Direct and indirect effects of excluding mammals in a tropical wet forest: Evidence from Costa Rica for altered seedling recruitment success
Ecologists have long sought to understand what factors regulate the distribution and abundance of plants across the globe and we have found that plant-animal interactions play a key role in these patterns. In the case of herbivory, we know direct effects (one-to-one interactions such as leaf loss and mortality) can cause landscape-scale restructuring of plant communities that can last for decades. However, indirect effects of plant-animal interactions (complex or passive processes such as trophic cascades and shifts in successional patterns) are less well understood. In this study, we tested the overarching hypothesis that mammal herbivory and physical disturbance of seedlings modifies patterns of plant succession in La Selva Biological Station, Costa Rica. Specifically, we predicted that experimentally excluding key mammals such as peccaries and agoutis would 1) increase seedling abundance, 2) decrease understory percent leaf cover, and 3) constrain height class diversity. We quantified these direct and indirect effects by comparing the woody seedling and sapling abundance, approximate percent leaf cover, and height structure between areas where mammal densities are high (natural abundances) and low (experimentally excluded via fences). We sampled vegetation in six 2x2m subplots within each of two mammal exclosures and two paired control (non-fence) plots that were established almost 20 years ago.
After almost two decades of reduced terrestrial mammal exposure, we observed a clear response in understory woody plant communities. Consistent with our predictions, we found three times more seedlings in the mammal exclosures than in the control plots, indicating the seed predators that were excluded acted primarily as suppressors of seedling regeneration. We also found a six-fold increase in leaf cover in the fenced plots when compared to the controls, which suggests that indirect effects such as physical trampling or leaf damage non-related to herbivory may strongly influence sapling recruitment success. There was greater height diversity (height class richness and evenness) in mammal exclosures, wherein two more height classes were represented on average than in the control plots, and fenced plots were characterized by significantly more variation in height. Heterogeneity in height structure may be both indicative of more even recruitment as well as may provide complexity of niche space that contributes to the maintenance of diversity in tropical plants. These results provide preliminary evidence that tropical mammals are having direct and indirect impacts on the survival and structure of woody plants in the seedlings and sapling stage—a key bottleneck of tropical forest succession.