SUMMARY

CONCEPT 13.1 Parasites typically feed on only one or a few host species, but host species have multiple parasite species.

13.1.1 Know why parasites are abundant and typically specialists.

Parasites usually feed on only one or a few host individuals during the course of their lives.

Parasites typically live on or in their hosts and feed on certain parts of the host's body.

Most host species are attacked by more than one parasite and their parasites have parasites.

13.1.2 Compare and contrast ectoparasites and endoparasites.

Some parasites are ectoparasites that live on the surface of their host; others are endoparasites that live within the body of their host.

Advantages of ectoparasitism include ease of dispersal from one host individual to another and avoidance of the host's immune system.

Advantages of endoparasitism include avoidance of natural enemies, ease of feeding, and protection from the external environment.

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

13.2.1 Describe the mechanisms organisms use for defending themselves against parasites.

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, as can defensive symbionts such as fungi and bacteria.

13.2.2 List the mechanisms that parasites use to circumvent host defenses.

Parasites have a broad suite of mechanisms that allow them to circumvent host defenses, from relatively simple counterdefenses against encapsulation to more complex counterdefenses that involve hundreds of genes.

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

13.3.1 Understand how host-parasite interactions can result in coevolution.

Host-parasite interactions show coevolution, in which populations of the host and parasite evolve together, each in response to selection pressure 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 respond to the new host genotype’s defenses may also increase in frequency.

13.3.2 Know why host-parasite interactions can result in life history trade-offs.

Host-parasite interactions can exhibit trade-offs in which a trait that improves host defenses or parasite counterdefenses has costs that reduce other aspects of the organism’s growth, survival, or reproduction.

CONCEPT 13.4 Hosts and parasites can have important effects on each other’s population dynamics.

13.4.1 Understand how parasites can influence the population dynamics of hosts.

Parasites can reduce the abundances of host populations, in some cases driving local host populations to extinction or changing the geographic distributions of host species.

Evidence suggests that parasites can also cause cycling of hosts’ populations.

13.4.2 Explain how simple models of host-pathogen dynamics can be used 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 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).

CONCEPT 13.5 Parasites can alter the outcomes of species interactions, thereby causing communities to change.

13.5.1 Know how parasites can affect the outcome of species interactions and community structure.

Parasites can affect the outcomes of interactions between their hosts and other species; for example, a host that is a dominant competitor may become an inferior competitor when infected by a parasite or a host may become more vulnerable to predation under these conditions.

The effects of parasites can alter the composition of ecological communities and change features of the physical environment.

13.5.2 Describe how climate change might influence host­pathogen relationships.

Ongoing climate change will likely cause the incidence of diseases to rise through both direct and indirect changes in the

abundances and distributions of pathogens.

REVIEW QUESTIONS

1. Define endoparasites and ectoparasites, giving an example of each. Describe some advantages and disadvantages associated with each of these two types of parasitism.

2. Given the effects that parasites can have on host individuals and host populations, would you expect that parasites could also alter the outcomes of species interactions and the composition of ecological communities? Explain.

3.

a. Summarize the mechanisms that host organisms use to kill parasites or reduce the severity of their attack.

b. With your answer to (a) as background material, do you think the following statement from a news report could be true?

The parasite has a mild effect on a plant species in Australia, but after it was introduced for the first time to Europe, it had devastating effects on European populations of the same plant species.

Explain your reasoning, and illustrate your argument with an example of how a plant defensive mechanism might work—or fail to work—in a situation such as this.

HONE YOUR PROBLEM-SOLVING SKILLS

As we learned in Concept 13.5, leishmaniasis is a serious disease caused by protists that reside in and are spread by sand flies. The disease is found both in humans and in rodents in the genus Neotoma (see Figure 13.20).

1. Why would you expect that reducing the infection by leishmaniasis of Neotoma rodents would have an effect on similar infection in humans?

2. To reduce the spread of leishmaniasis in Neotoma rodents, the control program recommends reducing the threshold density, S_t, in rodent populations. What is meant by the concept of a threshold density for the establishment and spread of a disease? Why is this concept important?

3. Suppose the control program determines that the threshold density (S_t) of leishmaniasis in Neotoma populations is 5,000 individuals. Using the data in the table, determine which Neotoma rodent populations will need to be reduced, and by how much, to decrease leishmaniasis infection.

4. What other measures besides reducing the host population sizes of both rodents and sand flies can be proposed to decrease the infection prevalence in humans?

LIST OF KEY TERMS

coevolution ectoparasite endoparasites host macroparasites microparasites parasites parasitoids pathogens threshold density