Abstract
We use a simple model of coupled carbon and nitrogen cycles in terrestrial ecosystems to examine how “explicitly representing grazers” vs. “having grazer effects implicitly aggregated in with other biogeochemical processes in the model” alters predicted responses to elevated carbon dioxide and warming. The aggregated approach can affect model predictions because grazer‐mediated processes can respond differently to changes in climate compared with the processes with which they are typically aggregated. We use small‐mammal grazers in a tundra as an example and find that the typical three‐to‐four‐year cycling frequency is too fast for the effects of cycle peaks and troughs to be fully manifested in the ecosystem biogeochemistry. We conclude that implicitly aggregating the effects of small‐mammal grazers with other processes results in an underestimation of ecosystem response to climate change, relative to estimations in which the grazer effects are explicitly represented. The magnitude of this underestimation increases with grazer density. We therefore recommend that grazing effects be incorporated explicitly when applying models of ecosystem response to global change.
Highlights
Despite evidence that animals can influence ecosystem carbon (C) and nutrient cycles (Schmitz 2014), the explicit incorporation of animals into terrestrial biogeochemical models is rare (Metclafe and Olofsson 2015)
To maintain mass balance in these models without explicit representation of animals, the effects of animals have to be implicitly aggregated into other biochemical processes through model calibration
Herbivore–vegetation models have been made for other ecosystems (e.g., Seagle and McNaughton 1993, Bennett 2003), we are aware of only one vegetation-dynamics model – ArcVeg – that explicitly addresses the effect of an Arctic herbivore on tundra biogeochemistry (Yu et al 2011). This model indicates that grazing dampens the increase in plant biomass expected from warming soils and the consequent increase in nutrient cycling (Yu et al 2011). These results suggest that the explicit inclusion of grazers in biogeochemical models could be necessary for predicting tundra responses to climate change
Summary
Despite evidence that animals can influence ecosystem carbon (C) and nutrient cycles (Schmitz 2014), the explicit incorporation of animals into terrestrial biogeochemical models is rare (Metclafe and Olofsson 2015). To maintain mass balance in these models without explicit representation of animals, the effects of animals have to be implicitly aggregated into other biochemical processes through model calibration (e.g., animal respiration included with other heterotrophic respiration). Animal-mediated processes can behave differently from the processes with which they are aggregated. Combining microbial and mammal respiration into a single value for heterotrophic respiration can cause problems, because warming generally increases respiration in microbes, but can slow respiration in mammals if the warming reduces the energy needed to maintain body temperature (Batzli et al 1980).
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