Chapter 13 Summary

CONCEPT 13.1 Parasites, which constitute roughly 50% of the species on Earth, typically feed on only one or a few host species.

• Parasites, including both macroparasites and microparasites, typically feed on only one or a few host individuals during the course of their lives; in addition, many parasites are closely adapted to particular host species.
• Some parasites are ectoparasites that live on the surface of their host; others are endoparasites the live within the body of their host.
• Endoparasitism and ectoparasitism each have advantages and disadvantages. It is easier for ectoparasites or their offspring to disperse from one host individual to another; however, ectoparasites are at greater risk from natural enemies than are endoparasites.

CONCEPT 13.2 Hosts have adaptations for defending themselves against parasites, and parasites have adaptations for overcoming host defenses.

• Many host organisms have immune systems that allow them to recognize and defend against endoparasites. Biochemical conditions inside the host’s body can also provide protection against parasites.
• In some birds, mammals, and fishes, females select their mates based on traits that indicate whether a male is well defended from parasites.
• Parasites have a broad suite of adaptations that allow them to circumvent host defenses, from relatively simple counterdefenses against encapsulation to more complex counterdefenses that involve hundreds of parasite genes.

CONCEPT 13.3 Host and parasite populations can evolve together, each in response to selection imposed by the other.

• Host–parasite interactions can result in coevolution, in which populations of the host and parasite evolve together, each in response to selection imposed by the other.
• Selection can favor a diversity of host and parasite genotypes. A rare host genotype may increase in frequency because few parasites can overcome its defenses; as a result, parasite genotypes that can cope with the now-common host genotype’s defenses may also increase in frequency.
• Host–parasite interactions can exhibit trade-offs in which a trait that improves host defenses or parasite counterdefenses reduces other aspects of the organism’s growth, survival, or reproduction.

CONCEPT 13.4 Parasites can reduce the sizes of host populations and alter the outcomes of species interactions, thereby causing communities to change.

• Parasites can reduce the abundances of host populations, in some cases driving local host populations to extinction or even changing the geographic distributions of host species.
• Evidence suggests that infection by parasites can cause host populations to cycle in abundance.
• Parasites can affect the outcomes of interactions between their hosts and other species; for example, a species that is a dominant competitor may become an inferior competitor when infected by a parasite.
• The effects of parasites can alter the composition of ecological communities and change features of the physical environment.

CONCEPT 13.5 Simple models of host–pathogen dynamics suggest ways to control the establishment and spread of diseases.

• Some models of host–pathogen population dynamics subdivide the host population into susceptible individuals, infected individuals, and recovered and immune individuals; track different host and pathogen genotypes; and take into account factors such as host age, latent periods, and vertical transmission.
• A simple mathematical model of host–pathogen dynamics (see Equation 13.1) yields an important insight: for a disease to become established and spread, the density of susceptible hosts must exceed a critical, threshold density.
• To control the spread of a disease, efforts may be made to lower the density of susceptible hosts (by slaughtering domesticated animals or undertaking vaccination programs) or to raise the threshold density (by increasing the recovery rate or decreasing the transmission rate).