What Is a Keystone Species?
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A keystone species is a species whose effect on its ecosystem is disproportionately large relative to its abundance. Remove it and the whole system reorganises, often dramatically. The name comes from the wedge shaped stone at the top of an arch: small, but take it out and everything collapses.
How the idea was found
The concept came from experiments by the ecologist Robert Paine in the 1960s on rocky shores in Washington state. He removed a single predatory starfish species from experimental plots and watched what happened.
Mussels, previously kept in check, took over completely and crowded out almost everything else. The number of species in the plot fell from around fifteen to one. The starfish was not abundant, but it was the reason the community had any diversity at all.
How keystone effects work
Keystone species influence systems through several different mechanisms:
- •Predation, keeping a dominant competitor from monopolising space
- •Engineering, physically altering habitat for other species
- •Mutualism, such as pollinators that many plants depend on
- •Resource provision, creating food or shelter others rely on
Sea otters and kelp forests
Sea otters eat sea urchins. Sea urchins eat kelp. Where otters were hunted out for the fur trade, urchin populations exploded and grazed kelp forests down to bare rock, a state called an urchin barren.
Kelp forests support enormous numbers of species, buffer coastlines from wave energy and store carbon. Where otters have returned, kelp has generally recovered along with everything that depends on it. A single predator was effectively holding an entire coastal ecosystem in place.
Beavers as ecosystem engineers
Beavers are the classic engineering keystone. By damming streams they create ponds and wetlands, which change water tables, trap sediment, create habitat for fish, amphibians, insects and birds, and reduce downstream flooding.
None of this is a side effect of eating or being eaten. The beaver reshapes the physical environment, and dozens of species benefit from a habitat that would not exist otherwise. Reintroduction projects in Europe have documented these changes appearing within a few years.
Why the concept matters for conservation
Conservation resources are limited, and the keystone idea suggests that protecting or restoring certain species delivers benefits far beyond that species alone.
It comes with caveats worth stating. Whether a species is keystone depends on context, and the same animal may be crucial in one system and unremarkable in another. Ecologists also warn against assuming any charismatic large animal is a keystone, since the label requires actual evidence of disproportionate effect rather than just importance.
Umbrella and indicator species
Conservation uses several related shortcuts, and they are worth distinguishing. An umbrella species has such large habitat requirements that protecting it incidentally protects many others sharing that range.
An indicator species is sensitive enough to environmental change that its condition signals the health of the wider system, which is why amphibians and lichens are monitored closely. A flagship species is chosen for public appeal to support fundraising and awareness. A single animal can be several of these at once, but the categories describe different roles and should not be used interchangeably.
The takeaway
A keystone species has an effect on its ecosystem far larger than its abundance suggests, whether through predation, engineering or mutualism, which is why removing one can restructure an entire community.