A keystone species is a species that has a disproportionate effect on its environment relative to its abundance. Such an organism plays a role in its ecosystem that is analogous to the role of a keystone in an arch. While the keystone feels the least pressure of any of the stones in an arch, the arch still collapses without it. Similarly, an ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of the ecosystem by measures of biomass or productivity. It has become a very popular concept in conservation biology.
Notes on the definition
The definition given here is somewhat qualitative in nature because there is not yet an accepted, rigorous definition. At issue is how to measure both abundance and impact and at what point to draw the line. Abundance in this context can be measured by biomass or productivity, among other metrics. Impact is even harder to define. It has been suggested that such keystone predators can be identified in ecosystems by their biomass dominance within ecological functional groups, even though they may be relatively rare in relation to the ecosystem as a whole.  The term is often misused in places where foundation species would be more appropriate. A keystone species is not simply one whose disappearance would alter the ecosystem. Removing any abundant species from any ecosystem will drastically alter that ecosystem by definition. What makes the concept of keystone species attractive to conservationists is that a species with a relatively tiny physical footprint can be critical to the health of its ecosystem.
Without a consensus on its exact definition, we are left to illustrate the concept of keystone species with a list of examples. A classic keystone species is a small predator that prevents a particular herbivorous species from eliminating dominant plant species. Since the prey numbers are low, the keystone predator numbers can be even lower and...
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