Trophic Cascades

A trophic cascade is a progression of indirect effects from predators across successively lower trophic levels (See Estes Estes, Crooks, Holt, 2001. Predators, ecological role of. Levin S, ed. Encyclopedia of Biodiversity, vol. 4. San Diego) the top down processes and other trophic interactions have broad ecosystem effects.

The reintroduction of wolves to Yellowstone National Park, in the northern U.S. Rocky Mountains, illustrates how large carnivores can have a cascading effect on lower trophic levels. In the early twentieth century, people developed a protectionist attitude towards ungulate herds, and thought wolves were pests. Starting in 1918, the Director of the Park Service ordered “extermination of mountain lions and other predatory animals” in Yellowstone. Approximately 136 wolves were killed, and thousands of other predators. By 1926, wolves had been eliminated from Yellowstone.

 

For the next 70 years, until 1995, Yellowstone did not have any wolves. During this period, there was also very little regeneration of new aspen, willow, and cottonwood trees. Elk populations began to show a marked increase in lame and sick megafauna. With no fear of wolves, elk could graze wherever they wanted, and for decades they proceeded to browse and kill nearly all the young cottonwood trees growing along the banks of the streams. This had a consequence of streambank erosion and higher water temperatures.

By the 1990s, the Federal government had reversed its views on wolves.  Scientists decided to bring them back to the area in 1995. The U.S. Fish and Wildlife Service brought in 15 gray wolves from Canada. The wolves thrived and there are now over 300 of their descendents living in the Greater Yellowstone Area. Wolves now influence elk through direct mortality, they have been hunting and eating elk. There has been an increase in regeneration of aspen, cottonwood, and willow. This illustrates how wolves also influence elk through non-lethal means. The fear of attack by wolves apparently prevents elk from browsing on young trees in wolves’ core areas, and also from streams.

The elk leftovers provide food for animals such as ravens, eagles, and bears. According to one ecologist, the reintroduction of wolves has affected more than 25 species in Yellowstone. Wolf-killed elk carcasses are consumed by other carnivores.

Adapted from: Wolves and the Ecology of Fear: Can Predation Risk Structure Ecosystems? By Ripple and Bestha. BioScience, August 2004 / Vol. 54 No. 8