Abstract
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.
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