In an ecosystem, a keystone predator is a predator which, amazingly, maintains high levels of species diversity through its feeding on the dominant secondary or primary consumers. To understand this, consider the following situation:

Species A and B are of roughly equal survival capability. As such, they are in constant rivalry for resources. Should at any time one gain an advantage over the other due to circumstantial factors, their population will experience a boost, providing them the advantage necessary to eventually consume such a disproportionate number of the ecological resources so as to eventually gain control over the entire niche.

Species C preys on either A or B, with no real preference for either. Due to the lack of real preference, it is driven largely to feed upon whichever is most numerous. This will create a tendency toward populational equilibrium between species A and B.

A popular example is the starfish Pisaster, which feeds primarily upon mussels and other creatures in certain intertidal communities. When the starfish is experimentally removed from this community, the mussels are known to "take over", and completely imbalance the local ecosystem.

An interesting, if tangential, analogy results from the comparison of keystone predation to regular government intervention in capitalist societies. Limits placed upon dominant corporations provide for the success of smaller corporations, increasing the overall richness of the economic community.

To complement Wagstaff's good description of a keystone predator (it's worthwhile considering the etymology of the term), it should be noted that the scenario he describes is just one of the potential situations in which a keystone predator maintains diversity in an ecosystem. In fact, it's not the most usual situation.

The example of Pisaster, provided as a good example of the dynamic, is not in fact a case where the non-predatory species are competitively equivalent. In fact, the mussels are the competitive dominants, and in the absence of the starfish the ecosystem rapidly becomes a mussel monoculture. However, the starfish preys preferentially on the mussel, thus keeping it's density down to reasonable levels and permitting competitively non-dominant species to flourish.

This situation where the keystone predator feeds upon the competitively dominant prey species is, in fact, the most commonly observed configuration of this kind of relationship.

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