The nutrient landscape of a plant community and its impact on niche partitioning in grasshoppers

While coexistence of specialized herbivores can be explained by differential host plant use, the coexistence of generalist herbivores may involve other mechanisms of resource partitioning. Different generalist herbivores likely require different ratios of important macronutrients (i.e. proteins and carbohydrates) and, as a result, are likely to feed on different ratios of various plant parts and species. The existence of different macronutrient ratios or ‘nutrient targets’ has been verified with coexisting species of Melanoplus grasshoppers, but has yet to be determined for an entire grasshopper community. In grasslands, grasshoppers are the dominant invertebrate herbivores with diverse communities that contain many generalist species with broadly overlapping diets. The restored mixed grass prairie of the Balcones Canyonlands National Wildlife Refuge (Burnet County, Texas) is an excellent example. The grasshopper community at this site contains at least 40 Acridid species that utilize a relatively simple grassland plant community. In the Balcones Canyonlands NWR we regularly surveyed the plant and grasshopper community and compared the temporal shifts in the Acridid community to changes in the plant community and dominant host plants’ protein and carbohydrate content. We combined the abundance of dominant host plants with the nutrient content to provide a visualization of the ‘nutrient landscape’ available to a foraging herbivore. Forbs generally contained higher protein to carbohydrate ratios than grasses but made up less biomass in the field. Plant quality decreased significantly from spring to summer even as grasshopper numbers increased. This study identified species which may partition resources on a nutrient basis.