Everyone, including members of Congress and Native American tribes, wants fewer wild horses on Western lands and tribal reservations. But there is little agreement on how the number should be reduced. The wild horses come from horses that were brought to America from Spain in the 16th century. By 1971, the number of wild horses had dropped sharply. So Congress passed a law to save them. Since then, the number of wild horses has increased three times in size. Experts predict there will be 115,000 wild horses by 2020. An average adult wild horse eats more than two-and-a-half kilograms of grass and drinks 75 liters of water a day. As a result, non-native plants begin growing in the affected areas. The horses’ actions have also caused soil erosion around water sources, threatening water quality.