OOS 46-4 - Modeling the zoogeochemical effects of herbivores on ecosystem carbon cycles

The effects of human-caused climate change are growing in severity and frequency, leading to a growing interest in natural solutions to manage, mitigate, and potentially reverse them. Among these natural solutions, targeting the cycle of carbon (C) in ecosystems has an intuitive appeal, as both living and non-living ecosystem components offer ways to capture atmospheric C and lock it in long-lived reservoirs. Mathematical models are key in forecasting how the responses of an ecosystem’s living and non-living components to climate change influence its C dynamics—i.e., budget, cycling, and storage. These models often assume that biomass-dominant trophic compartments in an ecosystem also dominate its C dynamics. However, a growing body of empirical evidence shows that low-biomass trophic compartments, like animals, may have more pervasive and consequential effects on the fate of ecosystem C than previously thought. In turn, this has prompted recent Earth System Models to begin integrating so-called zoogeochemical mechanisms that may influence the C cycle. Yet, the level of abstraction and spatial resolution at which these models operate means that we still lack a suite of quantitative tools to measure, forecast, and incorporate the impacts animals can have on an ecosystem’s C budget. Here, we map a dynamic ecosystem model with herbivores to a classic C budget model and use it to investigate herbivore zoogeochemical effects on an ecosystem’s C budget and cycle. We show that herbivore-induced trophic rewiring influences an ecosystem’s ability to capture and store atmospheric C. Compared to an herbivore-free ecosystem, ecosystems with herbivores store more C in both organic and inorganic stocks; especially, in long-lived soil and plant pools. Herbivore-induced acceleration of recycling pathways also leads to higher primary productivity—in turn increasing atmospheric C sequestration, compared to an herbivore-free ecosystem. Our results show that, despite low relative biomass, animals play a key role in shaping the balance of C in ecosystems. Therefore, coopting them and the existing management practices already in use worldwide may be instrumental in developing natural solutions to tackle climate change. As well, our results lend strength to emerging practices of returning animals to ecosystems they either disappeared or were extirpated from—so called “re-wilding” efforts. We offer our model as a further step to incorporate zoogeochemistry into efforts tackling human-caused climate change, as well as into theoretical and empirical frameworks of ecosystem functioning.