Chapter 12 Summary

CONCEPT 12.1 Most predators have broad diets, whereas a majority of herbivores have relatively narrow diets.

• Over half of the organisms on Earth are predators, herbivores, or parasites that sustain themselves by feeding on other organisms.
• Many predators move about their habitat in search of prey; others, known as sit-and-wait predators, remain in one place and attack or trap prey that move within striking distance.
• Most predators do not specialize on particular prey species; instead, they typically eat prey in relation to their availability across a broad range of prey species.
• Many herbivores specialize on particular plant parts, such as leaves (the most common food source), roots, stems, seeds, or internal fluids.
• Herbivorous insects, which constitute a majority of herbivore species, tend to have relatively narrow diets, feeding on only one or a few plant species.

CONCEPT 12.2 Organisms have evolved a wide range of adaptations that help them capture food and avoid being eaten.

• In response to strong selection pressures exerted by predators and herbivores, prey species and plants have evolved a rich variety of defensive mechanisms.
• Prey may rely on physical defenses, toxins, mimicry, or behavioral responses to escape predation.
• Plants cope with herbivory via masting and other forms of avoidance, compensation (a form of tolerance), and secondary chemicals that deter herbivores.
• Predators have a wide range of adaptations for overcoming the defenses of their prey, including physical features, toxins, mimicry, and tolerance or detoxification of prey toxins.
• Although a plant’s defensive mechanisms prevent most herbivores from eating it, typically some herbivores can overcome the plant’s defenses via structural, chemical, or behavioral means.

CONCEPT 12.3 Predation and herbivory affect ecological communities greatly, in some cases causing a shift from one community type to another.

• Predators and herbivores may have direct effects on the distributions and abundances of their food organisms as well as indirect effects on other species in their communities.
• Predators can cause dramatic declines in the distributions and abundances of their prey; similarly, herbivores can decimate their food plants.
• Exploitation can also alter the composition of ecological communities, in some cases changing one community type to another.

CONCEPT 12.4 Population cycles can be caused by feeding relations, such as a three-way interaction between predators, herbivores, and plants.

• Results from mathematical models, laboratory experiments, and field observations suggest that population cycles can be caused by exploitation.
• Coexistence of predators and prey can be influenced by several factors, including dispersal and habitat complexity.
• Evolutionary change can affect predator–prey population dynamics, in some cases producing unusual population cycles.