Understanding insect herbivore impact on forest ecosystems
Most forests currently take up more carbon dioxide than they release, helping to slow the build-up of CO2 in the atmosphere. The magnitude and duration of this effect is controlled by a range of factors that influence plant growth, of which soil nutrients are a key example. “One key driver of plant and soil nutrient cycling that has been overlooked is the activities of animals, particularly herbivores,” notes ECOHERB project coordinator Daniel Metcalfe from Umeå University in Sweden. “The few studies on herbivore impacts on ecosystem nutrient cycling have focused on large-bodied animals rather than insects, even though insects are extremely abundant in many terrestrial systems.”
Insect impacts on nutrient cycling
The ECOHERB project, funded by the European Research Council, aimed to fill this gap. It achieved this by building a global network of forest sites, where insect herbivore contributions to nutrient cycling could be quantified in a coordinated and systematic way. Monitoring stations were established to cover all forest types. Experiments were carried out to examine the effects of herbivore excreta on soil processes under different temperature and moisture conditions. This information is now being integrated into cutting-edge ecosystem models. “The basic approach I took was outlined by my earlier work, published just prior to the project,” says Metcalfe. “Through ECOHERB, I was able to dramatically upscale these tried-and-tested methods, in order to develop a better understanding of the role of insect herbivores in terrestrial ecosystems.” In practice, this involved having to overcome enormous bureaucratic and logistical obstacles, in order to establish and run intensive monitoring efforts in remote sites across dozens of countries. This was especially difficult during the latter years of the project, which spanned the COVID-19 pandemic. “The success of the project is a testament to the extremely talented and dedicated team of doctoral and postdoctoral researchers and in-country partners who were involved in the project,” adds Metcalfe.
Drivers of carbon and nutrient release
Despite these challenges, Metcalfe was able to provide the most detailed and comprehensive picture yet of the patterns in, and underlying drivers of, carbon and nutrient release from forests mediated by insect herbivores. “We were able to survey herbivore-mediated carbon and nutrient release patterns across relatively undisturbed forest systems,” he notes. “From this, we were able to identify the global distribution of insect herbivory, and better understand forest biogeochemistry.” In Hawaii for example, Metcalfe and his team were able to quantify how the nutrients released by herbivores affect some key ecosystem processes, and how these controls and processes were regulated by climate. “We are now working with several different modelling teams to use the data collected to develop and improve several different ecosystem models,” he remarks.
Large-scale monitoring efforts
Metcalfe’s work has helped to underline the importance of large-scale monitoring efforts, and demonstrate how initial research can be successfully developed and upscaled. “I believe that the methods we used, and the results we achieved, make a strong case for including insect herbivores, and animal activity more generally, in model simulations of ecosystem functions and processes,” he says. “I hope to continue working at many of the sites, building on the fantastic network of in-country partners who have made this work possible.”
Keywords
ECOHERB, ecosystems, forests, CO2, insect, atmospheric, nutrient
