During the 1980s, wolves were reintroduced to Yellowstone National Park after decades of absence due to attempted extermination in the early part of the twentieth century. Within 20 years, not only had the wolf population increased, but there was also a dramatic increase in other species, such as aspen, cottonwood, and willow trees, beavers, warblers, insects, and fish. These changes in Yellowstone Park are examples of how important a(n) ________ can be to an ecosystem.