Just two years after gray wolves were reintroduced to Yellowstone after what is believed to have been half a century's absence, they have dramatically made their presence felt. They have killed half the coyotes in the area, forced elk to become more vigilant and provided more opportunities for scavangers to share their kills. Because there are fewer coyotes, rodents are more plentiful, a boon for predators like hawks and bald eagles and overall biodiversity has sharply increased.