Interactive effects of insects and ungulate herbivores on primary production and diversity in a native northern grassland

Background/Question/Methods Grasslands are home to a diverse array of invertebrate and vertebrate herbivores. Though individual plants and patches may experience both forms of feeding, the effects of each herbivore guild on grasslands have traditionally been studied independently and focused primarily aboveground. However, these herbivore guilds may have interactive effects, impacting both root and shoot growth. We conducted a study to explore whether ungulate and insect herbivore impacts interact to affect primary productivity or local diversity. In 2004 and 2005, we conducted a two by two factorial experiment that paired a moderate-density, ungulate herbivory treatment with an insect suppression treatment in a rough-fescue prairie. Biotic and abiotic variables were extensively measured in both years, including: vegetation percent cover estimates, peak season aboveground biomass, light penetration, and insect abundance. In 2005 we measured root growth with ingrowth cores. We hypothesized ungulates would enhance belowground productivity and forb diversity, while insects would reduce overall aboveground biomass and forb diversity. Results/Conclusions Ungulate herbivores caused a 48% increase in root growth, a finding common to other grassland communities. However, this effect only occurred when insects were suppressed, such that insects prevented overcompensation in root growth in response to ungulates. Ungulate herbivory reduced the total aboveground biomass by 20% in both years, while insect herbivory never had an effect on total aboveground production. In both years, herbivore effects on grasses were consistent, but the effects on forbs varied between 2004 and 2005. In 2005, both guilds reduced forb biomass by approximately 25%. Neither insect nor ungulate exclusion altered any aspect of species diversity or plant community structure. These diversity and community structure results are consistent with a five-year insect suppression study in a nearby location. Even though both herbivore guilds have similar effects on aboveground communities, those aboveground affects interact with unpredictable results on belowground productivity. It is important, therefore, to consider the indirect effects of, and interactions between, aboveground herbivore guilds when measuring grassland belowground productivity. This likely extends to other measures of grassland function and diversity.