Chapter 15 — Parasitism & Mutualism

What is symbiosis?

An intimate, long-term association between two or more species; may be positive, negative, or neutral.

What is coevolution?

Reciprocal evolutionary changes between interacting species.

What is parasitism?

A relationship where the parasite benefits and the host is harmed.

How do parasites increase their fitness?

By exploiting hosts for food, habitat, and dispersal.

Do parasites usually kill the host?

No — they typically reduce host fitness but do not immediately kill.

How do parasites differ from their hosts?

They are smaller, specialized, and reproduce faster.

Are mosquitoes parasites? Why or why not?

No. Their feeding is episodic (noncontinuous), so they are not parasites.

What is cleptoparasitism?

Stealing food gathered by another organism.

What is a heavy load of parasites called?

An infection.

What is the outcome of an infection?

Disease

What are microparasites?

Small parasites with short generation times (viruses, bacteria, protozoans).

What are macroparasites?

Large parasites with long generation times (worms, lice, ticks, fungi).

What are parasitic plants?

Plants that extract nutrients from other plants.

What is a haustorium?

A modified parasitic root that penetrates the host plant.

What are hemiparasites?

Photosynthetic parasitic plants that rely on the host for water/nutrients.

What are holoparasites?

Nonphotosynthetic parasitic plants fully dependent on host.

What are ectoparasites?

Parasites that live on the host’s exterior (skin, feathers, hair).

What are endoparasites?

Parasites living inside the host (bloodstream, tissues, gills).

What must parasites do to infect hosts?

Gain entry and escape; move within host to point of infection.

What is direct transmission?

Parasite passed directly between hosts (contact, air, water).

Microparasites are usually transmitted how?

Directly (e.g., influenza, smallpox).

What is an example of direct macroparasite transmission?

Roundworm eggs expelled in feces.

What are vectors?

Organisms that transmit parasites between hosts.

Example of a vector in Lyme disease?

Black-legged tick.

Example of a vector in malaria?

Female Anopheles mosquito.

What is the definitive host?

The host where the parasite becomes an adult.

What is an intermediate host?

Host where parasite undergoes developmental stages.

Example of parasite with multiple hosts?

Meningeal worm: deer = definitive host; snails/slugs = intermediate.

Host behavioral defenses?

Grooming and preening.

What is the inflammatory response?

Histamine release, increased blood flow, inflammation.

Why do scabs or cysts form?

To isolate parasites.

How do parasites reduce host fitness?

Lower growth, lower reproduction, altered behavior.

Example of behavior change in infected hosts.

Killifish infected with trematodes swim erratically.

What happens when virulence is too high?

Host dies too soon → parasite dies too; selection favors lower virulence.

What is vertical transmission?

Parasites passed from mother to offspring.

Why does vertical transmission select for low virulence?

Parasite must keep host alive until reproduction.

Examples of parasites regulating populations.

Chestnut blight, Dutch elm disease, rinderpest, avian malaria.

What is commensalism?

One species benefits, the other is unaffected.

What is mutualism?

Both species benefit.

What is facultative mutualism?

Mutualism that is helpful but not required for survival.

What is obligate mutualism?

Species cannot live or reproduce without partner.

What are symbiotic mutualists?

Species living tightly together (corals & zooxanthellae).

Example of nonsymbiotic mutualism.

Pollination or seed dispersal.

What are the benefits of mutualism?

Resources, protection, reduced competition, dispersal.

Example of mutualism in nutrient transfer.

Ruminants with gut microbes.

How do Rhizobium bacteria benefit plants?

Fix atmospheric nitrogen into usable forms.

What do bacteria gain in legume mutualisms?

Carbon and nutrients from plant.

What are mycorrhizae?

Fungal–plant root mutualisms enhancing water & nutrient uptake.

What are ectomycorrhizae?

Fungi forming networks outside root cells.

What are endomycorrhizae (arbuscular)?

Fungi penetrating plant root cells.

How do plants attract pollinators?

Color, scent, nectar.

What are frugivores?

Fruit-eating animals that disperse seeds.

What are myrmecochores?

Plants whose seeds are dispersed by ants.

Example of mutualism in seed dispersal.

Clark’s nutcracker & whitebark pine.

How can mutualism influence population dynamics?

Mutualists increase each other’s carrying capacity and population growth.