Interactive effects of insects, mammals, and soil fertility on grassland plant community structure

Background/Question/Methods Herbivory is expected to have important effects on grassland plant community structure and biomass. However, the effects of herbivores may be mitigated by plant resource availability, which can influence plant compensatory response and plant defense, and may vary among herbivore taxonomic groups. Although a number of experiments have tested the relative importance and interactive effects of herbivory and plant resource availability in limiting plant biomass, little is known about how different herbivore taxonomic groups contribute to these patterns particularly in terrestrial systems. We tested for the additive or interactive effects of insect and non-bovine mammalian herbivores on plant community structure and biomass in a restored grassland in south-central Kansas. The experiment consisted of ninety-six, 3.5x4 m plots. Mammal access was restricted by fencing while insect abundance was reduced by applying a pyrethroid-based insecticide every two weeks during the growing season. These treatments were crossed with either ambient soil fertility or fertilizer addition treatments. We quantified plant community structure in the central 3x3.5 m area of each plot. Light penetration through the plant canopy was utilized as a non-destructive measure of plant biomass. Results/Conclusions After four growing seasons, we found herbivore access influenced light penetration through the plant canopy (a surrogate for total plant biomass) and these effects varied depending on herbivore taxonomic group and plant resource availability. Under ambient soil nutrients, the combination of insect and mammal herbivory decreased plant biomass and these herbivores' effects combined additively. However, under increased soil fertility, the specific combinations of the different herbivore groups resulted in higher, lower, or equal plant biomass compared to communities with ambient soil fertility. Additionally, the different herbivore groups altered plant species composition and abundance, but these changes were much smaller in ambient than fertilized communities even when adjusted for differences in biomass. These results suggest that different herbivore taxonomic groups can have unique effects on plant biomass and community structure and these impacts vary with soil fertility