Influences of native prairie dogs and domestic cattle on the native grasshopper assemblages of a desert grassland

Background/Question/Methods Black-tailed prairie dogs were historically abundant across Chihuahuan Desert grasslands, but they have been largely extirpated by humans within the past century, while over the same time, domestic cattle have become dominant on those same grasslands. Both prairie dogs and cattle are known to have significant effects on the on soils, vegetation, and grasshopper assemblages of those grasslands, but their comparative and interactive effects on grasshopper assemblages are not known. The purpose of this research was to determine the relative influences of native prairie dogs and exotic cattle on native grasshopper assemblages, in terms of affecting species composition and abundance. We are conducting a manipulative field experiment near Janos, Chihuahua, Mexico, at one of the world’s largest remaining black-tailed prairie dog colonies. The Janos grasslands also serve as communal rangeland for cattle, and the grasslands have been desertified by excessive cattle grazing over the last two centuries. We manipulated prairie dogs and cattle on a series of experimental study plots, and measured vegetation and grasshoppers on all study plots each year.

Results/Conclusions

Before treatments were imposed (2006), there were no differences in grasshopper abundance or species composition among the different study plots across replicated blocks. By the second year after treatments were imposed (2008), grasshopper abundance and species composition had changed significantly in response to prairie dog removal and less so to the addition of cattle. Overall grasshopper abundance increased significantly when prairie dogs were removed, and the grasshopper assemblages were dominated by forb-specialist species responding to increased forb foliage. However, overall grasshopper species diversity declined on those plots due to the dominance of a few species. Grasshopper assemblages on plots where prairie dogs remained were composed of many species that utilized bare soil and sparse vegetation for habitat, along with forb-specialist species, and those study plots had the highest grasshopper species diversity. Grasshopper assemblages appeared to be responding to changes in vegetation resulting from the manipulations of prairie dogs and cattle. This study demonstrates the important roles of both native and exotic keystone mammalian herbivore species in affecting important invertebrate herbivore species through the alteration of soils and vegetation.  This study further demonstrates that in those particular grasslands, native prairie dogs increase the species diversity of native grasshopper assemblages.