In late March 1995, Yellowstone National Park, United States of America, received a special delivery. Fourteen gray wolves (Canis lupus), flown in two months ago from the Canadian Rockies, were released into the park. Neighbouring Idaho received fifteen wolves. These forests had not heard wolf howls for sixty years, since wolves were exterminated by 1930s. Scientists had intended to reintroduce and conserve gray wolves in their original habitats. They did not foresee that the wolves, with blood on their teeth and claws, would restore leaves to the trees.
The wolves quickly reclaimed their spot as top predator. Ecologist William Ripple of Oregon State University has been studying the wolves since their return. He found that within a decade of their release, the wolves had cut elk—their main prey—numbers by half. The surviving elks avoided the wolves’ core range and stayed on the periphery. Woody trees like aspen and willow, chewed and trimmed by zealous elks, now grew tall and lush.
 Bangs, E.E., and Fritts, S.H. (1996). Reintroducing the gray wolf to central Idaho and Yellowstone National Park. Wildlife Society Bulletin 24, 402–413.
 Ripple, W.J., and Beschta, R.L. (2012). Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction. Biological Conservation 145, 205–213.