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Chapter 13 Outline

Case Study: Enslaver Parasites

Parasite Natural History

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

Ectoparasites live on the surface of their host
Endoparasites live inside their host
Endoparasitism and ectoparasitism have advantages and disadvantages

Defense and Counterdefense

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

Host immune systems and biochemical defenses can protect against parasites
Parasites have adaptations that circumvent host defenses

Coevolution

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

Selection can favor a diversity of host and parasite genotypes
There are costs to both host defenses and parasite counterdefenses

Ecological Effects of Parasites

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 drive host populations to extinction
Parasites can influence host population cycles
Parasites can alter ecological communities

Dynamics and Spread of Diseases

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

For a disease to spread, the density of susceptible hosts must exceed a critical threshold