The world’s largest terrestrial animals (megafauna) can play profound roles in seed dispersal. Yet, the term ‘megafauna’ is often used to encompass a diverse range of body sizes and physiologies of, primarily, herbivorous animals. To determine the extent to which these animals varied in their seed dispersal effectiveness (SDE), we compared the contribution of different megafauna for the large-fruited Platymitra macrocarpa (Annonaceae), in a tropical evergreen forest in Thailand. We quantified ‘seed dispersal effectiveness’ by measuring the quantity and quality contributions of all consumers of P. macrocarpa fruit. Seed dispersal quantity was the proportion of the crop consumed by each species. Quality was defined as the proportion of seeds handled by each animal taxon that survived to produce a 2-month seedling. Megafauna (elephants, sambar deer, bears) dispersed 78% of seeds that produced seedlings, with 21% dispersed by gibbons (a medium-sized frugivore). The main megafaunal consumers displayed different dispersal strategies. Elephants were the most effective dispersers (37% of seedlings) and they achieved this by being high-quality and low-quantity dispersers. Bears displayed a similar strategy but were especially rare visitors to the trees (24% of the total seedlings produced). Sambar were high-quantity dispersers, but most seeds they handled did not survive and they were responsible for only 17% of seedlings. Gibbons displayed a high SDE relative to their body size, but they probably cannot match the role of elephants despite being more regular consumers of the fruit. The low density and poor regeneration of P. macrocarpa in the study site suggest that current dispersal rates by megafauna are insufficient, possibly reflecting reduced or missing megafauna populations. We show that different megafaunal species disperse seeds in different ways and may make unique contributions to the reproductive success of the plant species.
In the forests of Khao Yai National Park, Thailand, gibbons (Hylobates lar) were the most effective seed disperser of the large-fruited Garcinia benthamii. The fruits were at the upper size limit of what the gibbons could process, however, and we found evidence that suggests these large fruits might be more efficiently dispersed by elephants, which were rare at our study site. Our study shows that primates can function as complementary or backup seed dispersers for some large-fruited species, but they might not be capable of replacing large dispersers, leading to potential changes in landscape-scale dispersal where these animals are absent.
Frugivores with disparate foraging behavior are considered to vary in their seed dispersal effectiveness (SDE). Measured SDEs for gibbons and macaques for a 'primate-fruit' were comparable despite the different foraging and movement behavior of the primates. This could help facilitate fruit trait convergence in diverse fruit-frugivore networks.Abstract in Thai is available in the online version of this article.
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