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.