The relevance of ants as seed rescuers of a primarily bird-dispersed tree in the Neotropical cerrado savanna

The scale at which seed dispersal operates has many implications for the spatial patterns of plant recruitment and diversity. We investigated the effect of short- (ants) and long-distance (birds) seed dispersal of the fleshy-fruited melastome, Miconia rubiginosa, in the Brazilian savanna. We estimated the contribution of dispersal vectors to the removal of the fruit crop from the canopy (birds), and once seeds have reached the cerrado floor (ants) over two fruiting seasons. Birds (13 species) removed up to 23.7% of the fruit crop from the crown, but dropped a substantial proportion of fruits beneath the parent plant. Birds removed a greater proportion of fruits from trees producing large fruit crops, as predicted by the fruit crop size hypothesis. However, up to 18.9% of the fruit crop fell beneath the parent plant as ripe fruit. Most fallen fruits were removed by ants (seven genera), which are likely to play a relatively important role in terms of the quantity of seeds dispersed, especially for plants producing small fruit crops (a conceptual model is presented). Birds and ants did not influence seed germination, but they differ in terms of the spatial scale of dispersal and deposition patterns. Ants probably play an important role in the local population dynamics of Miconia, whereas birds are responsible for long-distance dispersal associated with the colonization of new patches and metapopulation dynamics. By removing seeds from bird droppings, ants may also reshape at a finer scale the seed rain generated by primary dispersers. Indeed, seedlings and saplings of Miconia are more frequently found around leaf-cutter ant nests than in control areas away from ant nests or around large Miconia trees. The quantitative component of dispersal effectiveness by ants acting as rescuers of seeds that fail to be dispersed, or fall under parent trees, is probably more important than currently recognized in other systems.