The Wolf Effect

During the 1920s, hunting and trapping eliminated wolves from Yellowstone National Park. For decades, ecologists hypothesized that the loss of wolves - important predators of elk and other large grazing animals - had changed the
park ecosystem. But because there were no before and after data, it was impossible to test that hypothesis directly.

Then, in the mid 1990s, wolves were reintroduced to Yellowstone. Researchers watched park ecosystems carefully and sure enough, the number of elk in parts of the park began to fall just as predicted. But, unpredictably, forest and stream communities changed, too. Elk prefer to graze in high mountain slopes and valleys with streams. Do you think their preference might be affecting Yellowstone's plant communities? Why might life in Yellowstone's streams be affected by the presence or absence of plants along stream banks? Could a "wolf effect" be affecting organisms throughout the park's woods and streams? In this chapter, look for connections among Yellowstone's organisms and their environment. Then, solve the mystery.