What Is an Apex Predator?

An apex predator occupies the highest trophic level in its ecosystem. Energy flows from plants to herbivores to smaller predators and finally to it, with losses at each step, which is why apex predators are always relatively rare.

The definition is contextual rather than absolute. An animal can be an apex predator in one habitat and prey in another, and many are apex only as adults, since young individuals are vulnerable to a range of threats.

Apex predators exist in every ecosystem type:

• •Orcas and great white sharks in the ocean
• •Lions, tigers and wolves on land
• •Golden eagles and harpy eagles in the air
• •Saltwater crocodiles in rivers and estuaries
• •Polar bears across the Arctic ice

Apex predators exert top down control on ecosystems, an effect often called a trophic cascade. By limiting herbivore numbers and, just as importantly, changing herbivore behaviour, they affect vegetation, soil and water.

The behavioural part is easy to overlook. Prey animals avoid areas where predation risk is high, so grazing is spread out rather than concentrated. That landscape of fear can shape plant growth even when relatively few animals are actually killed.

The best known case is the reintroduction of wolves to Yellowstone National Park in 1995, after a seventy year absence. Elk numbers fell and elk behaviour changed, with less time spent browsing in exposed valleys and river banks.

Willow and aspen recovered in those areas, beavers returned, and river banks stabilised. The story is often told in a simplified form, and ecologists have pointed out that other factors including drought, bears and human hunting also contributed. The core finding still stands: removing or restoring a top predator changes far more than the prey species.

Apex predators are disproportionately at risk of extinction. They exist at low densities, need large territories, reproduce slowly and often come into direct conflict with people over livestock and land.

They also accumulate pollutants. Because contaminants concentrate at each step up the food chain, a process called biomagnification, apex predators end up with the highest burdens of persistent chemicals, which is why they are often used as indicators of ecosystem health.

Ecologists sometimes describe humans as a super predator, and the comparison is unflattering. Studies of harvest rates find that humans take adult prey at rates several times higher than other predators, and target large adults rather than the young, old and sick.

That difference matters ecologically. Most predators remove individuals that contribute least to a population's future, while human hunting and fishing often removes the largest breeding adults. This drives measurable evolutionary change, including declining body size and earlier maturation in heavily fished populations.

Removing an apex predator does not only affect herbivores. It can also release smaller predators that the top predator used to kill, chase away or compete with. Ecologists call this mesopredator release. With the larger predator gone, animals such as foxes, raccoons, feral cats or coyotes can become more numerous and more active, and because they eat small mammals, birds, eggs and reptiles, the effects can fall hardest on those smaller species. In some cases this has been linked to declines in ground-nesting birds and other vulnerable prey.

Researchers have studied this idea in several places. In North America, areas where wolves returned have been found to hold fewer coyotes, which in turn can benefit animals coyotes hunt. In Australia, some studies suggest that where dingoes are common, introduced foxes and cats are kept in check, which may help small native mammals. Evidence like this is often debated because many factors change at once, including habitat, human activity and disease. Even so, it shows why conservationists consider the whole predator community, not just the largest species.

People often use apex predator and keystone species as if they meant the same thing, but they describe different ideas. Apex refers to position: nothing preys on the animal as an adult. Keystone refers to influence: removing the species changes its community far more than its abundance would suggest. Many apex predators are keystone species, yet some keystone species sit in the middle of the food chain, and a few are not predators at all, such as beavers that reshape rivers with their dams.

Sea otters on the Pacific coast of North America show the difference clearly. Otters eat sea urchins, and urchins graze on kelp. Where otters are present, urchin numbers stay low and kelp forests flourish, sheltering fish and many other species. Where otters disappear, urchins can strip the kelp and leave bare rocky barrens. Yet otters are not apex predators, since orcas and sharks can eat them. Researchers in Alaska linked otter declines in the 1990s partly to increased predation by orcas, with knock-on losses of kelp.

Watching what an animal eats is slow and often impossible, especially in the ocean, so ecologists also use chemistry. Nitrogen comes in a common light form and a slightly heavier stable form, nitrogen-15, and animals tend to retain more of the heavier form than they excrete. As a result, each step up a food chain raises the proportion of nitrogen-15 in body tissue by a fairly predictable amount, roughly three to four parts per thousand. A small sample of muscle, fur, feather or blood can therefore reveal roughly how high an animal feeds, while carbon isotopes add clues about where its food came from.

This approach turns trophic level into a number. Plants are level 1, animals that eat only plants are level 2, and an animal with a mixed diet falls between whole numbers. Large ocean predators such as orcas and some sharks score among the highest values, well above 4. A study published in 2013 estimated the global average human trophic level at about 2.2, similar to that of an anchovy or a pig, because most human food comes from plants. That makes an instructive contrast with the idea of humans as super predators: our impact comes from the scale of our hunting and fishing, not from eating high up the chain.

Much of the theory began with a simple experiment on the rocky coast of Washington State. From 1963 the ecologist Robert Paine regularly prised ochre sea stars, the top predator of the shore, off one stretch of rock and threw them into deeper water, leaving a nearby stretch untouched for comparison. Without the sea stars, mussels spread unchecked and crowded out barnacles, limpets, seaweeds and other residents, and the number of species in the plot fell sharply. In 1969 Paine used this work to introduce the term keystone species, borrowing the image of the central stone that holds an arch together.

Testing the landscape of fear directly is harder, but researchers on small islands off British Columbia found a way. Large carnivores had largely disappeared there, and raccoons foraged boldly along the shore. For a month the team played recordings of barking dogs, standing in for large predators, on some shorelines and harmless seal and sea lion calls on others. Where raccoons heard dogs, they spent far less time foraging, and the crabs, fish and worms they normally ate became noticeably more abundant. Published in 2016, the experiment showed that fear alone, without any killing, could ripple down a food chain.

An apex predator sits at the top of its food chain with no natural predators as an adult, and because it controls both the numbers and the behaviour of everything below it, losing one can restructure an entire ecosystem.