Grasshopper herbivory modifies grass composition in northern mixed prairie

Background/Question/Methods

Insect herbivores often comprise more biomass than vertebrate herbivores in grassland ecosystems, but little longer term research has examined insect herbivore impacts on plant communities in grass dominated ecosystems. A manipulative cage experiment was conducted from 2000-2006 at a northern mixed-grass prairie site in eastern Montana. Treatments consisted of vegetation controls, Ageneotettix deorum herbivory (whitewhiskered grasshopper), and Melanoplus sanguinipes herbivory (migratory grasshopper). Vegetation composition was monitored using a 100 point frame in early summer prior to installation of cages. Plant damage was also assessed when cages were removed.

Results/Conclusions

Two grasses, western wheatgrass and blue grama, comprised over 97% of the grass point frame hits at the start of the experiment. Blue grama cover was reduced approximately 50% by the end of the 6 yr experiment with the gramnivorous feeding A. deorum, but was not affected by a larger bodied mixed grass and forb feeding grasshopper M. sanguinipes. Grasshoppers were maintained at similar densities that did not strongly reduce plant biomass. The results were likely driven by plant preference, as A. deorum selected grasses more in proportion to their abundance and M. sanguinipes selected against blue grama.  Blue grama could be less resistant to herbivory by a grass feeding grasshopper than cattle, but additional research is required. The results point to the need for research on the impact of abundant grasshopper herbivores on grassland function, in light of grasshoppers frequent dominance by biomass in grassland systems.