In his 1850 poem In Memoriam A.H.H., Alfred, Lord Tennyson wrote of “Nature, red in tooth and claw.” The phrase has endured as a powerful description of the brutality of wild nature. Thanks to nature documentaries and our human fascination with wild animals, most of us have witnessed plenty of images of this red-toothed nature: a lion chasing down a zebra, a hawk scooping a mouse into its talons, a shark sinking its teeth into a sea lion.

Predator–prey interactions are important in every ecosystem on Earth. After all, every living animal can be classified as either “predator” or “prey”—and often, both labels apply. To a fly, a toad is a fearsome predator. To a snake, that same toad may be choice prey.

Very few species enjoy the luxury of having no natural predators; these are called top predators. For the vast majority of animals, the threat of predation is simply a fact of life. Not surprisingly, that threat has been a powerful evolutionary force. Predator–prey dynamics have influenced both the evolution of individual organisms and the shape of the ecosystems in which they live.

On Isle Royale, an island in Lake Superior, ecologists have been studying moose and wolves since 1958 in the longest-running study of a single predator–prey system in the world. The moose arrived on the island about 100 years ago, presumably by swimming about 15 miles from the nearest shoreline. They arrived to find a predator-free paradise, and the moose population quickly exploded. Then, about 1950, a pair of wolves crossed an ice bridge to Isle Royale. As the wolf population grew, the moose population declined, until a delicate balance was established.

As a predator population grows, the prey population shrinks—a seemingly logical relationship. But predator–prey interactions are complex and sometimes even counter-intuitive. Removing a top predator can actually reduce biodiversity. Sea stars, for instance, prey on mussels in the rocky intertidal zone of the Pacific Northwest. When sea star numbers fall, mussel numbers increase. The mussels crowd out other species, such as barnacles and seaweed, which compete for space on the rocks. The result is a drop in the overall number of species in that habitat.

The same patterns have been found among birds of prey in the Italian Alps. These raptors—including the goshawk and four types of owl—are the top predators of their food chains. Spanish researchers found that locations where the birds were present had a greater diversity of trees, birds, and butterflies than did comparable control sites.

One reason that the predator and prey populations eventually achieve a delicate balance is that both are well adapted to their roles. Predation has exerted powerful evolutionary pressure that has influenced anatomy and physiology over the long term. That pressure has shaped predators by giving them claws, teeth, venom, and powerful muscles for hunting prey.

Predator–prey dynamics have influenced both the evolution of individual organisms and the shape of the ecosystems in which they live.

Of course, predation pressure has also shaped the form and function of prey. Some animals, including some insects and frogs, have evolved toxins to deter would-be predators. These toxic species often exhibit warning colors that tell the predators to steer clear. Other prey animals have evolved camouflage colors and forms to help them hide from hungry carnivores. Still others have developed protective behaviors, such as living together in herds for security against predation.

Often, predator and prey evolve in lockstep, driving each other’s adaptations. Clams may have evolved thicker shells to protect themselves from hungry crabs. In turn, the crabs evolved larger, stronger claws for cracking clamshells. This pattern of back-and-forth change is often described as an evolutionary arms race. And it has happened time and time again.

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The interactions between eaters and eaten have also influenced the evolution of physiological systems. The skills that a predator needs to hunt its prey and that its prey needs to escape depend in large part on adaptations of their sensory systems, musculoskeletal systems, nervous systems, and even their circulatory and respiratory systems.

Consider sensory systems. Animals rely on visual, auditory, tactile, and chemical stimuli to warn them of approaching predators—or to guide them to suitable prey. A bat relies on sonar to locate insects. A spider responds to the flutter of silk when an insect becomes ensnared in its web. Gazelles are always attentive, watching and listening for signs of a cheetah or lion in the grass.

When predators are nearby, prey species experience fear and anxiety. Stress hormones produced by prey animals can influence an animal’s physiology in a number of ways. In snowshoe hares, levels of the stress hormone cortisol increase when predators such as lynx and coyote are plentiful. The hormones trigger behaviors, such as alertness and fearfulness, which help the hares avoid becoming a lynx’s lunch. But the behavioral benefit comes at a cost: Research has shown that stressed hares give birth to fewer and smaller offspring.

In short, predation has left a physical imprint on both predator and prey, from nose to tail—their body shapes, their behaviors, their physiology, even their muscle fibers have been influenced over time by predator–prey interactions.

In the modern world, our own species, Homo sapiens, occupies a unique position at the top of the food web. While humans are occasionally killed by animals such as bears or sharks, we are, for the most part, no longer constrained by the fear of predation by other species. But it wasn’t always so. Some scientists have proposed that fear of predation among early humans led to the evolution of cooperative social behavior and large brains. Others have theorized that it was our hunting of other animals that led us to evolve big brains and the ability to work together. Perhaps both factors played a role. Either way, the importance of the predator–prey system can’t be ignored. To understand our roots, it seems, we must take a good look at nature, red in tooth and claw.

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CASE 7 QUESTIONS

Special sections in Chapters 35–41 discuss the following questions related to Case 7.