What is a keystone predator?

A keystone predator is known for its balancing effect on local populations in an ecosystem.

"Keystone predator" is an ecological term used to describe the basic principle by which a predator may be a balancing force on an ecosystem. Any species could potentially be a "keystone species", predator or no, given the right circumstances in the local ecology. The term "keystone" is a general analogy to bridge-building, meaning anything that, while small, is fundamental to the stability of a greater structure. Similarly, a keystone species is fundamental to an ecosystem, despite its proportionally small population.

In some cases, a keystone predator may in fact maintain necessary diversity in an ecosystem through selective predation. Perhaps the most common example is the starfish Pisaster ochraceus. Existing in Pacific Northwest intertidal communities, Pisaster has a predatory preference for the mussel Mytilus californicus over other local organisms. Experimental removal of the starfish from the community results in an extreme overpopulation of mussels, to the degree that most other organisms in the community are forced out of the competition for resources.

This happens because Pisaster is a balancing force in these communities; the mussels on their own are a more effective consumer than the other organisms. This means they find more of the resources and thus breed more quickly, their population growing to match the availability of resources. Heavy predation by the starfish in the community keeps the mussel population low, allowing other organisms to find their own niche.



Generally, a severe lack of diversity may have few short-term effects, but the smaller, more gradual effects of the population shift can have a devastating toll in the long term. For this reason, special care must be taken with identified keystone predators to keep them from being hunted out of an ecosystem. This is especially important because the number of predators in an ecosystem is necessarily less than the quantity of prey, and so the number of predators hunted to provide food or game for humans can put a serious strain on the local biological balance.

Additionally, a species not widely known to be structurally important to an ecosystem can still have a disproportionate effect on the ecological balance. In this way, many species are keystone species, to varying degrees. Even the slightest variation in the biological equilibrium that has been established over millions of years can have an impact greater than we can anticipate without a full understanding of the local environment.

There are some questions surrounding the use of the term "keystone species" in general, namely that it places an unnecessary focus on particular species and that its conditions are sometimes arbitrary: for instance, how is it one can say when a species has enough of an effect on its environment as compared to its population? Similarly, some definitions include that this effect must be constant, as well as disproportionate, while some significant effects may be produced only sporadically. It is often thought best that the discussion be restricted by general reference to the "keystone effect", referring to the phenomenon as a whole, rather than specifying which species display this effect most prominently.

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