What is a keystone species?

A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. The role that a keystone species plays in its ecosystem is analogous to the role of a keystone in an arch. An ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of the ecosystem by measures of biomass or productivity. Some examples of keystone species are: Sea otters that protect kelp forests from damage by sea urchins. Wolves prevent a single species from becoming dominant. Beavers are known as ecosystem engineers, and Elephants destroy trees, making room for the grass species and creating habitat for various small animal species.

Source