Drivers of Grassland Invertebrate Community Structure: Effects of Soil Nutrient Availability and Vertebrate Herbivores on Invertebrate Resource Limitation

The effects of nutrient availability and vertebrate herbivory on the plant community likely transfer to higher trophic levels. Herbivore biomass has been shown to correlate with primary productivity, and invertebrate herbivore biomass is expected to increase in response to an increase in plant tissue quality following nutrient additions. In addition, invertebrate herbivores compete with vertebrate herbivores for resources and may be negatively influenced by the impact of vertebrate herbivores on plant secondary chemistry, growth, physiology, or phenology. The impacts of nutrient additions and vertebrate herbivore removals on the invertebrate community can have important consequences for invertebrate herbivore feeding rates and preferences. However, no work has yet been done to test the simultaneous indirect effects of vertebrate herbivores and soil nutrients on invertebrate communities in a field setting. Here we address the effects of soil nutrient additions and vertebrate herbivore removals on the resource limitation of the invertebrate community.

We find that in a tallgrass prairie ecosystem (KNZ), N additions indirectly resulted in an increase in invertebrate herbivore abundance by increasing plant biomass; the increase in invertebrate herbivore abundance further fed up to increase invertebrate parasitoid abundance at this site. In a mixed-grass ecosystem, both N additions and vertebrate herbivore removals indirectly resulted in an increase in invertebrate herbivore abundance by increasing plant biomass, further leading to an increase in invertebrate predators and parasitoids. Finally, in a shortgrass steppe ecosystem (SGS), N and P additions resulted in changes in plant biomass, however did not influence invertebrate herbivore abundance. Feeding rates of invertebrate herbivores were also examined, and showed that while invertebrate herbivore abundance increased with nutrient additions in tall- and mixed-grass prairie ecosystems, the per capita feeding rates remained constant. These results provide preliminary evidence that invertebrate herbivores may be under relative resource limitation in areas with higher initial plant tissue quality (i.e. SGS), while those in areas with lower plant tissue quality (i.e. MIX, KNZ) may be under absolute resource limitation. These differences in resource limitation may have strong consequences for alteration in important community and ecosystem processes.