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Competition from horse appears to have been a factor in historical wildlife distribution and abundance. In archaeological studies in the Columbia River basin in North America, horse bones have been found alongside bison bones, indicating that bison were still present in the Columbia Basin at the time horses appeared, that is, sometime after about 1720. Researchers hypothesize that the introduction of the horse eventually led to the extinction of bison in the Columbia Basin in two ways. First, the horse acted as a direct foraging competitor to the bison. Horse and bison diets have considerable overlap, and competition for forage may have been high as horse numbers increased over time. Second, the introduction of horses improved the hunting efficiency of the Native Americans.