Keystone Species

A plant or animal the removal of which causes a significant rearrangement of energy flow relationships to occur within that ecosystem. In a classic experiment conducted by Dr. Mary Powers at The University of California at Berkeley (see:, excluding the top carnivore, the steelhead trout (Onchorynchus mykiss), from a portion of freestone river by placing wire baskets over portions of the benthos resulted in a collapse of the balance between herbivorous macro invertebrates, insectivorous fishes, such as the stickleback, and the top carnivores. Sticklebacks are favorite food items for steelhead and eliminating the possibility for predation resulted in the sticklebacks consuming all herbivorous insects within the confines of the basket. As the result, an overgrowth of filamentous algae occurred within the area of the basket. In this instance, the steelhead trout was the keystone species. Therefore, over harvesting predator species in riverain ecosystems results in energy flow rearrangements the outcome of which does not usually favor maintaining that environment as trout habitat. In limestone streams, the scud (crustacean amphipod) is a keystone species.