Ripple Effects

In Yellowstone National Park, invading lake trout have decimated the cutthroat trout that grizzly bears normally eat. Now, hungry grizzlies are preying on the calves of migrating elk instead.

Since lake trout took over Yellowstone Lake, the number of cutthroat trout has plunged by more than 90 percent. Grizzly bears that used to feast on spawning cutthroat trout have been forced to look elsewhere for food. Berries and other nutritious plants in the area are sparse, so the grizzlies don’t have many options.

A research team pored over previous studies and noted that the percentage of elk calf deaths caused by bears in the Yellowstone area has risen from about 12 to 41 percent since the 1980s. The change isn’t just due to a growing bear population, the team says, because the number of calves eaten per grizzly also has increased.

In Proceedings of the Royal Society B, the authors estimate that the grizzly population used to eat 5,656 kilograms of trout each year. After the lake trout invasion, that number dropped to 314 kilograms. Grizzlies would need to eat 297 elk calves to make up for the lost nutrition, the researchers say — enough to slow down elk population growth by 2 to 11 percent. And since cutthroat trout used to support many other species as well, they write, “the broader ecological consequences of lake trout invasion are potentially tremendous.” Roberta Kwok | 15 May 2013

Source: Middleton, A.D. et al. 2013. Grizzly bear predation links the loss of native trout to the demography of migratory elk in Yellowstone. Proceedings of the Royal Society B doi: 10.1098/rspb.2013.0870.

Image © Jean-Edouard Rozey |