What is a keystone species?

A keystone species is any species that plays a critical role in how the ecosystem functions. Couldn't one argue that every species is a keystone species, playing an important role? Well, a keystone species plays a disproportionately large role in maintaining the ecosystem structure and function.

Sea otters are a great example of a keystone species

For example, let's look at sea otters. They're a common example of a keystone species. Why? Well, because they feed on urchins. Urchins feed on kelp. By preying on the urchin population, sea otters prevent urchins from consuming too much kelp. Kelp forests are home to many species, including invertebrates, fish, and marine mammals. In addition to being important habitat, lots of other animals feed in the kelp forests, such as seals and sea lions. Kelp forests can be destroyed by urchins, but the sea otters feed on urchins, preventing this from happening. Without the sea otters to feed on the urchin, the urchin might consume massive amounts of kelp, destroying important habitat feeding waters for other animals. So the effect of this one species on the ecosystem is very large, larger than one would expect for a single species.

Many predators function as keystone species, limiting the population growth of multiple omnivores/herbivores. Wolves eat deer and deer eat small trees, so wolves protect this new growth. Jaguars, tiger sharks, mountain lions and sea stars are all predators that function as keystone species.

Pacific salmon is another good example of a keystone species and not because of what this species does during its life but because its death is so important. The salmon life cycle involves salmon returning to the freshwater streams they were born in to spawn (males release sperm and females release ova into the water) and then die. Like anything that's dead, the salmon then decompose in these waters. Their death provides a significant amount of nitrogen to the watershed (the area of land where all of the rain or water under the land drains to, click here for more info).  Salmon runs, when salmon return to the freshwater they were born in and spawn themselves, provide a crucial source of nutrients to areas that otherwise might have low productivity, and they act as a food source for many animals, including grizzlies and eagles.

Red mangroves

But let's not forget our green friends. Plants can function as keystone species too. Red mangroves, Rhizophora mangle, are found at the edge of the water. Their roots act as a nursery to fish and crustaceans. Red mangroves provide habitat for many species, such as manatees, birds, and fish, They protect against erosion and act as buffers from large waves, literally structuring the land around them by preventing soil erosion and acting as anchors.

The concept of a keystone species was first introduced by Robert Paine in 1969, who studied sea stars and noticed that removing this key predator had cascading effects. Since Paine's work, we've come to realize the importance keystone species have in their communities. These important species are often targets for conservation efforts, as conserving them protects many species and ecosystems.

Food for thought: Do some research on the reintroduction of wolves to Yellowstone National Park. How did the arrival of wolves in 1995 change Yellowstone? How is this example of a keystone species connected to conservation in Yellowstone?

If you want to learn more, check out this great resource from Nature: Keystone Species.