Interactions between trophic levels can lead to some surprising responses to global change factors. We have found that invertebrate herbivores increase in abundance with increased nutrient availability at three Nutrient Network (NutNet) sites spanning the North American Great Plains. However, their feeding rates decreased with nutrient additions, likely in response to increased plant tissue quality. This increase in abundance, but decrease in feeding rates resulted in no observable change in the amount of plant tissue removed by invertebrate herbivores with nutrient additions.
We are also examining the effects of herbivory on plant community composition under conditions of elevated nutrient availability. Within a tallgrass prairie ecosystem, we are removing invertebrate herbivores and adding nutrients in a factorial combination. Over five years of treatments, we have found that invertebrate herbivores suppress forb responses to nutrient additions, thus playing a large role in determining the trajectory of plant community shifts with chronic nutrient additions. This experiment is currently ongoing to investigate the role that fire and large native grazers may also play in the effects of invertebrate herbivores on plant community composition.