Large Herbivores Suppress Decomposer Abundance In A Semiarid Grazing Ecosystem

Abstract

Ecosystem-level studies of producer–decomposer interactions have focused primarily on plant production and soil texture as regulators of decomposer abundance but have rarely considered the role of grazers in mediating such interactions. Here, we conducted replicated exclosure experiments at both high and low levels of soil fertility to investigate the effects of large, mammalian grazers on decomposer biomass and activity patterns in a semiarid grazing ecosystem in Kenya. Within only two years of grazer exclusion, microbial biomass was greater in soil of fenced grassland across all levels of soil fertility. This consistent negative effect of grazers on microbial biomass occurred despite the fact that grazers stimulated aboveground plant production in nutrient-rich sites and depressed it in nutrient-poor sites. A consideration of all the potential pathways by which grazers influence decomposer populations suggests that observed grazing-induced reductions in microbial biomass were predominantly associated with a depression in the amount of plant carbon inputs to soils. Finally, across all study sites, microbial biomass was highly correlated with soil carbon content, suggesting that landscape-scale constraints on soil organic matter content and plant production overarch grazer effects on microbial abundance. Our results support previous ecosystem-level studies showing that microbial biomass and growth are constrained by plant production and soil C availability. In addition, our findings demonstrate that decomposer abundance can be influenced by an ecosystem's trophic structure, with significant reductions in microbial biomass occurring as a result of herbivores diverting plant carbon away from soils.