Megaherbivores facilitate large grazing herbivores and suppress small mammals through vegetation structure and cryptic pathways

Abstract

Variability in large mammalian herbivores can reshape smaller animal communities in complex ways. Accordingly, conservation strategies require a deeper understanding of the extent to which mesoherbivores (>20–1000 kg) and megaherbivores (>1000 kg) influence on animal communities are mediated by changes to vegetation structure, and the extent to which the influence of megaherbivores overlap with mesoherbivores. Using an exclusion experiment, we examined how megaherbivores altered mesoherbivore activity and how both groups influenced small mammals. We compared influences that were mediated by broad metrics of vegetation structure (i.e., indirect effects) with all other influences that were not mediated by vegetation structure (e.g., direct effects). We found megaherbivores were linked to increased mesoherbivore activity, particularly grazers, through both vegetation structure-mediated and other pathways. Alternatively, we found small mammals were suppressed where megaherbivores were present, and their population fluctuations were explained by both structure-mediated and other pathways. Small mammal abundance was higher where large herbivores were excluded compared to open plots, with no difference between plots excluding only megaherbivores and those excluding all herbivores. This suggests that these effects were produced by either megaherbivores alone, or by megaherbivores in conjunction with the increased mesoherbivore activity they facilitated. While small mammal abundance was linked to mesoherbivore-induced changes in grass biomass, their non-linear response, did not correspond with the exclusion of mesoherbivores. These findings highlight megaherbivores' capacity to reshape animal communities through interactions beyond broad metrics of vegetation structure, underscoring the importance of considering the varied influences of megaherbivores on mammal communities in conservation strategies. HIGHLIGHTS • The presence of Megaherbivores reduced small mammal densities. • Large herbivore activity increases in the presence of megaherbivores. • Responses were only partially due to megaherbivore changes to vegetation structure. • Megaherbivores' largest influences were not from vegetation structure. • Megaherbivores play a unique role in shaping animal communities.