Diet breadth and macronutrient regulation in coexisting generalist herbivores

Background/Question/Methods

Coexisting species of generalist grasshoppers, within the same genus and which have overlapping host plant use, have been shown to differentially regulate their protein and carbohydrate intake.  This behavior, which effectively places each species in its own nutritional niche, might provide a mechanism that facilitates stable coexistence of multiple generalist herbivores. To investigate this hypothesis we compared host plant use to macronutrient intake targets among dominant grasshopper species in Central Texas (Balcones Canyonlands National Wildlife Refuge). In 2011 and 2012 we collected grasshoppers from the field and documented, for each species, host plant use (via microscopic analysis of gut content).  Additionally, for each species, we reared them in the laboratory and subjected them to artificial diet experiments to determine the protein-carbohydrate intake target.

Results/Conclusions

Similar to other grasshopper communities, some species practiced monophagy, but most were polyphagous.  Polyphagous species had broadly overlapping diets, including eating host plants of monophagous species. Our laboratory feeding studies revealed that the different grasshopper species ate different absolute amounts of protein and carbohydrate, but self-selected diets with broadly similar protein-carbohydrate ratios. Interestingly, intake targets did not consistently reflect the nutrient composition of host plants. For instance, grasshoppers that feed largely on protein-rich forbs had carbohydrate-biased intake targets. Our results suggest that the use of nutritional niche partitioning as a mechanism to facilitate co-existence among generalist insect herbivores may only operate under certain conditions.  We discuss our findings in the context of insect herbivore body size, adaptive physiology, plant communities and latitudinal gradients.