Effects of drought stress on grassland plant traits and insect herbivore communities

Background/Question/Methods

While coexistence of specialized herbivores can be explained by differing host plant use, the diversity of generalist herbivore communities requires other mechanisms of resource partitioning. In grasslands, grasshoppers are the dominant invertebrate herbivores with communities that are usually diverse and contain many generalist species existing in close proximity with broadly overlapping diets.  Experiments using coexisting species of Melanoplusgrasshoppers have shown that these species significantly differ in their protein and carbohydrate regulation, despite the fact that they are generalists which feed on the same host plants. These species-specific nutrient requirements could represent different nutrient niches. To further investigate this hypothesis we manipulated plant quality in plots of grassland and tested correlations between the grasshopper community and plant macronutrient content. These plots, located in the Balcones Canyonlands National Wildlife Refuge, were either watered weekly from May to August with volumes simulating normal rainfall, or were allowed to wither as the severe Texas drought of 2011 progressed. By assaying the levels of digestible protein and carbohydrate in forbs and grasses I was able to accurately assess the effects of water stress on the plant macronutrient content and compare this with changes in the grasshopper community over the four months.

Results/Conclusions
As the 2011 drought progressed, the total density of grasshoppers decreased significantly in water-stressed plots, while densities in watered plots remained constant. Species richness in both treatments decreased towards the end of the summer, but to a greater extent in water-stressed plots. The grasshopper community was significantly different between the two treatments. This was correlated with changes in the biomass and species richness of the grassland plants as well as slight shifts in the protein and carbohydrate content. Our sampling identified changes in digestible plant protein and carbohydrates across the entire community of water stressed plants, however grass and forbs macronutrient content responded differently. Grasses were more resistant to water stress induced changes and therefore grass-feeding grasshopper species comprised a greater proportion of the grasshopper community in water stressed plots. This study identifies candidate species for future competition studies investigating the nutrient niche hypothesis. Herbivore species with similar host plant use, but different population responses to drought could be specializing on plant tissues with dissimilar macronutrient concentrations. If grasshopper communities respond to changes in host plant quality in a predictable way, new, more accurate models of rangeland grasshopper outbreaks could be developed to inform land management practitioners.