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 (20:55 hr) 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.

From structure to function in mutualistic interaction networks: Topologically important frugivores have greater potential as seed dispersers

“…Regarding frugivore body size, we found that both the smallest and the largest frugivores interacted with fewer plant species than medium-sized frugivores, which were better connected within networks (Appendix S6; Figure S6). This quadratic pattern contrasts with the positive linear relationship described in other studies (Correa et al, 2016;García et al, 2014;Palacio, Valderrama-Ardila, & Kattan, 2016), but is in line with cases where the structural role of large frugivores has been found to shrink when they specialize in large fruits inaccessible to small frugivores (Naniwadekar et al, 2019;Sebastián-González, Pires, Donatti, Guimarães, & Dirzo, 2017). That said, the relationship between the characteristics of frugivores and their topological position suggests that the link between frugivore structural and functional roles demonstrated in this work may result from a correspondence between matching traits (i.e.…”

“…There, alternatively, the ecological characteristics of frugivore species may be used as surrogates of their contribution to plant recruitment (Dennis & Westcott, 2006;Naniwadekar, Chaplod, Datta, Rathore, & Sridhar, 2019). For example, frugivore species abundance and the proportion of fruit in an animal's diet have been found to be good indicators of the quantity of fruits consumed by animals in frugivory networks (e.g.…”

Large frugivores matter more on an island: insights from island-mainland comparison of plant-frugivore communities

“…This is also one of the few studies that performed a paired comparison between an island and mainland 3 community. Modularity analysis revealed that the hornbill was the key seed disperser for the large-seeded 3 plants as has been demonstrated in previous studies (Naniwadekar et al 2019; Gopal et al 2020).…”

“…1B), is characterized by a species-poor frugivore assemblage, enabled us to examine the relative quantitative role of different species in seed dispersal. In the Asian tropics, small-bodied frugivores like bulbuls and barbets are known to play key roles in seed dispersal (Naniwadekar et al 2019). However, the Narcondam Island does not have any bulbuls and barbets and most of the dominant frugivores on the island are medium-and large-bodied avian frugivores (> 100 g) (Fig.…”

Biodiversity components mediate the response to forest loss and the effect on ecological processes of plant–frugivore assemblages

“…Namely, small‐sized drupes, like elder Sambucus nigra and blackberry, are mostly eaten by small birds such as blackcap, but also by other, larger species, such as bullfinch and Eurasian jay, which increase the degree of trait‐based functional diversity in local bird assemblages. Interestingly, tropical systems have shown the opposite pattern in that specialized interactions were mostly driven by large fruits (Naniwadekar, Chaplod, Datta, Rathore, & Sridhar, 2019).…”