What are keystone species and why do we need them?

When wolves were hunted to near extinction in parts of the US and Europe, the environment rapidly changed. Deer or elk no longer had a predator, allowing them to overgraze and trample vegetated areas where they once dared not tread. Higher populations of the hoofed mammal altered landscapes and destroyed habitats that supported other species such as songbirds. Soil erosion caused rivers to change course, impacting marine ecosystems. 

This outsized impact on biodiversity is the common trait of keystone species. But as many become endangered through hunting or habitat loss, the environment they once regulated is suffering.

What exactly is a keystone species?

In the early 1960s, US ecologist Robert Paine removed starfish from an area of coast and watched as the mussels that the sea star — which are actually predators — usually ate became a monoculture. Marine species like flowering anemones and shellfish were soon wiped out, causing Paine to realize the importance of starfish in regulating seashore biodiversity.

He referred to the predator as a keystone species, describing it as analogous to the keystone in an archway that holds all the other stones in place.

Keystone species are often thought of as predators at the top of the food chain like wolves. But some are way down the bottom. Krill — among the most plentiful food sources on the planet — are Antarctic shrimp that regulate the food web of the Southern Ocean by providing feed for diverse species from whales to penguins and birds. Without krill, whole ocean ecosystems would be thrown out of balance.

Other keystone species are known as ecosystem engineers — like beavers who build dams, creating deep pools and habitats for young fish, turtles and frogs.

Keystone decline and biodiversity impacts

Many keystone species are endangered or threatened, including jaguars in the Americas, or ivory tree coral in the Caribbean, which provides essential food and shelter for thousands of invertebrate and fish species.

The jaguar that once ranged from Mexico to Argentina is the largest feline in the Americas and an apex predator. Such animals at the top of the food chain keep herbivores like deer and giant capybara rodents in check. This preserves vegetation and limits soil erosion, but vitally ensures that prey species don't become a monoculture.

Extinct in Uruguay and El Salvador and critically endangered in Argentina, with only round 200 remaining, the jaguar's importance as a keystone species has inspired a program to rewild Argentinian wetlands by reintroducing the feline for the first time in 70 years.

Elephants, meanwhile, aren't predators but rather ecosystem engineers who maintain the savanna grasslands of Africa. As they clear shrubs and uproot small acacia trees, elephants maintain habitat for grazing animals such as antelope, impala and gazelle. And when the largest land mammal digs into soil to create waterholes, it allows species such as zebras and giraffes to survive drought.

But these vital ecosystem functions are being lost as elephants become endangered due to poaching for ivory.

Sphagnum mosses might be much less conspicuous, but they are also a keystone species due to their ability to maintain the peatlands that regulate climate. Bog moss, which is a species of bryophyte, is a thick spongy plant that retains water during drought and helps slow down decomposition in peatlands, locking in planet-heating CO2. 

This keystone climate function is on the decline, however, with 22.5% of bryophyte species threatened in Europe alone. "Without these bryophytes, peatland ecosystems cannot function effectively," stated the United Nations Convention on Biodiversity.

What happens when keystone species return?

The sea otter is a keystone species and central cog in coastal ecosystems that was nearly driven to extinction in the 19th century. But after hunting otters for fur was banned internationally in 1911, populations of the cuddly creatures grew, especially along the western coast of North America.

Returned otters fed on the sea urchins and crustaceans that had overrun kelp forests in the absence of a predator. The undersea forests could regenerate and provide habitat for diverse ocean species, including fish and invertebrates like squid and shrimp. Protected sea otters are today maintaining biodiverse marine life from California to Alaska.

Wolves, meanwhile, had been hunted out of what is today Yellowstone National Park some 70 years before they were reintroduced in 1995. The idea was to rebalance a struggling ecosystem.

The reintroduction of the predator into the park located mostly in the US state of Wyoming had rapid biodiversity benefits.

Its presence limited the range and impact of grazing elk and deer, allowing diverse vegetation and trees such as willows and aspen to recover. Animal carcasses left behind by wolf kills provided food for a diversity of species such as ravens, eagles and bears. Meanwhile, as coyotes retreated from wolf territories, other small predators and rodents could benefit, according to the US National Park Service.

With hoofed animals less dominant and destructive, soil erosion rapidly decreased, allowing riverbank biodiversity to regenerate. This in turn brought back ecosystem building beavers.

"It is like kicking a pebble down a mountain slope where conditions were just right that a falling pebble could trigger an avalanche of change," noted Doug Smith, a wildlife biologist who leads the Yellowstone Wolf Project.

Edited by: Tamsin Walker and Jennifer Collins