Plant biodiversity on shortgrass steppe after 55 years of zero, light, moderate, or heavy cattle grazing

Shortgrass steppe rangeland near Nunn, Colorado, USA, has been lightly, moderately, or heavily grazed by cattle, or protected from grazing in exclosures, for 55 years. Plant species biodiversity and evenness were greatest in lightly- and moderately-grazed pastures. Both pastures were dominated by the warm-season shortgrass Bouteloua gracilis, but the cool-season midgrasses Pascopyrum smithii and Stipa comata contributed significantly to biomass production on the lightly-grazed pasture, as they did in the exclosures. Diversity was least in the exclosures, which were strongly dominated by the cactus Opuntia polyacantha. Buchloe dactyloides, another warm-season shortgrass, and Bouteloua gracilis were co-dominants under heavy grazing, and diversity was intermediate. Plant community structure and diversity were controlled by selective grazing by cattle and soil disturbance by cattle and rodents. Shortgrass steppe moderately or heavily grazed by cattle was similar to and probably as sustainable as steppe grazed for millenia by bison and other wild ungulates.