One of the big surprises to biologists when wolves were returned to Yellowstone National Park is that the fishing got better – a lot better. It turned out that the elk which liked to stand in the creeks prior to the return of the wolves would eat all of the willows and small plants along the creek.
When the wolves returned, the elk wanted better visibility around them, and so they abandoned the creeks and stayed either in the forests or out in the meadows where they could see predators coming. The result was that trees were now given the opportunity to grow along the streams, insects would fall from the branches, fish would feed on them and the trout got bigger and more abundant.
The local biologists never anticipated that. This points out that nature is more complex that we think. In fact a former Chief of the U.S. Forest Service Jack Ward Thomas reported that nature is more complex than the human mind can think.