In the complex network of ecological relationships, every living population is interdependent. A historical instance of this interdependence is the 1920 policy that permitted the widespread killing of wolves, resulting in an ecological breakdown. When wolf numbers dwindled, the number of elk grew quickly because there were no wolves to hunt them. This led to the elk eating too much of the trees such as willows and aspens. Beavers, who need these trees to build their dams and live in the water, were negatively impacted by this overgrazing. Significant changes in the population of a species can have catastrophic effects on its ecosystem, and start a series of downstream impacts. This occurrence is called a trophic cascade.