An example of apex predators affecting an ecosystem is in Yellowstone National Park. After the gray wolf was reintroduced, in 1995, researchers noticed big changes occurring in the Greater Yellowstone Ecosystem. Elk, the primary prey of the gray wolf, became less abundant and changed their behavior. This freed the riparian (river) zones from constant grazing. This allowed willows, aspens, and cottonwoods to grow, so creating a habitat for beaver, moose, and scores of other species. In addition to the effects on prey species, the gray wolf's presence also affected the park's grizzly bear, a vulnerable species. The bears, emerging from hibernation, chose to scavenge off wolf kills after fasting for months. They can also eat wolf kills in autumn to prepare for hibernation. As grizzly bears give birth during hibernation, a greater food supply may improve the mother’s nutrition and increase the number of cubs. Dozens of other species, including eagles, ravens, magpies, coyotes, and black bears, have been seen scavenging from wolf kills.