Fecundity and Seed Dispersal of a Tropical Tree

Abstract

We observed feeding assemblages of birds at a Costa Rican dry—forest population of Casearia corymbosa (Flacourtiaceae) in order to distinguish effective dispersers from a incidental visitors and to examine the relationship between fruit production and depletion by birds at individual plants. On the basis of 1956 recorded visits by birds, the Yellow—green Vireo (Vireo flavoviridis) was the most reliable disperser throughout the season, accounting for 65% of the arillate seeds removed. Other common visitors were the Streaked Flycatcher (Myiodynastes maculatus) 12%, Golden—fronted Woodpecker (Melanerpes aurifrons) 9%, Pale—throated Flycatcher (Myiarchus nuttingi) 8%. All stripped off the oily arils in the crop and regurgitated viable seeds from perches in surrounding forest. Sporadic visitation by 10 other trogons and passerines accounted for the remainder of fruit utilized by birds. Capsules opened early in the morning, and fruit accessible to birds was depleted by early afternoon. At 17 closely monitored plants, birds removed 91% of the accessible arillate seeds. Crop sizes ranged from 1—2700, with dispersal success highest among plants of intermediate fecundity. Variance was attributable to individuals with low fecundity, often missed by birds on all but the most productive days, and to an exceptionally fecund tree that satiated its visitors. The dispersal ecology of the small dry—forest tree offers a unique comparison to that of a more fecund overstory ecotype in wet forest. In secondary dry forest, a protocooperative relationship exists between several bird species and this plant, with an abundant and efficient vireo responsible for most dispersal.In the wet forest, dispersal occurs through a mutualistic relationship with a cotinga (Tityra semifasciata), while most common visitors are "fruit thieves" of little benefit to the plant. These differences are considered in light of comparative ecology of the 2 forms.