Importance of Primary and Secondary Seed Dispersal in the Malagasy Treecommiphora Guillaumini

We studied the dispersal system of the tree Commiphora guillaumini (Burseraceae) in a dry deciduous forest of western Madagascar by measuring tree visitation, fruit consumption, and seed dispersal rates of primary dispersers and the modification of the primary seed shadow by secondary dispersers. The seed dispersal system of C. guillaumini is unusually simple because primary seed dispersal and secondary seed dispersal are each effectively carried out by a single species: primary dispersal by the Lesser Vasa Parrot, Coracopsis nigra, and secondary dispersal by the ant Aphaenogaster swammerdami. The quality of seed dispersal by each disperser was measured by comparing seed distributions with seedling distributions. Although parrots dispersed only 7.9 ± 6.0% (mean ± 1 sd) of the seeds away from the crown, these seeds had a probability of becoming established as seedlings that was 35.4 times higher than that of seeds that were dropped beneath the crown or that simply fell from the crown. Ants secondarily dispersed 45.1 ± 23.2% of the seeds. However, ants increased the establishment success of seedlings more weakly and less consistently than did parrots. Establishment success of seeds dispersed by ants was 4.5 times higher in one year but only 1.1 times higher in another year than that of seeds not dispersed by ants. Although secondary seed dispersal by ants played an important role in the redistribution of seeds and affected the establishment of seedlings, the importance of primary dispersers exceeded that of secondary dispersers by an order of magnitude.