Ch 8 Negative species interactions— infection and parasitism Pathogens and parasites have negative impacts on species Compartment models are useful for analyzing how diseases affect populations – Parameters of compartment models – epidemics Pathogens and parasites affect individual organisms by reducing reproductive output and increasing mortality – Effects on reproduction – Effects on mortality Diseases can reduce populations – Brucellosis in ungulates – Rabies in wild mammals – Myxomatosis in the European rabbit Pathogens can become more or less virulent through evolution, and their hosts can evolve resistance – Evolution of virulence – Coevolution in disease systems 1

Parasites Among Us Symbionts are organisms that live in or on other organisms. – A parasite consumes the tissues or body fluids of the organism on which it lives, its host. – Pathogens are parasites that cause diseases. More than half of the millions of species that live on Earth are symbionts.

Figure 13.3 The Human Body as Habitat

Size – Macroparasites are large, such as arthropods and worms. – Microparasites are microscopic, such as bacteria. Location – Ectoparasites live on the outer body surface of the host. – Endoparasites live within the host, in the alimentary canal, or within cells or tissues Parasite Basics Concept 13.1: Parasites, which constitute roughly 50% of the species on Earth, typically feed on only one or a few host species.

Figure 13.4 Many Species Are Host to More Than One Parasite Species

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Defense and Counterdefense Host organisms have many kinds of defense mechanisms. Protective outer coverings include skin and exoskeletons. Many parasites that do gain entry are killed by the host’s immune system. Concept 13.2: Hosts have adaptations for defending themselves against parasites, and parasites have adaptations for overcoming host defenses.

Figure 13.8 Using Plants to Fight Parasites

Coevolution When a parasite and its host each possess specific adaptations, it suggests that the strong selection pressure hosts and parasites impose on each other has caused both of their populations to evolve. Concept 13.3: Host and parasite populations can evolve together, each in response to selection imposed by the other.

Figure Coevolution of the European Rabbit and the Myxoma Virus (Part 1)

Figure Coevolution of the European Rabbit and the Myxoma Virus (Part 2)

Figure Adaptation by Parasites to Local Host Populations

Ecological Effects of Parasites Reduce survival or reproduction of host Alter species interactions Alter population cycles Concept 13.4: Parasites can reduce the sizes of host populations and alter the outcomes of species interactions, thereby causing communities to change.

Figure Parasites Can Reduce Host Reproduction (Part 1)

Figure Parasites Can Reduce Host Reproduction (Part 2)

Figure Parasites Can Reduce Their Host’s Geographic Range

Figure Parasites Can Alter the Outcome of Competition

Figure Parasite Removal Reduces Host Population Fluctuations

Applications: Disease and Pest Control Concept 13.5: Simple models of host– pathogen dynamics suggest ways to control the establishment and spread of diseases.

Containing Disease Key is keeping the number of susceptible individuals below the threshold. Susceptible domestic animals are sometimes slaughtered to reduce density, especially if the disease can also affect humans, such as bird flu. Quarantines may also be imposed

Figure Vaccination Reduces the Incidence of Disease

Figure Determining Threshold Population Densities

Other Types of Parasites Nest parasites Parasitoids