High in the Sierra Nevada, divers searched a clear blue lake for rainbow trout. When they found a fish, they killed it. The divers did not use fishing poles and did not eat the fish—they were state fisheries biologists who had killed over three thousand trout one summer to help save a native amphibian, the mountain yellow-legged frog—whose tadpoles are the trout’s favorite food—from extinction.

The rainbow trout found in that lake and in virtually all freshwater lakes and streams in the Sierra were planted there over the years by the State of California for the enjoyment of recreational anglers. However, many biologists maintain that the introduction of non-native species—exotics—which scientists call “bioinvasion,” threatens biological diversity. Don C. Schmitz claims that “exotic species have contributed to the...