Avian frugivore species richness influences dietary niche breadth and network properties in Asian wet tropical forests

Understanding how local species richness shapes ecological interactions is key to predicting the resilience of ecological communities. However, it remains unclear whether and how increasing species richness systematically alters niche breadth and the structure of interaction networks, independent of environmental, methodological, and phylogenetic confounding effects. To address this gap, we examined how variation in frugivore species richness influences dietary niche breadth and plant–avian frugivore network organisation across six undisturbed lowland wet tropical forest sites spanning four biodiversity hotspots in south and south‐east Asia. These sites encompassed a seven‐fold gradient in frugivore richness while maintaining broadly comparable forest type and phylogenetic composition. We conducted a total of 4267 h of focal watches on 551 fruiting individuals belonging to 133 bird‐dispersed fleshy‐fruited woody plant species to record over 34 000 feeding visits by 138 avian frugivore species. Joint species distribution models, fit using the hierarchical modelling of species communities (HMSC) framework, showed that frugivore visitation was primarily structured by fruit and seed morphology, with seed size contributing to the greatest proportion of explained variation (42%), followed by fruit width (23.4%), fruit crop size (19.7%), and pulp lipid content (14.9%). Across sites, frugivores in species‐rich communities exhibited narrower dietary breadth. Correspondingly, plant–frugivore networks showed reduced connectance and nestedness and increased modularity with increasing frugivore richness. These results highlight the role of fruit and seed morphology in shaping plant–frugivore interactions and indicate that higher frugivore diversity may reduce dietary niche breadth, likely through increased interspecific competition, potentially leading to the reorganisation of mutualistic network structure. Our findings are consistent with the hypothesis that niche differentiation contributes to the organisation of ecological networks and suggest that local variation in species richness is associated with changes in the architecture of tropical plant–frugivore assemblages.