Direct and indirect effects of plant and frugivore diversity on structural and functional components of fruit removal by birds
Seed dispersal is an important ecosystem function, but it is contentious how structural and functional diversity of plant and bird communities are associated with seed-dispersal functions. We used structural equation models to test how structural (i.e., abundance, species richness) and functional diversity (i.e., functional dispersion and community-weighted means of functional traits) of fruiting plants and frugivorous birds directly and indirectly influence the respective components of fruit removal. We recorded plant and bird diversity in point counts and observed plant–frugivore interactions in a tropical mountain forest in Ecuador. We also recorded plant and bird morphological traits to calculate measures of functional diversity. We found that fruit abundance had a positive direct effect on bird abundance, which directly and indirectly mediated the abundance of removed fruits. Plant and bird species richness were only directly related to the richness of the removed fruits. Functional dispersion of the plant community was positively associated to that of the bird community and to that of the removed fruits. Consistently, we found positive associations between community-weighted means of plant and bird traits and between community-weighted means of plant traits and that of plants with removed fruits. In contrast, community-weighted means of the bird community were unrelated to that of the removed fruits. Overall, our results suggest that plant abundance directly and indirectly influences fruit removal, likely because of avian fruit tracking. However, we did not find strong links between the functional diversity of the frugivore community and removed fruits, suggesting that other factors in addition to plant–animal trait matching might be important for the functional diversity of removed fruits. Our findings highlight the importance of frugivore abundance for maintaining seed dispersal by animals in tropical forests.
KeywordsSeed dispersal Plant–frugivore interaction networks Functional trait diversity Species identity Structural equation models Tropical montane forest
We thank the German Research Foundation (DFG) for funding our projects in the framework of the Research Bundle 823–825 “Platform for Biodiversity and Ecosystem Monitoring and Research in South Ecuador” (PAK 825/1) and the Research Unit FOR2730 “Environmental changes in biodiversity hotspot ecosystems of South Ecuador: RESPonse and feedback effECTs”. The Ecuadorian Ministry of the Environment (MAE) kindly provided permission to conduct research. We thank Nature and Culture International (NCI), Felix Matt, Jörg Zeilinger and Catherine Vits for logistic support. We are grateful to Agustín Carrasco, Patricio Estrella, Dagmar Hanz and Nina Gunselmann for their help in fieldwork. We thank Dagmar Hanz, Larissa Nowak and Anna Phillips for their help measuring bird traits. Three anonymous reviewers provided valuable comments on an earlier version of this manuscript.
Author contribution statement
MS, ELN, KBG, MQ, VS conceived and designed the study. MQ and VS collected the data. JH identified the botanical samples. MQ analyzed the data. MQ, MS, ELN led the writing of the manuscript with editorial advice from KBG, JH and CIE.
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