Limited effects of above- and belowground insects on community structure and function in a species-rich grassland

Do above- and belowground insects differentially impact plant community structure and function in a diverse native grassland? Rough fescue prairie in Alberta, Canada. Above- and belowground insects were suppressed with insecticides for 5 years using a randomised block design. During this experiment, a severe drought began in 2001 and ended in 2003. Aboveground plant growth was measured as cover and biomass from 2001 to 2005. Root demography was measured in 2002 using a minirhizotron. Mixed models and repeated measures ANOVA were used to determine treatment effects on a number of response variables. MRBP were used to test for treatment effects on community composition. Five years of insect suppression had few significant effects on plant growth, species richness or community composition, and were limited primarily to the severe drought in 2002. During the drought, insect attack increased root mortality, reduced plant cover, and altered community composition. Following the drought, plant responses were unaffected by insecticide application for the remainder of the experiment. Five years of insect suppression had only minor effects on community structure and function in this diverse native grassland. There was no indication that these effects increased over time. The results are counter to studies conducted in productive old-field communities that revealed large effects of insects on community structure. We suggest that the unique features of this system, such as high species evenness, abundance of generalist herbivores, and a lack of competition for light among plants, limited the potential for insects to greatly impact community-level processes.