How ecosystem functioning in tropical forests varies with anthropogenic land use and environmental change

Abstract

The combined effect of habitat degradation and shifts in climate are pushing many species towards local and global extinctions. Often, the most sensitive species to disturbance are the most important for the maintenance of healthy ecosystems. With conservation efforts focussing on biodiversity, can we assume that ecosystem functions will also be preserved? We aimed to test these assumptions by disrupting trophic interactions along natural and anthropogenic gradients of diversity with vertebrate exclusions. In chapter two, we focus on shifts in biotic distribution that occur naturally with elevation. Vertebrate exclusion caused strong trophic cascades on herbivory in pristine tropical forests, but only at high altitudes. In contrast, at lower elevations, forests showed little response to predator exclusion, with no effect on herbivore density or subsequent plant damage. We hypothesis that habitat in the warmer and wetter lowlands harbours higher levels of diversity within mid and low trophic levels and this may confer some resilience to disturbance. In chapters three and four, we investigate how distance to edge and habitat loss effects the above processes. Edge effects significantly mediated trophic cascades, which occurred only within the forest interior. Changes in vertebrate community composition along interior to edge gradients are likely to be responsible for these findings. The effect of exclusion on both arthropod densities was mediated by habitat loss, although the proportion of plant damage was not. The generality of plant-herbivore interactions may make them robust to perturbation, hence the neutral response of herbivory to habitat loss. Our findings provide insight into how the provision of ecosystem function is mediated by biodiversity loss. This information can be vital in identifying which areas of habitat might lose functionality in the wake of disturbance. We 6 suggest that conservation efforts are refocussed to target ecosystems in which functions are reliant on vulnerable specialist species.