Exploring Fruit-Frugivore Interactions and Post-Dispersal Seed Fate of Large-Seeded Plants in the Lowland Rainforests of Indonesian Borneo

Abstract

Interactions between frugivores and the fruits they disperse play a crucial role in shaping plant population dynamics and ecosystem functioning. Large-bodied frugivores are particularly important as they are primary dispersers of fruits containing large seeds. Unfortunately, these frugivores are often preferentially targeted by hunters, and their loss can have profound effects on forest diversity, structure, and function. Despite the critical importance of understanding the ecological impacts of frugivore loss, many tropical regions lack community-level studies that assess region-specific patterns. Biodiverse Southeast Asian forests are facing significant challenges due to human-induced land-use changes and declining wildlife populations, especially among large-bodied species. Consequently, opportunities to study the vital relationships between diverse plants and frugivore species are rapidly diminishing. Comprehensive community-level assessments are imperative in the remaining areas that still retain diverse populations of large-bodied frugivores. The Cabang Panti Research Station in West Kalimantan, Indonesian Borneo, provides a unique opportunity for such research, as it has remained largely unaffected by anthropogenic influences. At this site, I characterized and evaluated fruit-frugivore interactions, specifically focusing on large-seeded plant species that depend on mutualistic interactions with large-bodied animals. Chapter 2 of my study analyzes a network comprising 19 large-seeded liana and tree species and 25 interacting animal species. I assess interaction magnitudes, directions, and the impact of plant and animal traits on positive, neutral, and negative interactions. The results demonstrate a modular yet less nested (compared to networks in other SE Asia forests) frugivore-plant interaction network, showcasing a moderate level of specialization. Results show the differential impact of seed weight and animal body weight on the likelihood of positive and neutral interactions. In Chapter 3, I examine post-dispersal seed fate for 15 large-seeded liana and tree species. My findings note that seeds placed farther from parents experienced lower predation rates, while seeds in high-density clusters under the canopy fared better than medium- and low-density clusters. Vertebrate seed predators, both large and small generalists, significantly affect seed mortality. Chapter 4 represents the first multi-species assessment of hunting threats to 33 mammal and bird species that feed on fruits and seeds in Indonesian Borneo. My findings underscore that hunting disproportionately targets specific animal species in the region, potentially leading to the local extinction of at least one species, Rusa unicolor. In summary, this study reveals a diverse frugivore and predator assemblage, emphasizing context-dependent interactions within fruit-frugivore and seed-predator networks. It also highlights a higher number of antagonistic interactions experienced by the studied plants compared to mutualistic interactions. Furthermore, some of the limited mutualistic interactions observed between the studied plants and large-bodied frugivores involve animals targeted by hunting.