Effects of seed size and toucan regurgitation on the germination of the tropical tree Eugenia uniflora

Abstract Understanding the quality of seed dispersal effectiveness of frugivorous species can elucidate how endozoochory structures tropical forests. Large seeds, containing more resources for growth, and gut passage by frugivores, which remove seed pulp, both typically enhance the speed and probability of germination of tropical seeds. However, the interaction of seed size and gut passage has not been well studied. We assessed the role of two species of toucans (Ramphastos spp.) in seed germination of the tropical tree Eugenia uniflora, which produces seeds that vary considerably in size (3.7–14.3 mm), using 151 control and 137 regurgitated seeds in germination trials. We found that toucan regurgitation did not increase germination success, although 93.4% germinated compared to 76.8% of control seeds; however, larger seeds germinated more often at faster rates. Although only marginally significant, germination rates were 3.6× faster when seeds were both large and regurgitated by toucans, demonstrating that toucan regurgitation can disproportionally benefit larger E. uniflora seeds. As tropical forests are increasingly disturbed and fragmented by human activities, the ability of toucans to continue providing seed dispersal services to degraded habitats may be vital to the persistence of many tropical plants that contain larger seeds and depend on larger dispersers.