As many as 30 million bison may once have lived on the North American plains, but after about 1840 their numbers began to decline and by 1880 only a few hundred survived. The end of a three-hundred year climate cycle called the little Ice Age in the 1840s brought hotter, drier weather to the plains, slowing the growth of prairie grasses; before long, there were more buffalo than the land could support, and the herds grew smaller. Indian people still relied on the buffalo and killed up to 250,000 per year, while huge numbers of wild and domesticated horses, as many as 2 million, competed for range. By the 1850s and 1860s, habitat began disappearing to towns, farms, railroads, and livestock raising. In the 1870s, world demand for leather and a nationwide economic depression brought thousands of out-of-work adventurers onto the plains as hide hunters, and hunting pressures drove the herds to near extinction. Buffalo Bill, who first became known for hunting buffalo to feed Kansas Pacific Railroad workers and for leading buffalo hunts for European royalty, later worked to save them by preserving a small herd from which many living buffalo descend. The effort to save the buffalo that began in the 1880s has been called the beginning of the modern conservation movement. Bison and buffalo are both correct names for Bison bison; the word buffalo grew from the term boeufs used by the first Europeans who saw them. Full-grown bison stand over 6 feet high at the shoulder and weigh over a ton, and the giant herds were actually made up of many smaller groups of 100 or fewer led by females to good grazing. Beginning in the 1880s, people such as Buffalo Bill and Charles J. "Buffalo" Jones became conservationists, and many bison in national parks and grasslands descend from animals they saved; today there are more than 250,000 buffalo.