Elk or wapiti ("Cervus canadensis") are the most abundant large mammal found in Yellowstone; paleontological evidence confirms their continuous presence for at least 1,000 years. Yellowstone National Park was established in 1872, when market hunting of all large grazing animals was rampant. Not until after 1886, when the United States Army was called in to protect the park and wildlife slaughter was brought under control, did the large animals increase in number. In recent years, however, Yellowstone's elk population has plummeted. The Northern Herd, the only herd that winters in the park, has declined from nearly 20,000 animals in 1994 to less than 4,000 in 2013. Ecologists have linked this decline to a declining population of cutthroat trout in Yellowstone Lake, caused by invasive lake trout. With less fish to eat, grizzly bears began to eat more elk calves, causing a steep decline in elk numbers.