Effect of forest use on trophic levels of small mammals: an analysis using stable isotopes

The degradation of tropical forests is progressing rapidly and its ecological effects on wild animals are a global concern. We evaluated the hypothesis that small mammals in highly degraded forest occupy higher trophic levels than those in somewhat degraded forests, as indicated by diets high in consumers such as insects, in a tropical rain forest in Sarawak, Malaysia. After correcting for differences in the delta N-15 values for primary production among the study sites, the delta N-15(corrected) values for rats and mice (Muridae) differed significantly among forest types. Rats and mice in more degraded forest had higher delta N-15(corrected) values than those in less degraded or primary forest: in contrast, treeshrews (Tupaiidae) and squirrels (Sciuridae) showed no significant differences in the delta N-15(corrected) values among forest types. We found significant positive correlations between canopy openness and the delta N-15(corrected) values for one species of squirrel and two species of rats. This hypothesis was supported for small mammals that have normal dietary preferences for plants, i.e., omnivorous rodents, but not for those that normally prefer insects, i.e., treeshrews. The delta N-15 values for omnivorous mammals may be useful as an indicator of changes in food-web structure in response to forest disturbance.