Rewiring the Carbon Cycle: A Theoretical Framework for Animal-Driven Ecosystem Carbon Sequestration

Most carbon cycle models do not consider animal-mediated effects, focusing instead on carbon exchanges among plants, microbes, and the atmosphere. Yet, a growing body of empirical evidence from diverse ecosystems points to pervasive animal effects on ecosystem carbon cycling and shows that ignoring them could lead to misrepresentation of an ecosystem's carbon cycle. We develop a new theoretical framework to account for animal effects on ecosystem carbon cycling. We combine a classic ecosystem compartment modeling approach with a classic carbon model to account for carbon flux and storage among plant, animal, and soil microbial trophic compartments. We show, by way of numerical analyses of steady state conditions across three competing scenarios, that animal presence alters the dominant pathways of control over carbon storage and capture. This altered control arises via direct, consumptive effects and especially via indirect, non-consumptive pathways by instigating faster nutrient recycling. This leads to a quantitative change in the ecosystem's carbon balance, enhancing the amount of carbon captured and stored in the ecosystem. Further, the indirect pathways appear especially important in enabling these effects because of their sensitivity to the structure of the ecosystem's food chain. The modeling shows that animals could play a larger role in ecosystem carbon cycle than previously thought. Our framework provides further guidance for empirical research aimed at quantifying animal-mediated control of carbon cycling and to inform the development of nature-based climate change solutions that leverage animal influence on the carbon cycle to help mitigate climate change. The mathematical tools that scientists use to account for the amount of carbon captured and stored in ecosystems rarely consider the effects of animals. This stems from the assumption that because animals are much rarer than plants and microbes in ecosystems their potential influence ought to be minimal. Yet, field studies have begun to show that this assumption may not be accurate. Here, we report on modeling analyses that incorporate insights from field research about the different ways that animals can affect carbon uptake and storage in ecosystems through their interactions with plants and microbes. Our analyses show that animal presence fundamentally changes the relationships between plants, microbes, and the environment. In turn, this leads to large changes in the amount of carbon captured and stored in ecosystems relative to conditions that exclude animals. Hence, animals can be allies in fighting climate change as a key addition to the growing portfolio of nature-based climate change solutions. Animals rewire ecosystem food webs and thereby qualitatively change the dominant control over carbon cycling Animal-mediated top-down control of the carbon cycle quantitatively changes key ecosystem processes, increasing carbon sequestration Leveraging animal-driven carbon capture may lead to more accurate ecosystem carbon budgets, enhancing nature-based climate change solutions