Mammal defaunation leads to biotic homogenization of plant communities in tropical rainforests

Biotic homogenization is the process in which species communities become increasingly similar across different regions over time. This phenomenon has substantial ecological, evolutionary, and economic implications, primarily driven by human activities such as habitat destruction, invasive species introduction, and climate change. An underexplored driver of biotic homogenization is defaunation, particularly the loss or population decline of large herbivorous mammals and its consequences on plant communities. In this study, we examined how defaunation of medium- to large-sized mammals, such as tapirs and peccaries, affects taxonomic biotic homogenization in seedling and sapling communities in tropical rainforests of South America. Using data from a 13-year mammal-exclosure experiment across four forest sites in the Brazilian Atlantic Forest, we investigated the effects of defaunation on both alpha and beta diversity to understand how it might contribute to biotic homogenization. Our results indicate that defaunation significantly increased alpha diversity in exclusion plots over time, contrary to expectations, with more pronounced effects at forest sites hosting more complete mammal assemblies, that is, with greater mammal abundance and diversity. In contrast, beta diversity decreased as exclusion treatments led to more spatially homogeneous plant communities, particularly at the site where exclusion treatment prevents access to the plant community by the most complete mammal assembly. This homogenization was driven by reduced species turnover and the dominance of a few plant species that thrive in the absence of mammal herbivores, including a palm Euterpe edulis, a bamboo Merostachys neesii, and a fern Polybotrya cylindrica. These findings suggest that the removal of medium- to large-sized mammal herbivores can lead to both increased local species richness and decreased spatial heterogeneity, reshaping plant community structure across tropical forest landscapes. Our study highlights the critical role of large-bodied herbivores in maintaining biodiversity at multiple scales and underscores the ecological consequences of their functional loss. This work provides essential insights for conservation efforts aimed at mitigating the impacts of defaunation and preserving the resilience of tropical forest ecosystems, positioning defaunation as a significant anthropogenic driver of biotic homogenization.