28.11: Predator-Prey Interactions

Predators consume prey for energy. Predators that acquire prey and prey that avoid predation both increase their chances of survival and reproduction (i.e., fitness). Routine predator-prey interactions elicit mutual adaptations that improve predator offenses, such as claws, teeth, and speed, as well as prey defenses, including crypsis, aposematism, and mimicry. Thus, predator-prey interactions resemble an evolutionary arms race.

Although predation is commonly associated with carnivory, for example, cheetahs hunting gazelles, a closely related type of interaction exists. Herbivory is the consumption of plants by animals known as herbivores. Plants typically deter herbivores by employing an array of defenses, including morphological defenses like an acacia tree’s thorns, and chemical defenses such as a milkweed’s toxins. However, some herbivores evolve adaptations to bypass plant defenses. Giraffes, for example, have long, dexterous tongues that allow them to consume the acacia’s leaves while avoiding its thorns. Monarch butterfly caterpillars evolved immunity to milkweed toxins, and instead ingest milkweed to store the toxins in their tissues as a defense against their own predators.

Predator and prey population sizes can increase and decrease in cycles, due in part to predation. For instance, the lynx and snowshoe hare populations in northern Canada cycle about every 10 years, with the lynx population changes lagging 1-2 years behind the hare population. As the hare population increases, the lynx population—which prefers to feed on snowshoe hares—increases as well. However, as lynx capture hares, the hare population begins to decline. Scarcity of hares eventually reduces the lynx population, allowing hares to thrive and the cycle to repeat. Other factors, such as vegetation availability and predation by other predators, also impact the hare population cycle by limiting its peak population size and growth rate.

All organisms need energy to survive. Gazelles, for example, are herbivores that feed on vegetation, while cheetahs are carnivores that consume gazelles. This type of interaction is called predation, where one organism, the predator, gains energy by consuming another organism, the prey.

A predator's acute senses, such as sight, smell, and hearing, enable them to detect prey. Specialized physical features, such as teeth or claws, facilitate the capture and consumption of prey.

Routine predator-prey interactions cause prey to evolve traits that allow them to escape detection or capture.

For example, most birds evolved exceptional eyesight, including color vision, to hunt prey. A defense known as crypsis allows prey to avoid visual detection by blending in with their environment. Larva and adult peppered moths, for instance, evolved body and wing colorings that closely match their host trees' branches and bark.

Rather than hide, monarch butterflies advertise their toxicity with ornate, brightly-colored wings. Such warning coloration, or aposematism, acts as a prey's visual cue to predators that it is hazardous or inedible. Monarch caterpillars ingest milkweed, which makes them toxic as adults. Birds that ignore the monarch's warning experience its bad taste or become nauseous and avoid future contact.

When a species' aposematism is effective, other species that share the same predators may evolve to copy, or mimic, that coloration.

Mullerian mimicry occurs when harmful species with similar aposematic appearances share the costs of predator education. Viceroy butterflies, for example, are toxic and closely mimic the monarch's appearance. Predators that try one species learn to avoid the other, rather than risk another unpleasant feeding experience.

Alternatively, Batesian mimicry occurs when a harmless species mimics a harmful species. Predators typically steer clear of the bright, tricolor, banded pattern observed on venomous coral snakes. Non-venomous king snakes exploit this by mimicking the coral snake's appearance.

Predator-prey interactions resemble an arms race. As prey evolve to avoid predation, predators evolve in response, like the cheetah's enhanced speed to better capture prey. Such reciprocal natural selection between interacting species is known as coevolution.