This study will quantitatively examine the contribution to seed dispersal and seedling establishment of understory shrub species by fruit-eating birds in a Costa Rican wet lowland forest. Detailed data will be collected on the distribution, fruiting phenology and seed germination and seedling establishment requirements of two species of shrubby Melastomatacea and of Rubiaceae, both dominant families in the understory at the site. These data will be combined with dietary and behavioral data of three species of understory frugivorous birds into a model to quantify seedling establishment of each plant species as a result of dispersal by each bird species. Populations of the four shrub species will be studied using standard measurement and phenology methods, and also through quantification of light environments by hemispherical canopy photographs and field germination and seedling survival trials. Bird populations will be studied by direct observation of color-banded birds, fecal sample analysis, and radio-tracking individuals of each species throughout the year. The combination of plant and bird data into a mathematical model represents the first time that this approach has been applied to a lowland understory system. Because of the difficulty or impossibility of following the fates of individual seeds dispersed from an understory plant, this model will provide a powerful tool for quantifying interactions between understory plants and understory frugivorous birds, a species rich, but little understood system .
