Modelling herbivory impacts on vegetation structure and productivity

Abstract

Animal herbivory can have large and diverse impacts on vegetation and hence on the state and function of ecosystems. Despite this, quantitative understanding of vegetation responses to consumption of green leaf tissue by herbivores is currently lacking. The large-scale impacts of changes in herbivore abundance on ecosystem function have yet to be investigated. Process-based modelling can help to quantify how animals affect important processes, such as ecosystem carbon cycling. To do so, we linked the dynamic global vegetation model LPJ-GUESS with Madingley, a model of multi-trophic functional diversity. This implementation allows us to simulate feedbacks between the availability of green vegetation biomass, herbivory and the whole trophic chain in response to monthly consumption of leaf biomass. In the coupled model system, we see an overall reduction in ecosystem productivity (NPP -5.2 %), leaf area index (-9.0 %) and carbon mass (-9.7 %), compared to the stand-alone version of LPJ-GUESS, with the highest impact on carbon mass in the boreal ecosystems (-42 %). We observe ecosystem composition to shift from boreal coniferous forests (without animals) to boreal mixed forests (with animals), as well as a general increase in herbaceous vegetation. Indirect effects like an increased light transfer facilitating growth of lower canopy layers are also captured by the model system. Overall, the results of this study underpin the important role of animals in ecosystem functioning and highlight the important contribution of process-based modelling towards a better understanding of complex food web interconnections.