In 1995, ecologists started to reintroduce the wolf into Yellowstone Park, where the wolf had been eliminated intentionally half a century earlier  Rangers expected a rise in the number of wolf would place limits on the elk population and on the populations of many mid-size mammals
One unexpected consequence was a dramatic rise in the population of foxes, which in turn helped to control many rodent populations
The main predators of the fox were coyotes, on which the wolf preys.