Secondary seed dispersal by ants in a fragmented Neotropical savanna: Edge effects on ant-derived benefits to seedlings

In tropical habitats nearly 90% of the shrubs and trees bear fleshy fruits that attract vertebrates for seed dispersal. Thus most studies on seed dispersal of tropical species have hitherto focused on fruit consumption and seed deposition patterns generated by primary, vertebrate seed dispersers. However, large amounts of the fruit crop produced by tropical trees fall to the ground and are consumed by ground-dwelling ants. Data from the Atlantic rainforest and the “cerrado” savanna in Brazil show that ants markedly affect seed germination and fate, and influence patterns of seedling recruitment and survival in primarily vertebrate-dispersed species. We provide evidence that fragmentation of the cerrado savanna affects the outcome of such interactions. Potential benefits afforded by secondary dispersal by ants to a bird-dispersed shrub (Erythroxylum pelleterianum) collapse at the fragment border. Although overall seedling survival (1 yr) in the interior of the savanna was greater in nests than in control areas without nests, this benefit disappears at the edge of the fragment. Variation in temperature and humidity levels, differences in the composition of the ant fauna, and stability of ant nests, likely account for the contrasting patterns observed.  The positive effects from secondary dispersal by ants enhance the complex, two-phase nature of the dispersal ecology of tropical plants. This study highlights the vulnerability of such dispersal systems to anthropogenic disturbance. Cerrado fragmentation is likely to modify plant community structure and composition in the long run through disruption of important mutualisms. Conservation of biotic interactions should thus be an integral part of strategies to maintain viable cerrado communities.