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The raised stone at the apex of the arch pictured above is the keystone, and in historical architecture is considered so important that it is often decorated with a lion’s head, family seal or other ornament. Without this piece the whole arch will crumble.
Likewise, a keystone species is one that plays an overly important role in a biological ecosystem. Remove this one animal and every other member of the community disappears as well. In Antarctic waters the role of the keystone species is played by a tiny shrimp-like creature called a krill.
