Correct Answer: Gray Wolf
Explanation: Yellowstone’s last wolf pack was killed in 1926. The loss of this keystone species significantly affected the Greater Yellowstone Ecosystem. With the wolves’ removal and nothing to control their populations, elk numbers skyrocketed. Elk ate aspens and willows, which had severe repercussions on the many species that relied upon the trees, such as beavers. The elk browsed on shrubs along the banks of streams, and, unshaded, the streams rose in temperature and left cold-water fish species to suffer. In 1995, wolves were restored to the park, and this change for the better became quickly apparent. The carcasses of wolf kills provided food for other animals such as foxes, bears and bald eagles. Elk populations dropped back to a sustainable number, and willows and aspens are now thriving.
Photo Credit—Red fox: Wilma McKenzie; Coyote: Henry Holdsworth; Bobcat: Sean Beckett