Any species that extensively shapes its physical environment, creating new habitat for other species in the process, is called an ecosystem engineer. Examples include beavers, whose dams form wetlands upstream; American alligators, whose burrows serve as drinking water sources and hiding places for other animals; and coral, whose massive reefs serve as literal bedrock for one of earth’s most diverse ecosystems. Some ecosystem engineers play such an outsized role in the ecological stability and biological diversity of their communities that they are also considered keystone species.

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