The effects of herbivore density on soil nutrients and tree growth in tropical forest fragments

The role of herbivores in nutrient cycling in tropical forest ecosystems remains poorly understood. This study investigates several aspects of nutrient cycling along a gradient in herbivore (Alouatta seniculus, red howler monkey) density among small landbridge islands in Lago Guri, Venezuela. Specifically, two contrasting hypotheses were addressed: (1) herbivores increase the availability of soil nutrients and subsequently primary productivity, and (2) herbivores decrease nutrient availability and primary productivity because they increase the dominance of non-preferred, nutrient-poor, tree species. Although C:N increased with herbivore density, the annual increase in basal area (an indicator of aboveground productivity) increased with herbivore density. According to an analysis of the tree communities on the study islands, herbivory may also be causing a shift in the tree community toward non-preferred species, and thus, over a longer time scale, both nutrient availability and productivity are expected to decline. The influence of herbivores on nutrient cycling and plant productivity has important implications for conservation and rates of carbon sequestration in tropical forests.