A keystone species fills a role that no other species can and has a major impact on ecosystems. They have low functional redundancy, meaning that their disappearance could cause an ecosystem to radically change. How Might You Add Keystone Species to the Concept Map?
Robert Paine coined the term keystone species when he studied a tidal pool and observed the effects of predatory sea stars on mussels and other marine organisms. He physically removed the sea stars to see what would happen.
1. They are a source of food for other species
A keystone species is usually a predator but can also be herbivores or mutualists. For example, bees are a keystone species because they take nectar from flowers and then spread pollen from one flower to another, increasing the chances of fertilization. They also help plants grow by spreading nutrients like nitrogen and phosphorus.
Often, keystone species have an influence that is out of proportion to their population or biomass. This is because they have a strong impact on other species in an ecosystem. For example, a study by zoologist Robert Paine found that when he removed Pisaster ochraceus sea stars from tidal areas on Tatoosh Island, Washington, the ecosystem changed drastically in just a few years.
Besides being important sources of food, keystone species can also serve as habitat builders. For example, sagebrush in the Western United States provides shelter and cover for wildlife and adds moisture to the soil. In addition, keystone species can be “nutrient vectors,” transferring nutrients from one habitat to another. Grizzly bears that eat salmon, for example, can leave behind carcasses that resorb and fertilize the soil.
2. They are a source of pollination
Species like bees and hummingbirds are keystone pollinators, providing vital services for plants such as vegetables, fruits, and crops that make our lives possible.
Keystone species may also act as “ecosystem engineers,” modifying or shaping their habitats. For example, grizzly bears that prey on salmon can deposit carcasses in the soil, fertilizing it with nutrients not available locally.
The keystone species concept arose from the work of American zoology professor Robert Paine, who in 1966 conducted an experiment along the rocky Pacific coast. He removed top predator species — sea stars (Pisaster ochraceus) — from a section of the tidal plain on Tatoosh Island in Washington state, and observed that the mussel population quickly exploded. Without the sea stars to keep them in check, mussels crowded out other species such as barnacles and limpets, and wiped out the biodiversity of the tidal plain within months.
3. They are a source of habitat
A keystone species is a creature that is hardly replaceable in its ecosystem, which means other creatures are dependent on it. This makes it important in a network and is called its importance-uniqueness value (TF5), or TI.
Several categories of keystone species exist and are often divided into predators, ecosystem engineers, and mutualists. A typical example would be a predator that keeps a prey population in check; if this animal were removed from the ecosystem, the prey species could suddenly explode in numbers and put immense strain on the next organism down the food chain.
A great example of a keystone species is a beaver, which creates habitat in river ecosystems by removing trees along the banks. This enables other species to thrive in the area. Without beavers, the ecosystem is at risk of collapse and invasive species may invade the region. (See this fascinating video about a wolf-beaver pair in the Greater Yellowstone Ecosystem of Montana, Idaho and Wyoming).) A similar example of an ecosystem engineer is a crocodilian, like the saltwater crocodile.
4. They are a predator
While predators are commonly considered keystone species, prey species can also serve this role. For example, sea otters control populations of the sea urchin that destroy kelp forests in Pacific Northwest ecosystems, which are vital habitats for crabs, snails, fish, and other marine animals. Using the concept map, you can represent these creatures as central hubs with numerous incoming and outgoing arrows that link them to other organisms in their ecosystem. You can even place them with heavier weighting to highlight their outsized influence.
Another important feature of keystone species is that they have low functional redundancy – if they were to disappear, no other species could fill their ecological niche and maintain the balance of the ecosystem. This is especially true of apex or dominant species, like sharks in reef ecosystems. They keep populations of herbivorous fish, which graze on algae, in check so the reef can thrive. Without these top-down predators, the reef would degrade rapidly.