Soil nutrient additions increase invertebrate herbivore abundances, but not herbivory, across three grassland systems
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Resource availability may influence invertebrate communities, with important consequences for ecosystem function, such as biomass production. We assessed: (1) the effects of experimental soil nutrient additions on invertebrate abundances and feeding rates and (2) the resultant changes in the effects of invertebrates on aboveground plant biomass at three grassland sites spanning the North American Central Plains, across which plant tissue chemistry and biomass vary. Invertebrate communities and rates of herbivory were sampled within a long-term nutrient-addition experiment established at each site along the broad Central Plains precipitation gradient. Additionally, the effects of invertebrates on aboveground plant biomass were determined under ambient and elevated nutrient conditions. At the more mesic sites, invertebrate herbivore abundances increased and their per capita rate of herbivory decreased with nutrient additions. In contrast, at the semi-arid site where plant biomass is low and plant nutrient concentrations are high, invertebrate herbivore abundances did not vary and per capita rates of herbivory increased with nutrient additions. No change in the effect of invertebrate herbivores on aboveground plant biomass was observed at any of the sites. In sum, nutrient additions induced shifts in both plant biomass and leaf nutrient content, which altered invertebrate abundances and feeding rate. However, due to the inverse relationship between changes in herbivore abundance and per capita rates of herbivory, nutrient additions did not alter the effect of invertebrates on aboveground biomass. Overall, we suggest that this inverse response of herbivore abundance and per capita feeding rate may buffer ecosystems against changes in invertebrate damage in response to fluctuations in nutrient levels.
KeywordsNitrogen Per capita herbivory rate Phosphorus Plant tissue chemistry Central Plains
The authors are grateful for field assistance from M. Avolio, D. Blumenthal, C. Brown, L. Dev, K. Harmony, J. Klein, A. Kuhl, B. La Pierre, A. Potter, R. Ramundo, A. Joern, and the SGS field crew. O. Schmitz, K. Gross, K. Burghardt, and three anonymous reviewers provided helpful feedback on earlier drafts of the manuscript. Funding was provided by a Yale Institute for Biospheric Studies Center for Field Ecology Pilot Grant, a Yale Institute for Biospheric Studies Dissertation Improvement Grant, and a grant from the Lee Pierce Fund to K. La Pierre, the Konza Prairie LTER, and the Shortgrass Steppe LTER. This work was generated using data from three sites within the Nutrient Network collaborative experiment, funded at the site scale by individual researchers and coordinated through Research Coordination Network funding from the National Science Foundation to E. Borer and E. Seabloom (NSF-DEB-1042132). N fertilizer was donated by Crop Production Services, Loveland, Colorado. K. La Pierre was supported by an National Science Foundation Graduate Research Fellowship.
Author contribution statement
K. J. L. and M. D. S. conceived and designed the experiments. K. J. L. performed the experiments and analyzed the data. K. J. L. wrote the manuscript with editorial input from M. D. S.
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