Insect Herbivores, Nutrient Cycling and Plant Productivity

Summary

We review the various ways in which insect herbivores affect ecosystem function, focusing particularly on their impacts on decomposition, nutrient cycling and plant productivity. Many of the most profound effects of insects on these processes occur below ground and until recently have been considered relatively inaccessible to study. However, new approaches, particularly the development of stable isotope techniques and more inventive uses of controlled environment studies, have allowed significant advances in our understanding of the role of soil biota in both below- and aboveground processes. Undoubtedly, one of their most important roles is to physically break up organic matter within the soil and make it accessible to the microbial component, but these new techniques mean we now know far more about the more indirect effects of soil biota as well, such as the impact of root-feeding herbivores on the rhizosphere. In addition, the development of more quantitative techniques for assessing canopy herbivory and the impacts of defoliation on nitrogen cycling in forests has provided new evidence for the impact of aboveground foliovores on soil processes. We draw on the literature and on our own work to summarize the ‘state of play’. We highlight some of the progress that has been made in the study of the role of insect herbivores in ecosystem function, examine some intriguing interactions between insect herbivores and other organisms, and draw attention to some neglected impacts on ecosystem processes. Thus we highlight gaps in our current understanding and hence areas that future research might profitably examine.