Keystone species have a major influence on the structure of their ecological community. The profound influence of keystone species occurs because of their position and activity within the food chain/web. In the sense meant here, a "major influence" means that removal of a keystone species would result in a large change in the abundance, and even the local extirpation, of one or more species in the community. This would fundamentally change the structure of the overall community in terms of species composition, productivity, and other characteristics. Such changes would have substantial effects on all of the species that are present, and could allow new species to invade the community.

The original use of the word "keystone" was in architecture. An architectural keystone is a wedge-shaped stone that is strategically located at the summit of an arch. The keystone serves to lock all other elements of...