Keystone Species: How Animals Shape Entire Ecosystems

Ecosystems are held together by thousands of connections, yet at times a single species determines whether they remain stable or collapse. Biologists call these “keystone species”. Understanding them explains why wolves can alter rivers, beavers construct landscapes, and coral reefs die when one small control mechanism no longer works.

What experts mean by a keystone species

The term “keystone species” refers to a species whose influence on an ecosystem is far greater than its numbers alone would suggest. Remove it and the entire community of living organisms changes, often dramatically.

A keystone species is like an inconspicuous supporting pillar in a house: while it remains in place, it barely attracts attention. If it gives way, the whole structure collapses.

They are often apex predators, animals at the top of the food chain, such as wolves. When they disappear, prey populations can surge, plants are overgrazed, soils erode and rivers shift their course. Predators are not the only species able to play this part, however.

Species that receive little attention can also occupy a keystone position: burrowing rodents, habitat-building engineers such as beavers, or plants that provide shelter. Through their behaviour, they transform their surroundings so profoundly that countless other species depend on them.

Keystone allies: when two species support the system together

In many situations, several species share this supporting role. Researchers refer to them as “keystone mutualists”: species that benefit one another and, together, keep the system stable.

• Pollinators such as particular wild bees and their associated flowering plants
• Fig trees and their specialised fig wasps
• Corals and the algae living within their tissues

If one side of such a partnership fails, the other begins to falter too, along with an entire habitat.

Four striking examples of keystone species

Beavers: furry water engineers

Beavers are classic “ecosystem engineers”. They dam streams and build dams and lodges from timber, mud and stones. In doing so, they create ponds and wetlands that would not otherwise exist.

These newly formed wet areas provide habitat for:

• Amphibians such as frogs and newts
• Numerous insect species
• Fish that require calm water
• Water birds and small mammals

For centuries, people hunted beavers almost to extinction for their fur, meat and the scent substance castoreum. Only strict legal protection enabled their return. Entire wetlands returned with them, illustrating how closely human use of nature and ecosystem stability are connected.

Grey wolves: how a predator restores balance

The case of grey wolves in Yellowstone National Park is regarded as a textbook example of a keystone species’ power. Wolves were eradicated there during the 1920s. As a result, elk herds grew substantially, stripped young trees bare and caused riparian woodland to decline. Birds, beavers and many other species also became less common.

Wolves were reintroduced in 1995. Over the following decades, the picture changed:

• Elk numbers fell slightly and the animals became healthier because weaker individuals were killed first.
• Young willows and poplars were able to regrow.
• Beavers again had access to building material and new sites.
• Further insects, fish and birds returned alongside the wetlands.

Researchers call this a “trophic cascade” – a chain of knock-on effects running through every level of the food chain.

Even so, wolves remain controversial. In regions where livestock is grazed, conflict regularly resurfaces. This is precisely where short-term economic interests collide sharply with the long-term stability of ecosystems.

Prairie dogs: small rodents with enormous influence

At first glance, prairie dogs seem harmless and rather endearing. Yet in North American grasslands they are genuinely central figures. Their digging loosens the soil, affects water movement and creates a network of burrows.

These colonies benefit:

• Birds of prey and foxes that hunt prairie dogs
• Snakes and other animals that use the burrows
• Plants that grow more effectively in loosened soil

Studies show that biodiversity falls markedly where prairie dogs are systematically controlled. Entire grassland landscapes become poorer because a key building block has been removed.

Kelp forests at sea: when algae become architects

Plants – or, more precisely, large brown algae – can also be keystone species. So-called kelp forests grow along many coasts: towering stands of algae reaching from the seabed almost to the surface.

They form a three-dimensional underwater landscape. Its different “storeys” are inhabited by:

• Snails, mussels and sea urchins
• Crabs, prawns and octopuses
• Numerous fish species, from juvenile fish to predators

Kelp grows extraordinarily quickly and can withstand disruption surprisingly well, provided the pressure is neither permanent nor coming from several directions at once. Overfishing, pollution, warming waters and intensive seaweed harvesting can overwhelm even these robust systems.

The more pressures operate simultaneously, the more difficult recovery becomes – even for habitats that are usually resilient.

How keystone species shape entire landscapes

Whether in savannah, reef or forest, every ecosystem contains a small number of species carrying a disproportionate share of responsibility, often without being noticed.

Elephants in the savannah

In East African savannahs, elephants act as enormous landscape designers. They break shrubs, fell trees and create trampling-path-wide routes through the grass. This keeps open areas clear and prevents the savannah from becoming entirely overgrown with scrub.

Notably, the greatest plant diversity occurs in areas with moderate elephant populations. Where elephants have become scarce, for example because of poaching, only a few plant species dominate. Where their numbers are extremely high, they consume too much vegetation. The issue is therefore one of a shifting balance.

Coral reefs and parrotfish

Coral reefs are biodiversity hotspots. Corals themselves are animals and rank among the keystone species of these habitats. Their calcium carbonate skeletons create the framework into which countless other species fit.

One often overlooked hero within this system is the parrotfish. It eats algae from the coral surface and keeps it clear. Without this cleaning service, algae overgrow the corals and the reef tips into decline. At the same time, the fish produces new substrate through its “coral sand”.

Rising water temperatures, overfishing and pollution place immense pressure on corals. Many reefs are losing their vivid colours and increasingly resemble dead rocky deserts. They already show how severe collapse can be when several stress factors act at the same time.

What this means for climate, politics and everyday life

Keystone species also play a central role in the climate crisis. Intact kelp forests, coral reefs, wetlands and forests store enormous amounts of carbon. When one of these systems collapses, more CO₂ enters the atmosphere, creating a vicious circle.

Researchers identify four core aims for effective nature conservation:

• Preserve biodiversity rather than protecting only individual symbols
• Identify and conserve keystone species specifically
• Connect habitats so species can move and migrate
• Coordinate conservation measures from local to international level

Remarkably, around 80 per cent of global biodiversity is found in areas managed by Indigenous communities. Their knowledge of landscapes, animal migration routes and appropriate use is often much more finely tuned than modern administrative plans.

Some experts describe humans themselves as a “hyper-keystone species”. No other living being intervenes so forcefully in so many ecosystems through agriculture, fishing, industry, transport and consumption. The question, then, is not whether we exert influence, but how consciously we do so.

Initial steps can be put into practice easily in everyday life: eat less meat from intensive grazing systems, shop with fish sustainability in mind, protect nearby wetlands, and support politically the restoration of nature and protected areas. These choices may appear modest, but they help precisely those species on which entire networks depend.