Defaunation impacts on seed survival and its effect on the biomass of future tropical forests

Abstract

Large animal species, which provide important ecological functions such as dispersal of seeds or top-down control of seed predators, are very vulnerable in fragmented forests, being unable to survive in small fragments, and facing increasing hunting pressure. The loss of large animals affects two main ecological processes crucial for the tree reproductive cycle: seed dispersal of large seeds (e.g. provided by tapirs) and control of seed predator population (e.g. provided by large cats). The changes in both processes are expected to increase seed mortality since seeds are not dispersed away from conspecifics (causing increased pre-dispersal mortality due to negative density dependent effects) and/or face increased predation after a dispersal event (post-dispersal mortality). Although an extensive body of empirical knowledge exists on seed predation, the link between seed loss and adult tree community composition and structure is not well established, as well as the temporal scale seed changes affect adults. Using an individual-based forest model (FORMIND), we evaluate the long-term consequences of increased pre and post-dispersal seed mortality on the future forest biomass retention of a Brazilian northeastern Atlantic forest. Our results show that forest biomass is significantly affected after 80-93% pre-dispersal loss of large seeds, or post-dispersal predation densities of 20-25 predators per parent tree. Large-seeded tree species are at increased risk of local extinction causing up to 26.2% loss of forest biomass when both pre and post-dispersal processes are combined. However, these changes can last up to 100 years after the occurrence of defaunation. In summary we conclude that large animal loss has the potential to reduce future forest biomass and tree species-richness by impacting seed survival, and should be considered in the planning of biodiversity friendly landscapes as well as in calculations of the global carbon budget.