How do birds and ants contribute to the recruitment of a tropical tree?

In most tropical ecosystems, birds play a crucial role as primary dispersers (Phase 1) by removing seeds from the plant crown. Following primary dispersal, ground-dwelling animals, notably ants, often disperse seeds secondarily (Phase 2). However, the relative contributions of each phase to seed dispersal effectiveness until plant establishment remains unclear. We combined observational and manipulative experiments to quantify the contributions of birds and ants to seed dispersal effectiveness of the ornithochoric tree Xylopia sericea in the Brazilian Atlantic Rainforest. Birds dispersed 17.2% of diaspores, with a dispersal distance (6.6 m ± 6.7 m) twenty times greater than that of ants (0.3 m ± 0.5 m). Notably, birds often dispersed seeds beyond the parental canopy, where seedling survival is higher. Ants were quantitatively more important to seed dispersal, with relative diaspore removal 221.3% higher than birds. Despite the limited dispersal-distance by ants, proximity to ant nests increased seedling survival. Phase 1 led to the establishment of 16.26 seedlings per 100 diaspores produced, and Phase 2 increased the number of seedlings established by 28.6% (N = 20.91). Ants enhanced seed recruitment and improved seedling survival both away and near the parental plant, likely due to ant nests providing favorable microsites for establishment. Conversely, plants distant from ant nests performed better when away from the parental plant, underscoring the importance of birds in seed dispersal. Birds promote long-distance dispersal and ants redistribute diaspores to microsites that increases establishment, so the dispersal of X. sericea is highly dependent on both dispersers.

Abstract in Portuguese is available with online material.