A species whose impact on its community is disproportionately large relative to its abundance. This is generally because it alone fulfils some crucial functional role in the community, the continuation of which is essential for the survival of numerous other species. Classic examples are the beaver (Castor spp.), whose dam building creates the unique beaver ponds on which many other species depend, and the bison (Bos bison), responsible for a mosaic-like grazing pattern that underpinned the biodiversity of the grasslands of North America. Compare foundation species.