed dispersal by ants along an extensive elevational gradient (256–2025 m) in the Great Smoky Mountains National Park, USA. Specifically, we asked whether variation in ant community composition affected seed removals and seed dispersal distance of Trillium undulatum and Hexastylis arifolia, two common understory herbs found throughout the southern Appalachian Mountains. We also examined variation in myrmecochore abundance, specifically Trillium spp. and H. arifolia, along the same elevational gradient. Measures of ant community and climate variables strongly covaried with elevation, while Trillium species richness and abundance did not. We found that seed removals decreased with elevation, but seed dispersal distance did not depend on elevation. The most important variables predicting seed removals were average annual temperature and the abundance of Aphaenogaster rudis, both of which varied along the elevational gradient. Seed dispersal by ants did not depend on ant community composition, but was dominated by one species, A. rudis, which occurred at every site and removed the vast majority of all observed seeds in this study. Though the ant fauna in the Great Smoky Mountains National Park is diverse, dispersal of T. undulatum and H. arifolia, and likely other myrmecochores, is driven by one ant species, A. rudis.