http://www.bbc.com/earth/story/20170516-when-wolves-return-to-the-wild-everything-changes
late March 1995, the USA's Yellowstone National Park received a special delivery. Fourteen grey wolves, flown in two months earlier from the Canadian Rockies, were released into the park. Neighbouring Idaho received fifteen wolves. Wolves howling in the snow, not heard in these forests since their extermination 60 years before, trumpeted their return.
Scientists had intended to reintroduce and conserve grey 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.
In fact, it seems the return of the wolves has had remarkable consequences for the entire Yellowstone ecosystem. The story illustrates how the presence or absence of a top predator can utterly reshape an ecosystem.
In Yellowstone, 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 the number of elk – their main prey – by half. The surviving elks avoided the wolves' core range and stayed on the periphery. Woody trees like aspen and willow, which had been chewed and trimmed by zealous elks, now grew tall and lush.
more at
http://www.bbc.com/earth/story/20170516-when-wolves-return-to-the-wild-everything-changes 
