Question

Wolves were reintroduced to Yellowstone National Park in 1995. They began to hunt the abundant elk population, so elk numbers decreased. The elk carcasses provided food for other predators, like foxes, coyotes, and bears, so all of their populations increased. Elk used to overgraze on willow plants; now that willows are growing back, beaver numbers are increasing, because they prefer the slow-moving water that willows cause.

Which of these species is a keystone species?

(1 point)

willows

elk

wolves

bears

Answer 1

Wolves — they have a disproportionate effect on the ecosystem (a trophic cascade): by reducing elk numbers and changing elk behavior they allowed willows and beavers to recover, which altered the whole community.