The parasite which causes Whirling Disease is one example of the increasingly difficult problems caused by exotic species around the world. Yellowstone Cutthroat Trout has seen dramatic population declines and other native species - even otters, herons, and grizzly bear, may be at risk. The problem, caused by a parasite which came from Europe and was introduced into the Rocky Mountains early this century, defies an easy solution. Nonetheless, researchers continue to test and apply new strategies in the hopes of restoring native populations.