How does avian seed dispersal shape the structure of early successional tropical forests

Frugivores shape plant communities via seed dispersal of fleshy-fruited plant species. However, the structural characteristics that frugivores impart to plant communities are little understood. Evaluating how frugivores structure plant communities via the nonproportional use of available fruit resources is critical to understand the functioning of ecosystems where fleshy-fruited plant species are dominant, such as tropical forests. We performed a seed-addition field experiment to investigate how frugivorous birds shape the composition and richness of forests during early stages of secondary succession in cleared areas in Puerto Rico. The experiment tested whether the bird-generated seed rain and the subsequent early successional plant communities were proportional representations of the fleshy-fruited species that dominated the surrounding community. Experimental treatments consisted of patches with (a) seed-additions by wild birds attracted to experimental patches with pole perches, (b) manual seed-additions proportional to fruit abundance at the local scale (50m from experimental plots) and (c) manual seed-additions proportional to fruit abundance at the landscape scale (entire study site). Birds' seed-additions differed in composition and abundance to expectations based on fruit availability at local and landscape scales. Treatments with seeds added by birds had the highest species richness in both the seed rain and the emergence stages despite how, on average, the monthly richness in the landscape-scale treatment was double that of birds and the local-scale treatment. This phenomenon was explained by the highest heterogeneity from the bird seed-addition treatment across months, and the lowest seed per capita emergence rates in landscape treatments. Rather than reflecting relative fruit abundance, birds biased seed rain and per capita emergence towards a non-random mixture of both small- and large-seeded species, resulting in richer and distinct plant communities. Because frugivory and seed dispersal patterns depart from random encounters between frugivores and plants in communities, successional forests are characterized by an overrepresentation of proportionally rare plant species, and decreases in the dominance of many common species. Thus, for regenerating tropical forests, frugivory can function as mechanism that promotes persistence of rare plant species and their coexistence with more abundant plants.