Dispersal success of a specialized tropical tree depends on complex interactions among diverse mammalian frugivores
The study of seed dispersal in tropical forest communities is complicated by the high diversity of frugivores and the complex interactions among species and their environments. Determining which species are effective dispersers and which are opportunists with neutral or even negative effects on fruiting plants is a major problem which requires detailed studies focused on particular plant species. In this study we focused on seed dispersal of the wild rambutan (Nephelium melliferum) which supplies energy-rich fruits to primates and other mammals in a seasonal evergreen forest in central Thailand. We hypothesized that gibbons (Hylobates lar) were the most important dispersers and were capable of carrying and defecating seeds away from the tree crown where seeds or seedlings could escape increased distance or density-dependent mortality. We determined the seed dispersal effectiveness of all major arboreal consumers of Nephelium fruit (gibbons, pig-tail monkeys, squirrels) by using data on fruit production and consumption from fruit/seed traps under the canopies of eight sample trees, observations on animals feeding in fruiting trees, and seed deposition data at varying distances from tree crowns. We assessed the survival of seeds and seedlings in relation to distance from the crown by experiments and transect counts. Camera traps were used to detect consumption of seeds on the ground by terrestrial consumers. About half of the ripe fruit crop was harvested by squirrels which gnawed off the outer husk, ate the pulp and dropped intact seeds to the ground. Monkeys chewed and sucked off the pulp and dropped most seeds to the ground not far from the tree. Seeds and intact fruits dropped by squirrels and monkeys supported a large variety of terrestrial mammals most of which were seed predators. Gibbons, which swallow the seed with pulp attached, consumed only 16 % of the fruit crop but were the most effective and reliable seed dispersers overall, and appear to be the most specialized seed dispersal mutualist. Distance-dependent mortality was found in first-year seedlings, as well as evidence that further mortality must space out older saplings prior to recruitment into the tree population, which validates the importance of dispersal by gibbons. However, the great majority of fruits of Nephelium melliferum were consumed by mammalian opportunists and seed predators which interacted with one another in ways which depend on composition of the local community.