Herbivorous Vertebrates Decrease CO2 Sequestration Across Global Terrestrial Ecosystems

Abstract

Herbivorous vertebrates are potent drivers of terrestrial carbon fluxes, yet the mechanistic pathways underlying this regulation remain unresolved. Here, we synthesized data from 138 studies to systematically untangle the complex mechanisms by which herbivorous vertebrates regulate carbon cycling via distinct body sizes and behavioral disturbance patterns (grazing, excretion, trampling, and burrowing). Synthesizing these data reveals that herbivorous vertebrates induce a global “flux contraction,” driving a 23.6% reduction in the absolute magnitude of Net Ecosystem Exchange (|NEE|) by concurrently lowering Gross Primary Production (GPP, 15.4%) and Ecosystem Respiration (RE, 9.3%). This asymmetric inhibition, where production declined more sharply than respiration, suggests that herbivorous vertebrates primarily constrain the overall magnitude of terrestrial carbon sequestration. Specifically, large mammals diminished |NEE| by 21.7%, acting as strong suppressors of net exchange, while small mammals reduced RE by 17.1%. Notably, birds exerted the strongest constraints, reducing |NEE|, GPP, and RE by 37.5%, 38.7%, and 28.6%, respectively. Among disturbance modes, grazing was the dominant driver, reducing |NEE| and GPP by 24.0% and 12.9%. In contrast, trampling and excretion stimulated soil respiration (RS) via physical priming and nutrient pulses, thereby modulating the net grazing-induced suppression, whereas burrowing had minimal net impact on CO₂ fluxes due to offsetting effects. These influences were most pronounced in wetlands, where herbivorous vertebrates compromised the net CO₂ sink (a 30.9% reduction in |NEE|) while simultaneously amplifying methane (CH₄) emissions by 73.5%. This study establishes herbivorous vertebrates as critical ecosystem engineers, highlighting the urgent need to integrate biotic effects into biogeochemical models to refine natural climate solutions.