Trout fishing is almost nonexistent is some American states because of a foreign parasite. The Nile perch, a voracious predator introduced to Lake Victoria as a food fish, has already extinguished over one hundred species of native cichlid fish there. A parasite can be similarly devastating. The sea lamprey reached the Great Lakes through a series of canals and, in combination with overfishing, led to the extinction of three endemic fishes. The European parasite that causes whirling disease in fishes, introduced to rainbow trout in a hatchery in Pennsylvania, has now spread to many states and devastated the rainbow trout sport fishery in Montana and Colorado.
Herbivores can wreak great damage.
The first sailors to land on the remote Atlantic island of St. Helena in the 16th century introduced goats, which quickly extinguished over half the endemic plant species. Some alien species eliminate native species by breeding with them, altering the gene pool. Such competition for resources is not easy to observe, but the end result is the loss of a native species. Introduced mallards, for instance, are driving the native Hawaiian duck to a sort of genetic extinction by breeding with them. Of 26 animal species that have gone extinct since being listed under the Endangered Species Act, at least three were wholly or partly lost because of hybridization with invaders.