Predators and prey

Predation, one animal killing and eating another, is among the strongest forces in evolution. Predators that catch prey more efficiently leave more offspring, and so do prey that escape more often. The result is an evolutionary arms race in which each side keeps adapting to the other.

This process explains many of the most striking features of animals: the cheetah's speed, the owl's silent flight, the tortoise's shell and the porcupine's quills.

• Speed: cheetahs can briefly exceed 100 kilometres per hour, but only in short sprints.
• Stealth: many cats, owls and snakes rely on ambush and camouflage rather than chase.
• Senses: eagles have extremely sharp vision, and pit vipers detect the body heat of prey.
• Teamwork: wolves, lions and orcas hunt in groups to take prey larger than themselves.
• Venom: snakes, spiders and cone snails subdue prey chemically.

Prey animals avoid being eaten in many ways. Camouflage helps them avoid being seen, while some harmless species mimic dangerous ones, like hoverflies that look like wasps. Poisonous animals, such as poison dart frogs, often advertise with bright warning colours, a strategy called aposematism.

Living in groups offers safety in numbers: more eyes spot danger, and each individual is less likely to be the one caught. Many prey species also have eyes on the sides of their heads for a wide field of view, and stotting, a stiff-legged bouncing jump by gazelles, may signal to predators that the animal is fit and not worth chasing.

Predator and prey numbers can rise and fall together in cycles. A classic example comes from fur-trading records of the Hudson's Bay Company, which showed Canada lynx and snowshoe hare populations rising and falling roughly every ten years.

When hares are abundant, lynx thrive and multiply. More lynx then eat more hares, and hare numbers fall, followed by a crash in lynx. Researchers have since found that hare food supplies also play a role, which shows how predator and prey dynamics are woven into the wider ecosystem.

Top predators can have effects that ripple down through the food web. Sea otters eat sea urchins; where otters were hunted out, urchins multiplied and grazed away kelp forests, and where otters returned, kelp forests recovered.

Wolves were eliminated from Yellowstone National Park by the 1920s and reintroduced in 1995. Elk numbers declined and their behaviour changed, and in some areas willows and aspens began to recover. Scientists still debate how strong these effects are, since weather, bears, hunting and other factors also matter, but the case shows that predators can influence far more than the animals they eat.

Large predators such as tigers, lions, wolves and sharks have declined sharply in many regions because of habitat loss, prey depletion and conflict with people over livestock and safety. Because they need large territories and reproduce slowly, they are especially vulnerable.

Conservation efforts try to reduce conflict through measures such as livestock guarding dogs, predator-proof enclosures, compensation for losses and wildlife corridors that connect habitats. Where these succeed, predators and people can share landscapes.