The geographic distribution of seed-dispersal mutualisms in North America

Abstract

It is often difficult to determine the relative importance of different sorts of species interactions in shaping community structure or how communities function because we lack basic information on patterns of occurrence of biotic interactions. Here we determine the geographic distribution of seed-dispersal mutualisms across North America, and then test the hypotheses that those patterns are correlated with mean annual precipitation and latitude. We analyze the floras of 197 sites across North America to identify which species of native seed plants are dispersed by animals in one of three types of plant-animal mutualisms: frugivory, scatter hoarding, and ants. We identified 1655 plant species that are involved in these seed-dispersal mutualisms. 16.5 ± 6.5% of all seed plants across North America are dispersed by animals; 10.0 ± 4.2% by frugivores, 3.7 ± 1.7% by scatter hoarders, and 3.9 ± 2.2% by ants. Secondary dispersal by a different mode and vector (e.g., wind dispersal followed by scatter hoarding, or ballistic dispersal followed by myrmecochory) occurred in 16.8% of all plant mutualist records. Although each mode of dispersal showed a different pattern, the prevalence of seed-dispersal mutualisms increased with mean annual precipitation. The prevalence of seed dispersal by frugivory and scatter-hoarding decreased with increasing latitude. The prevalence of seed-dispersal interactions varies dramatically across North America. The center of greatest diversity for all three types of seed-dispersal mutualisms is the eastern United States, roughly coincident with the eastern deciduous forest. Knowing the distribution of species interactions improves our understanding of how the structure and functioning of communities varies across environmental gradients.