Answers to Review Questions

1. Ectoparasites live on the surface of their host, whereas endoparasites live inside the body of their host. Examples of ectoparasites include plants such as dodder and fungi such as rusts and smuts; examples of endoparasites include tapeworms and bacterial pathogens such as Mycobacterium tuberculosis.

2. Parasites can greatly reduce the growth, reproduction, or survival of host individuals, thereby reducing the growth rate of host populations. As a result, we would expect that parasites could also alter both the outcomes of species interactions and the composition of ecological communities. For example, if two plant species compete for resources and one typically outcompetes the other, a parasite that reduces the performance of the superior competitor may cause a competitive reversal in which the inferior competitor becomes the superior competitor. Such changes in the outcome of species interactions can cause changes in the relative abundances of the interacting species, thus altering the ecological community.

3.

a. Host organisms have a wide range of defensive mechanisms that include a protective outer covering, an immune system that kills or limits the effectiveness of the parasite, and biochemical conditions inside the host's body that reduce the ability of the parasite to grow or reproduce.

b. The statement could be true if the plant populations in Australia possessed specific defensive features that limited the ability of the parasite to grow or reproduce, yet the populations in Europe lacked such adaptations. Among many other possible examples, plants in the Australian populations might possess a specific allele that enabled them to kill or disable the parasite—hence causing the parasite to have mild effects there—whereas plants in the European populations might lack this allele, making them more vulnerable to parasite attack.