Spatial Patterns of Seed Dispersal and Predation of Two Myrmecochorous Forest Herbs

Abstract

Hepatica acutiloba and Trillium nivale are spring—flowering deciduous forest herbs whose seeds are dispersed by ants. We studied patterns of seed dispersal by ants and seed predation by rodents of these two herbs at three spatial scales: (1) among three woodlots that differed in density and size of Hepatica and/or Trillium populations, (2) between two sites within each woodlot located inside and outside a natural population, and (3) between replicate quadrats within each site. We estimated ant numbers and rodent presence using baits. In contrast to other studies done on other ant—dispersed species, seed predation by rodents was rare or nonexistent for both species even though rodents were present at all sites. Seed removal rates by ants were quite patchy at the scale of a few square metres within sites, but generally were slower where seed populations were dense and extensive. This indicates that ants become satiated during and immediately after seeds are released from parent plants growing in dense populations. Hence, seed dispersal is often ant—limited within well established populations of myrmecochores. Edaphic—topographic factors also play an important indirect role in seed dispersal through their effect on local ant numbers. The evolutionary role of seed predation on myrmecochory, and the demographic implications of density—dependent seed dispersal are discussed.