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If you were strolling along Washington state’s Makah Bay shoreline in 1963, you might have come across an odd site — a man using a crowbar to pry ochre sea stars from tidal pool rocks and hurtling them into the ocean. That man was Robert Paine, a University of Washington ecologist and professor whose research on the bay’s tidal pools led to a groundbreaking understanding of how the loss of a single species can drastically alter an ecosystem. Paine later created the label “keystone species” to define species that are critical to other organisms' survival within the system.
When an ecosystem loses one or more of its keystone species, it sets off a negative chain of events that alters the system’s structure and biodiversity, often leading to its destruction. When traveling to a new destination, it's important to take note of some of the incredible plants and animals around the world that are paramount to Earth's continued growth and biodiversity. Here are 14 keystone species and why they are significant to our planet.
What Are Keystone Species?
Keystone species can be animals, plants, bacteria, and even fungi that have a disproportionately powerful influence on an ecosystem. Keystone species aren’t typically the largest or most abundant species, but help control populations of other organisms or supply food and shelter.
When Paine removed the sea stars (a predatory starfish that fed on barnacles attached to rocks), the barnacle and mussel populations significantly increased in his test pools. The mussels crowded out algae, limpets, and other organisms, which reduced the food supply for other species. Within a year, the tidal pools’ diversity was halved — some species fled to nearby habitats, and some died out completely. Ecosystems worldwide lose their keystone species due to natural causes, climate change, pollution, over-hunting, or other damages caused by humans.
Different Types of Keystone Species
Like the sea stars in Paine’s study, some keystone species are predators that control populations, while others create or change habitats in beneficial ways. Other keystone species can be organisms that have symbiotic (mutually beneficial) relationships. Scientists often group keystone species into categories by type, and some fall into multiple categories. Here’s a list of keystone species broken down by type.
Paine discovered another predatory keystone species in Washington’s waters — the sea otter. Sea otters love to eat sea urchins, which feed on kelp. Kelp forests provide shelter, food, nursery habitats, and hunting grounds for various marine organisms. When sea otter populations diminished in kelp forests, sea urchin populations expanded, and they decimated kelp forests at a rate of up to 30 feet per month.
Sharks prey on sick and weakened fish species and scavenge the seafloor for dead carcasses. Sharks control the spread of disease and prey on other predatory fish, allowing populations of herbivorous fish farther down the food chain to survive. Herbivorous fish eat algae that can smother coral. Sharks also patrol seagrass beds in search of turtles, which feed on the seagrass. If too many turtles populate a grass bed, they can destroy the habitat for the small marine creatures that call it home.
These predators keep elk and other herbivore populations in check, preventing them from overgrazing and destroying habitats for other organisms that rely on the meadows in the Greater Yellowstone Ecosystem (GYE). Late in the 19th century, the U.S. government eliminated gray wolves from this diverse ecosystem that spans Montana, Wyoming, and Idaho to protect ranchers’ livestock and other herds.
The removal of gray wolves in the region started a top-down trophic cascade, an ecosystem change that occurs when an apex predator joins or leaves. The elk population exploded, and grasses and reeds couldn’t grow fast enough to keep up with their grazing needs. The lack of food also impacted other species such as birds, fish, and beavers. Physical changes also occurred when stream banks lost the plants and root systems that kept their soil intact, altering wetland habitats. Wolves have been reintroduced to the GYE, and as elk populations shrink, other species are making a comeback.
Not all predators are carnivores — African elephants control the tree populations in savannas such as the Serengeti plains in Tanzania. These massive, powerful herbivores eat shrubs and even topple and uproot small trees, allowing grasses to thrive. If the tree population remained unchecked and the savannas transformed into forests, dozens of species such as zebras, antelopes, and wildebeests that rely on the grasslands for survival would suffer. The number of predators, such as hyenas and lions that prey on grazing animals, would also diminish.
Animals that are considered prey can also be keystone species because they provide a balance between predators and prey.
