Nonredundancy in the dispersal network of a generalist tropical forest tree

Plant species with generalized dispersal mutualisms are considered to be robust to local frugivore extinctions because of redundancy between dispersal agents. However, real redundancy can only occur if frugivores have similar foraging and ranging patterns and if fruit is a limiting resource. We evaluated the quantitative and qualitative contributions of seed dispersers for an endochorus mast-fruiting species, Prunus javanica (Rosaceae) in Khao Yai National Park, Thailand, to evaluate the potential redundancy of dispersers. Data were collected from tree watches, seed/fruit traps, and seed transects under and away from fruiting trees, feeding and seed deposition by gibbons (Hylobates lar), and evaluations of seed and first-year seedling survival. We identified three clusters of dispersers within the network. Most (>80%) frugivore species observed were small birds and squirrels that were not functional dispersers, dropping most seeds under or very near the tree crown, where seedling survival was ultimately nil. Monkeys (Macaca leonina) were low-quality, short-range dispersers, but they dispersed large numbers of seeds and were responsible for 67% of surviving first-year seedlings. Gibbons and Oriental Pied Hornbills (Anthracoceros albirostris) handled few fruits, but they provided the highest quality service by carrying most seeds away from the canopy to medium and long distances, respectively. Although there was overlap in the deposition patterns of the functional dispersers, they displayed complementary, rather than redundant, roles in seed dispersal. Satiation of all functional dispersers further limited their capacity to “replace” one another. Redundancy must be evaluated at the community level because each type of disperser may shift to different species in the non-masting years of P. javanica. Our results underscore the need for research on broader spatial and temporal scales, which combines studies of dispersal and plant recruitment, to better understand mechanisms that maintain network stability.

Kim R. McConkey & Warren Y. Brockelman
Mo Singto