Competition between generalist herbivores: Effects of nutrient regulation and body size

Multiple species of generalist coexisting herbivores with overlapping diets can reach high population densities simultaneously with no adequate explanation of how coexistence is maintained when food appears to be limiting. A possible explanation, the nutrient niche hypothesis, suggests that different species could regulate for macronutrients differently, thus lessening competition for plant tissue with similar protein and carbohydrate content. We investigated this hypothesis in greenhouse microcosms using three species of generalist grasshoppers with overlapping diets, but either similar or dissimilar nutrient intake targets. Wild caught grasshopper nymphs (Balcones Canyonlands National Wildlife Refuge, TX) were caged in either monoculture, or mixed species treatments on a standard mixed plant community with one grass and two forb species. Analysis of cage population decline, final body size, and lipid content revealed competitive differences between species that did not support the predictions of the nutrient niche hypothesis. In contrast, body size was a good predictor of competitive outcomes. The smallest grasshopper in this study, also the most common grasshopper species in the field, was the weakest competitor; the strongest competitor was the largest bodied species, which is less common. Our study highlights how body size may play a large role in the outcome of scramble competition between generalist insect herbivores.