Monkey Dispersal and Waste of a Neotropical Fruit

Abstract

Foraging birds and mammals were observed at a Panamanian rain forest tree Tetragastris panamensis (Burseraceae) in order to determine (1) relative importance of different dispersal agents and (2) the relationship between fruit production and dispersal. The system is of interest because fruits provide little but sugar, fruits are not depleted by animals (i.e., are superabundant), and the tree is visited by a variety of vertebrates with different effects on seed dissemination. Three mammals (Alouatta palliata, Cebus capucinus, Nasua narica) account for 97% of all seeds removed from the trees; the howler monkey (Alouatta) is alone responsible for 74% of the seeds removed. Seeds from the feces of these mammals are fully viable. In addition, two diurnal mammals, three nocturnal mammals, and nine birds ingest and disperse small numbers of seeds, but these species are numerically insignificant. Two parrots (Amazona farinosa and A. autumnalis) are important fruit thieves and seed predators, while three mammals and three additional birds kill or drop small numbers of seeds near the parent trees. Assemblage richness increases markedly with crop size, but most of 25 visitors are of little significance to the plant. Use by three monkeys closely reflects their relative abundance; the abundance of the coatimundi (Nasua) is unknown. This system is characterized by enormous seed waste. Of >430,000 seeds produced by 19 study trees in 1978, <4% had even a remote chance of establishment. The remainder were killed by parrots (6%), were dropped or spontaneously fell under parent trees (66%), or were doomed to intense competition in fecal clumps (24%). The three primary dispersal agents indirectly caused most seed and seedling mortality of dropping seeds under the tree crown or leaving them in fecal clumps, precluding survival of any but a small fraction of seedlings. Crop size ranged from 165 to 100,000 arillate seeds per season, but fruits appeared superabundant on all trees. Numbers of seeds dispersed increases dramatically with crop size, but the proportion dispersed was highest at plants with crops of intermediate size. The proportion of seeds killed by parrots increased with crop size. Disproportionate mortality of seedlings in high densities near adult trees indicated density—dependent mortality among juveniles. Wide geographical ranges of this tree and members of the frugivore assemblage, use of other food resources by common foragers, richness of the assemblage, variation in species visitation at individual trees, irregular annual fruits production, and obvious "inefficiecy" of the dominant dispersal agents from the perspective of the plant suggest that Tetragastris has a generalized dispersal strategy and is not closely coevolved with particular frugivores. A generalized dispersal system is expected from a tree, such as this one, that produces sugary fruit incapable of providing a balanced diet.