Cascading Effects of Overhunting on the Functional Tree Composition of Amazonian Forests

The depletion of tropical frugivorous vertebrates due to overhunting may impair natural forest regeneration. Yet our understanding of how the loss of seed dispersers and browsers along hunting pressure gradients can shift plant community composition and, consequently, trait distributions, is still limited. We assessed the cascading effects of hunting pressure on forest composition by examining species-level responses and how these are translated into potential shifts in community traits at different life stages. We sampled 4784 trees and 6132 saplings across 30 forest plots along a gradient of hunting pressure in western Brazilian Amazonia, and compiled plant species data on dispersal mode, seed size, wood density, and leaf mass per area (LMA). We tested how hunting pressure affects sapling recruitment probability and sapling-to-tree (S:T) abundance ratios based on dispersal syndromes and varying seed sizes. We also evaluated whether hunting influences community-weighted mean (CWM) wood density, seed size, and LMA of saplings and adult cohorts. Our results show that overhunted forests exhibit significantly lower sapling recruitment probabilities and sapling-to-tree (S:T) abundance ratios for large-seeded endozoochorous species, particularly those bearing seeds larger than 18 mm. In contrast, abiotically dispersed and scatter-hoarded species exhibited increased recruitment success under high hunting pressure. Hunting pressure had no significant effect on CWMs of wood density, seed size, or LMA for trees and saplings. In our study landscape, up to 249 plant species, encompassing ~⅓ of the species surveyed, may be experiencing seed dispersal limitation and impaired sapling recruitment in heavily hunted forests. Yet, these species-level responses did not scale up to wholesale changes in community-wide plant functional composition, potentially due to time-lag effects. Our study demonstrates that defaunation driven by overhunting triggers early functional shifts in tropical forests by altering plant recruitment patterns, especially for animal-dispersed species, potentially leading to long-term changes in forest structure and carbon storage capacity.