Seed Predation and Seed Number in Scheelea Palm Fruits

Two Scheelea palm populations in Central America were studied: Scheelea rostrata in Costa Rica and Scheelea zonensis in the Panama Canal Zone. Most of the fruits in both populations contained one seed, but small proportions of the fruits on each tree contained two or three seeds. Compared to single—seeded fruits, seedlings from multiseeded fruits are apparently disadvantaged because of less endosperm and competition among seedlings emerging from the same indehiscent fruit. However, since seed predation of Scheelea is heavy, we hypothesized that differential seed predation may counteract this selection pressure against the production of multiseeded fruits and thus maintain multiseeded fruits in Scheelea populations. At least 61% of the fruits in the Costa Rica population and 81% of the fruits in the Canal Zone population were attacked by seed predators. Beetles (Family: Bruchidae) were the predominant predators in the Costa Rica population (84% of attacked fruits), whereas rodents predominated in the Canal Zone population (78%of attacked fruits). Attack by rodents was independent of seed number, but data from Costa Rica suggested that bruchids concentrated predation slightly on one—seeded fruits. Because seed predators were often restricted to one seed, seed survivorship in attacked fruits was directly related to seed number (0%, 22%, and 41% for attacked 1—, 2—, and 3—seeded fruits, respectively, in the Costa Rica population). As a result of an interaction of these aspects of predation, the proportion of seeds that escaped predation was also directly related to seed number (39%, 54%, and 60% for 1—, 2—, and 3—seeded fruits, respectively, in the Costa Rica population). The major difference in predation between the two populations appeared to be the predominance of bruchid predation in the Costa Rica population, which allowed a greater survivorship in attacked multiseeded fruits (29% survivorship of bruchid attack versus 12% survivorship of rodent attack). This apparent stronger selection pressure for producing multiseeded fruits coincided with a significantly greater proportion of multiseeded fruits in the Costa Rica population (27%) compared to the Canal Zone population (6%). These findings support the hypothesis that seed predation is a significant factor in maintaining multiseeded fruits in Scheelea populations.