Birds and ants provide complementary seed dispersal in a neotropical savanna

Abstract

A substantial portion of the crop of fruiting trees falls beneath parent plants as a result of dispersal failure. Such diaspores are considered as waste because the likelihood of plant recruitment is usually very small close to parent trees. However, many animals may rescue fallen diaspores and provide them with another chance of dispersal and establishment. We investigated the effectiveness of two broad types of seed dispersal vectors for the regeneration of Xylopia aromatica in the Brazilian cerrado savanna: birds that remove diaspores from plant canopies and ants that harvest diaspores on the ground under the parent plant (as rescuers) or from bird feces (as secondary dispersers). Birds removed a mean of 32% of the crop from plant canopies, but removal was independent of crop size. A large part of the crop (mean of 25%) landed beneath parent plants or was dropped after manipulation by vertebrate frugivores as viable diaspores. Ants from at least five genera removed most fallen diaspores (up to 83%) within 24 h. Ants influenced the fate of a large amount of the crop, and for some trees ants removed as many diaspores as birds. Large ants rescued some diaspores to distances beyond the parent plant crown, but birds may remove diaspores 40-fold farther. However, seedlings of X. aromatica were only found close to nests of large ants, probably due to diaspore rescuing and/or directed secondary dispersal by certain ant groups following primary dispersal by birds. Although an unknown percentage of seeds was lost to granivorous ants, diaspore removal by ants potentially enhances the likelihood of plant recruitment due to distance-related benefits and directed dispersal to ant nests, while birds play a premier role in long-distance seed dispersal and metapopulation dynamics. Synthesis. Birds and ants provide complementary seed dispersal at different spatial scales to X. aromatica. Since ants remove most fallen diaspores beneath parent plants, the use of diaspore removal rates from plant canopy as a surrogate of plant fitness may be misleading. By acting as secondary dispersers, ants may also provide a fine-tuned dispersal following long-distance dispersal by birds (i.e. diplochory). © 2010 The Authors. Journal compilation © 2010 British Ecological Society.