Herbivores strongly influence the structure and composition of vegetation, but their impacts have not been reviewed in the context of terrestrial carbon storage. Understanding how herbivores influence the size of terrestrial carbon pools is critical for predicting how land use changes might alter carbon cycling. We evaluate the effects of plant biomass consumption by large mammalian herbivores (>10 kg adult biomass), and the responses of ecosystems to these herbivores, on carbon stocks in temperate and tropical regions, and the Arctic. We calculate the difference in above- and below-ground carbon stocks resulting from herbivore exclusion using the results of 107 studies from 51 vegetation types.

Results/Conclusions

The mean effect of herbivore removal on carbon stocks per unit land area is positive but relatively small across vegetation types and the range of possible effects overlaps with zero in the four carbon pools we consider in our study: above- and below-ground biomass, litter, and soil. We estimate that if all large herbivores were removed from the vegetation types sampled in our review, increases in terrestrial carbon stocks would be several orders of magnitude less than many of the natural and human-influenced sources of carbon emissions reported in global carbon budgets. Most of the estimated reductions in carbon stocks due to herbivores were within above-ground biomass and the effects of herbivore removal were larger in tropical grasslands and shrublands, and pastures, than in forests. We lack estimates for the effects of herbivores on below-ground biomass and soil carbon levels in many regions, including those with high herbivore densities, and upwards revisions of our estimates may be necessary. Removing herbivores from large areas is neither desirable nor feasible, but our estimates provide a starting point for a discussion on the magnitude of herbivory on global carbon stocks, particularly given that humans manage many populations of large herbivores.