American bison are big. They can weigh as much as 2,000 pounds and they eat a lot — so much in fact that they can change the ecosystems where they graze.

Dr. Matthew D. Moran, a professor at Hendrix College, decided to find out how bison grazing affects insects and other arthropods at the Tallgrass Prairie Preserve in Osage Colorado by taking samples in grazed and ungrazed areas. The results of his study, which are published in the journal Environmental Entomology, show that arthropod abundance and diversity are both higher in areas where bison are grazing.

This may seem counterintuitive at first, since more grazing bison means less grass for insects to eat. However, bison manure contains nitrogen, an important plant nutrient that increases the quality of the grass.

“The increase in herbivores and reduction in plant biomass from grazing resulted in an arthropod herbivore load almost three times higher in grazed plots compared with controls,” he wrote. “Ungulate grazing, although decreasing available biomass to other consumers, enhances plant quality by increasing nitrogen level in plants.”

Even though the bison reduced the amount of grass by 50% or more, the overall abundance and diversity of arthropod herbivores and carnivores increased significantly, especially among sap-feeders. In addition, Dr. Moran suggests that this may also be beneficial to birds and and other animals that eat insects.

“Arthropods are important dietary components of many other consumers such as birds and small mammals. In fact, grazing can increase insectivorous bird abundance,” he wrote. “This study indicates that bison are important for maintaining arthropod communities in tallgrass prairies.”

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– Bison Grazing Increases Arthropod Abundance and Diversity in a Tallgrass Prairie