Endozoochory, a mutualistic interaction between plants and frugivores, is one of the key processes responsible for maintenance of tropical biodiversity. Islands, which have a smaller subset of plants and frugivores when compared with mainland communities, offer an interesting setting to understand the organization of plant–frugivore communities vis‐a‐vis the mainland sites. We examined the relative influence of functional traits and phylogenetic relationships on the plant–seed disperser interactions on an island and a mainland site. The island site allowed us to investigate the organization of the plant–seed disperser community in the natural absence of key frugivore groups (bulbuls and barbets) of Asian tropics. The endemic Narcondam Hornbill was the most abundant frugivore on the island and played a central role in the community. Species strength of frugivores (a measure of relevance of frugivores for plants) was positively associated with their abundance. Among plants, figs had the highest species strength and played a central role in the community. Island‐mainland comparison revealed that the island plant–seed disperser community was more asymmetric, connected, and nested as compared to the mainland community. Neither phylogenetic relationships nor functional traits (after controlling for phylogenetic relationships) were able to explain the patterns of interactions between plants and frugivores on the island or the mainland pointing toward the diffused nature of plant–frugivore interactions. The diffused nature is a likely consequence of plasticity in foraging behavior and trait convergence that contribute to governing the interactions between plants and frugivores. This is one of the few studies to compare the plant–seed disperser communities between a tropical island and mainland and demonstrates key role played by a point‐endemic frugivore in seed dispersal on island.
Mutualistic interactions, like seed dispersal, are important for maintaining plant diversity in tropics. Islands, which have a smaller subset of plants and frugivores, when compared with mainland communities offer an interesting setting to understand the relative role of ecological and evolutionary processes in influencing the interactions between plants and frugivores. In this study, we examined the relative influence of functional traits and phylogenetic relationships on the plant-seed disperser interactions on an island and a mainland site. The island site allowed us to investigate the organization of the plant-seed disperser community in the natural absence of key frugivore groups (bulbuls and barbets) of Asian tropics. The largest frugivore on the island, the point endemic Narcondam Hornbill, was also the most abundant frugivore on the island and played a significant quantitative and central role in the plant-seed disperser community. Species strength of frugivores was positively associated with frugivore abundance. Among plants, figs had the highest species strength and played a central role in the community during the study period. We found that the island had among the highest reported densities of hornbills and figs in the world. Island-mainland comparison revealed that the island plant-seed disperser community was more asymmetric, connected and nested as compared to the mainland community with which it shared certain plant and frugivore species. Neither functional traits nor phylogenetic relationships were able to explain the patterns of interactions between plants and frugivores on the island or the mainland. Diffused nature of interactions between plants and frugivores, trait convergence and plasticity in foraging behavior likely contribute to shaping the interactions between plants and frugivores. This study underscores the need to study plant-seed disperser communities on tropical islands, particularly in the Indian Ocean, which have inadequate representation in the literature.
Oceanic islands, due to their evolutionary history and isolation, hold a disproportionately high proportion of endemic species. However, their evolutionary history also makes them vulnerable to extinctions, with most known extinctions occurring on islands. Plant-animal interactions are particularly important on islands, as island systems generally have low redundancy and are more vulnerable to disruption either via extinction or by invasive species. Here, we examined the fruit removal and seed predation of a keystone palm, Caryota mitis, on the remote oceanic island of Narcondam. The island endemic Narcondam hornbill (Rhyticeros narcondami), was the sole seed disperser of the Caryota mitis (90 hours; N = 15 trees), indicating a lack of redundancy in seed dispersal of the palm on this island. While the invasive rodent, Rattus cf. tiomanicus was the sole predator of the Caryota mitis seeds in the forest (N = 15 individual fruiting palms, 416 trap nights). Overall, 17.1% of the seeds placed (N = 375 seeds) were removed. Seeds placed under and away from the canopy, and at different densities (2 plots with 10 seeds each; 1 plot with 5 seeds, respectively), showed similar removal rates. This is indicative of ambient seed predation and the lack of safe sites for the regeneration of Caryota mitis, with potential deleterious effects on the subsequent stages of the 'seed dispersal cycle'. Here, from a data deficient site, we provide baseline information on the plant-frugivore interaction of a keystone palm and the potential impacts by an invasive rodent.
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