Since the 19th century, humans have impacted Banff's environment through introduction of non-native species, controls on other species, and development in the Bow Valley, among other human activities. Bison once lived in the valleys of Banff, but were hunted by indigenous people and the last bison were killed off in 1850s. In 2017 a small herd of sixteen plains Bison were reintroduced into an eastern section of the park. Elk are not indigenous to Banff, and were introduced in 1917 with 57 elk brought in from Yellowstone National Park. The introduction of elk to Banff, combined with controls on coyote and wolves by Parks Canada beginning in the 1930s, has caused imbalance of the ecosystem. Other species that have been displaced from the Bow Valley include grizzly bears, cougars, lynx, wolverine, otter, and moose. Beginning in 1985, gray wolves were recolonizing areas in the Bow Valley. However, the wolf population has struggled, with 32 wolf deaths along the Trans-Canada Highway between 1987 and 2000, leaving only 31 wolves in the area.