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Noting that bison herds appear to graze selectively in areas occupied by prairie dog colonies, Coppock hypothesized that the attraction is due to improved forage quality: prairie dogs` own grazing activities alter herbage dynamics, plant species composition, and nutrient cycling. The colonies` territory contains relatively little low-quality mature standing herbage and is characterized by communities of plants that are high in crude protein and highly digestible. However, such effects are not produced only by prairie dogs. Willms showed that sites selectively grazed by cattle had reduced standing dead plant material, altered species composition, and increased soil nitrates, ammonium, and available phosphorus. In fact, long-term, intensive use by any grazer will cause comparable changes in plant communities.