Large frugivores matter: Insights from network and seed dispersal effectiveness approaches.

Abstract

While large avian frugivores are known to be key dispersers for large-seeded tree species, their role in community-wide plant-disperser networks is still poorly known. Large avian frugivores are also among the most threatened due to anthropogenic impacts. We evaluated the role of large avian frugivores in a plant-disperser community by (a) determining whether the plant-disperser community was modular, with a distinct community of large frugivores (thereby highlighting their importance), (b) determining relative qualitative and quantitative roles played by large-bodied frugivores vis-à-vis other frugivores and (c) determining impacts of large-bodied frugivore loss on the plant-disperser community. The study was carried out at a tropical forest site in north-east India, which is part of the Eastern Himalaya Biodiversity Hotspot. We collected tree watch data (20:55hr) from 46 tree species, which represented 85% of tree species that are predominantly bird-dispersed in the area. We found that the plant-disperser community was modular, with a distinct module of large-seeded tree species and large frugivores. Intermediate-sized frugivores such as barbets and bulbuls were the most connected, while large-sized frugivores, such as hornbills and imperial pigeons, were moderately well connected. Qualitative and quantitative roles played by different dispersers varied across the gradient of frugivore body size. Hornbills, the largest avian frugivores, consumed a significantly greater number of fruits and swallowed larger proportions of fruits compared with other avian groups. In comparison with similar-sized frugivores, imperial pigeons fed on larger-sized fruits, highlighting their importance for dispersal of large-seeded plants. Under simulated extinction scenarios, larger extinction cascades were not necessarily caused by larger frugivores; however, extinctions of certain large-bodied frugivores (hornbills, imperial pigeons) caused extinction cascades. Integrating information from networks and seed dispersal effectiveness approaches enabled a better understanding of large frugivore role in a plant-disperser community. While large-bodied frugivores may not be playing a central role in plant-disperser communities, they are crucial as seed dispersal service providers for large-seeded plants. In conjunction with the reported local extinctions of large frugivores like hornbills from the south Asian region, this study's findings highlight the irreplaceable quantitative and qualitative impacts that tropical plant communities are likely to experience in the future.