What is a Keystone?
A keystone is a wedge-shaped stone in the center of a stone arch that keeps the whole arch up. A keystone species is a species that enables many other species to exist in a particular area.
The beaver is an example of a keystone species. The beaver, by damming a stream, creates a habitat for fish, amphibians, turtles, many insect species, and numerous aquatic plants.
Beavers feed on the bark of trees and shrubs. In the search for food and branches for their dam and lodge, they can chew down numerous trees in a forest. This converts a small portion of the forest which has a whole species cohort of its own into grass and shrubland with their own unique species.