Unlike many predator keystone species that aren’t abundant in a habitat, snowshoe hares are plentiful in Boreal forests that surround the northern latitudes just below the Arctic Circle. The hares serve as the primary food source for many predators, such as the endangered Canada lynx, foxes, coyotes, hawks, and owls. Studies have found a 10-year fluctuation cycle of hare, lynx, and willow populations. When hare populations grow, the animals overgraze on willows. Lynx populations increase as they prey on the bounty of hares. As hare numbers decrease, willows regrow. But when hare numbers become too low, lynx populations fall. Then the cycle slowly repeats.
Ecosystem engineers are organisms that create, significantly modify, maintain, or destroy a habitat. Two types of ecosystem engineers exist: one alters the environment as it grows, and the other physically transforms the environment.
As trees grow, their branches and leaves create habitats and provide food for other species, such as birds, squirrels, and insects. Trees are keystone species in many ecosystems. Red mangrove trees play a dual role — their exposed roots hold shorelines intact and reduce erosion, as well as provide a haven for juvenile fish and other small creatures.
Like trees, this iconic desert cactus provides habitats and nesting opportunities for birds such as hawks and woodpeckers. Bats, bees, and birds feed on their flower nectar and pollen. The cactus' juicy fruit is a staple food during the dry season for many organisms.
As coral polyps grow, they leave behind their hard exoskeleton, which causes reefs to form. Millions of species rely on reef ecosystems for survival, from the tiniest algae to sponges, seahorses, anemones, crustaceans, sea turtles, and large fish and fish predators, such as sharks and dolphins. Coral reefs also protect coastlines from damaging storm waves and erosion. More than half a billion people rely on coral reefs for food, income, and protection. Corals are an absolutely essential keystone species for marine ecosystems.
These industrious furry builders chew through old or dead trees that line riverbanks and use them to create dams. Removing the old or dead trees allows healthier new trees to thrive. Beaver dams divert river flow, which creates vital wetland habitats for a variety of plants and animals.
These plump little rodents burrow miles of tunnels under grasslands, which encourages water filtration, aerates the soil, and distributes nutrients. They feed on the grasses, preventing overgrowth and allowing other plant species to share the habitat. The healthier habitat enhances diversity as more wildlife, insects, and birds move in.
One of the grizzly bear’s favorite foods is salmon from the river. If salmon populations significantly increase, they overgraze on seaweed and destroy the food supply and habitat for other river species. Grizzly bears (and wolves) drag fish carcasses far into the forest to consume them. The leftovers decompose and fertilize the soil with nutrients that would otherwise be unavailable, enhancing the growth of trees and shrubs. The bears also dig up roots, which aerates and mixes the soil. They also distribute berry seeds and pine nuts in their feces.
These tiny insects create surprisingly massive mounds, with some piles reaching almost 100 feet tall. The mounds can last for decades, so many animals such as barbets and mongeese build nests within the mounds to raise their young. The mounds also contain essential nutrients for plants and trees, such as acacias.
When two or more species engage in reciprocal interactions that benefit both, they are considered mutualists.
Bees and Hummingbirds
Both organisms collect nectar from flowers, and while doing so, pollen attaches to their bodies. As they move from flower to flower, they help spread pollen, enhancing the plants’ chances for fertilization and growth. Bees play an enormous role in pollinating fruits, vegetables, and other crops humans and animals consume. The plants they pollinate also help produce seeds, berries, nuts, and fruits that countless species rely on for survival. Bees and hummingbirds benefit because they rely on the plants' pollen and nectar for food.
Consequences of Lost Keystone Species
Often, a keystone species’ value to an ecosystem isn’t entirely recognized until it’s gone. In some instances, such as the gray wolf, the species can be reintroduced, and the ecosystem’s health improves. Coral reefs are of particular concern because they play such a vital role in marine ecosystems that millions of species, including humans, need to thrive and survive.