Herbivore effects on plant quality and quantity in a shortgrass prairie.

The Earth's grasslands have experienced extensive alterations to their grazing regimes over the course of human history. We asked how native grassland herbivores (bison, prairie dogs, and grasshoppers) and a non-native herbivore that has become dominant (cattle) affect seasonal patterns of plant and soil elemental chemistry and aboveground plant biomass in a shortgrass prairie in the North American Northern Great Plains. To quantify herbivore effects, we sampled plants and soils across 4 months of the growing season in 15 grassland sites comprising five herbivore regimes with varying densities of bison, cattle, prairie dogs, and grasshoppers. Prairie dogs had the strongest herbivore effects on grass and soil chemistry, increasing grass N, K, and Mg, and increasing soil C and N. Both bison and cattle grazing increased grass Mg and decreased grass Si. Sites with higher grasshopper densities had higher soil P. Finally, the seasonal trajectory of aboveground plant biomass was altered by the use of insecticides in prairie dog towns, with the biomass at these sites peaking near the end of the growing season. Plant biomass peaked in mid-summer in all other herbivore regimes, with declines in the late growing season. This suggests that Orthopteran herbivores, taxa that tend to eat more in the late season when they are often in the adult stage, may have an overlooked contribution to seasonal aboveground plant biomass trajectories in temperate grasslands. Conservation and rewilding of grassland herbivores can maintain the critical nutrient cycling services that these taxa provide.