Keystone Species

What Is a Keystone Species?

A keystone species is a species whose effect on the structure and function of an ecological community is disproportionately large compared with its abundance. In other words, the species may not be the most common organism in the ecosystem, but its presence can have a major influence on many other populations.

The term comes from the keystone of an arch. A keystone is one relatively small part of the structure, yet removing it can cause the rest of the arch to become unstable. Ecologists use the term in a similar way: removing a keystone species can produce changes that spread well beyond that species itself.

Why Are Keystone Species Important?

Ecological communities are networks of interacting populations. Species compete for resources, consume one another, provide habitat, alter the physical environment, and influence the abundance of other organisms. A keystone species can strongly affect several of these relationships at once.

Imagine a predator that keeps an herbivore population from becoming too large. If the predator is present, plants may experience less grazing pressure, allowing more plant biomass and habitat to remain available. Those plants can then support insects, birds, and other organisms. The predator may therefore influence organisms that it never eats.

This is an example of an indirect effect. Keystone species are especially important because their influence can extend through a community rather than stopping with their immediate interaction.

Keystone Predators

Some of the best-known keystone species are predators. A predator can have a disproportionately large effect when it prevents another population from becoming dominant.

Suppose a predator feeds on a herbivore that consumes vegetation. If the predator disappears, the herbivore population may increase. More herbivores can then consume more plants, changing the amount of vegetation available to other organisms. The predator's effect has therefore traveled through several levels of the community.

Importantly, not every predator is a keystone species. A predator is considered keystone when its effect on community structure is unusually large. The ecological evidence—not simply the fact that it is a predator—determines whether the label is appropriate.

Keystone Species and Ecosystem Engineers

Some organisms influence communities by changing the physical environment. These organisms are often called ecosystem engineers. They may build structures, create shelter, modify water flow, change soil, or otherwise alter conditions that other organisms use.

Keystone species and ecosystem engineers are related concepts, but they are not identical. A keystone species is defined by the magnitude of its effect on community structure. An ecosystem engineer is defined by its ability to modify the physical environment. A species can fit both descriptions, but one term should not automatically be substituted for the other.

What Happens When a Keystone Species Is Removed?

Removing a keystone species can cause a series of population changes. The exact outcome depends on the ecosystem, but the important pattern is that the effects can be much larger than expected from the abundance of the removed species.

For a keystone predator, removal may allow prey to increase. Increased prey pressure can then reduce vegetation or another resource. Other species that depend on that vegetation may decline, move away, or face greater competition. In this way, one removal can reorganize community structure.

Ecologists must be careful not to assume that every ecosystem will respond in exactly the same way. Multiple species interactions and abiotic factors can modify the result. The key idea is the potential for disproportionately large community-level effects.

Keystone Species and Trophic Cascades

A keystone species can be associated with a trophic cascade, especially when it is a predator. A trophic cascade is a chain of effects that moves through multiple trophic levels. For example, a predator can reduce herbivores, which can reduce grazing pressure on producers.

These terms should not be treated as synonyms. Keystone species describes the ecological importance of a species. Trophic cascade describes a pattern of effects moving through trophic levels. A keystone species can trigger a trophic cascade, but not every trophic cascade necessarily involves a keystone species.

How Ecologists Identify a Keystone Species

Calling a species “keystone” requires more than noticing that it interacts with many organisms. Ecologists look for evidence that changing the abundance or presence of that species produces a disproportionately large change in community structure.

Researchers may compare communities where the species is present with communities where it is absent, or they may conduct carefully designed experiments in which a species is removed or its abundance is changed. They can then measure responses such as population abundance, species richness, vegetation cover, or resource availability.

This experimental approach is important because ecosystems contain many variables. A population may change for several reasons, so scientists need evidence that the focal species is actually responsible for the observed community response.

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