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: http://ib.berkeley.edu/labs/power/index.shtml),
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.