As Return of Wolves Changes Ecosystem points out, removing apex predators from the top of the food chain can have far-reaching effects:
After wolves were reintroduced to Yellowstone National Park in 1995 and 1996, researchers noticed they were most successful bringing down elk where the prey had to deal with a change in terrain, such as crossing a stream. Elk soon learned to avoid those areas.‘When you remove the wolves, the elk are able to browse unimpeded wherever they want, as long as they want,’ said co-author Robert L. Beschta. ‘Now that the wolves are back, the ecology of fear comes into play.’
Comparing old photographs and other descriptions of the area with present conditions, Ripple and Beschta found streamside vegetation sharply declined in the mid-1920s, about the time the last wolves were killed.
Vegetation along streams prevents erosion, cools the water for fish, cycles nutrients through the food web, and provides habitat for birds and amphibians.