Under the keystone predator hypothesis, a predator severely limits the abundance of a competitively dominant prey species, allowing competitively subordinate species coexist in part of the habitat, maintaining the species richness, and the spatial patterns comprising the community. Now widely applied to both aquatic and terrestrial habitats, the hypothesis was based on the experimental findings of the Mytilus—Pisaster system. The removal of predacious sea star Pisaster precipitated the downward expansion of the mussel bed, which overgrew, shaded, and crushed the algae and sedentary invertebrates occupying the lower level. This result was the first operational definition of a keystone predator.
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