The correct statement regarding the reintroduction of wolves to Yellowstone National Park and its effect on the beaver population is:
The wolves decreased the population by eating beavers.
However, this needs clarification. While wolves primarily prey on large ungulates (such as elk), their presence indirectly benefited the beaver population by controlling the elk population. With fewer elk, there was less overbrowsing on riparian vegetation (such as willow and cottonwood), which beavers rely on for building dams and lodges. Therefore, the reintroduction of wolves ultimately led to an increase in beaver populations in Yellowstone, but they did not directly increase it by preying on beavers themselves. Instead, wolves help maintain a balanced ecosystem that supports the habitats beavers need.