Ecology: Lecture Ecology: Lecture 1616

Parasitism and Mutualism

Lecture overviewLecture overview

Basics of parasitism Characteristics of parasites Hosts as habitat Life cycles Dynamics of transmission Host responses

Basics of parasitismBasics of parasitism

Defined: Condition whereby two organisms live closely together and one derives its nourishment (or other resource) at the expense of the other.

Impacts on individuals How do parasites affect their hosts? What determines the degree of

impact?

Basics of parasitismBasics of parasitism

Impacts on populations What did early ecologists think

about the role of parasites? What did Aldo Leopold hypothesize? Current: Many studies show effects

of parasites on Host’s birth, death and growth rates Host’s mating success Host’s susceptibility to predation

Characteristics of Characteristics of parasitesparasites

Key groups of parasites Micro vs.

macroparasites Location of parasites

Ectoparasites: outside, may burrow (i.e.ticks, fleas, gill parasites)

Endoparasites: live inside body (i.e. tapeworms)

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ParasitoidsParasitoids

Intermediate between predators and parasites Eventually kill their host, but it takes

many to kill one (and only one…)

Parasitoid wasps on caterpillar

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Hosts as habitatHosts as habitatLocation as related to Location as related to

needs/strategiesneeds/strategies

Trematode parasites in snail gonads Lipid-rich environment!

Photos: USGS “Soundwave” program

Hosts as habitatHosts as habitatLocation as related to Location as related to

needs/strategiesneeds/strategies

Tapeworms in digestive tract Absorbs ready-

to-use nutrients! No digestive

tract of its own

Hosts as habitatHosts as habitatLocation as related to Location as related to

needs/strategiesneeds/strategies Schistosoma “blood fluke” in

intestinal blood vessels Direct access to nutrients Location by intestine: eggs released into

intestine/exit with feces

University of YorkDepartment of Biology

Host entryHost entry

Via food and drink Trichinosis: Eating

raw or undercooked pork

Cholera: contaminated water supply and food Campbell, Reese “Biology”

M. Courtney-Clarke, Photo Researchers, Inc.

Host entryHost entry

Via burrowing Schistosoma

mansoni burrows through feet or ankles of wading person

Host entryHost entry

Via insect bite Plasmodium, the

malaria parasite, enters from the salivary glands of the mosquito into the bloodstream

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Host entry: CandiruHost entry: Candiru

Via body passageways…

Host exitHost exit

Via the feces Schistosoma mansoni

Host exit: bot flyHost exit: bot fly

Host entryHost entry

Via insect bite Malaria: A few

Plasmodium in, many Plasmodium out…

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Life cycle: single hostLife cycle: single host“Direct transmission”“Direct transmission”

Single host; may exit host Mechanisms of transfer

Direct host-to-host contact (lice) Bites (rabies) Vectors: transfers parasite from host to

host Example: Bot fly uses mosquito vector for

transfer to mammalian host… Mosquito not infected.

NOTE: For malaria, the mosquito is infected; it is a secondary host as well as a vector.

Life cycle: multiple hostsLife cycle: multiple hosts“Indirect transmission”“Indirect transmission”

Human Definitive host:

location of sexual reproduction

Snail Intermediate host:

asexual reproduction only

See outline and own notes for more detail

Dynamics of transmissionDynamics of transmission

Direct transmission tends to favor high population densities.

Exception: introduced parasites may initially spread rapidly due to lack of developed defenses.

In these cases, high rate of spread may be independent of density

Dynamics of transmissionDynamics of transmission

Multiple host parasites: Success linked to Effectiveness of transfer Availability of both species to complete

life cycle

Dynamics of transmissionDynamics of transmission

Advantages of multiple hosts One host scarce; parasite can persist in other

host Both asexual and sexual reproduction occur

Why an advantage to have both types?

Disadvantages of multiple hosts Disruption of transmission prevents completion

of life cycle Schistosomiasis can be prevented by wearing waders

Loss of one host eventual crash of the parasite population. Eradication of mosquitoes reduces malaria.

Host response to Host response to parasitismparasitism

Biochemical Inflammation Immune response

In some cases, hosts can become resistant to the parasite (Schistosomiasis)

Abnormal growths Cysts may form around the invading

parasite (Plant galls [Fig. 17.10])

Galls on plantsGalls on plants

Host response to Host response to parasitismparasitism

Sterility Example: parasite within snail gonad

tissue Also see text for nematode parasite in

fungus-eating flies. Behavioral changes

Example 1: Ant parasitized by the liver fluke Dicrocoelium dendriticum

Example 2: Killfish infected with a particular trematode (fluke relative)

Mutualism (briefly)Mutualism (briefly)

Defined: A relationship between two species in which both benefit

Types of mutualistic relationships Obligate symbiotic mutualism: a

permanent and obligatory relationship where it is sometimes difficult to tell where one organism ends and the other begins Example: coral animals and their protist

(zooxanthellae) symbionts

Obligate symbiotic mutualism: Obligate symbiotic mutualism: coral reefscoral reefs

Reef and coral photos courtesy of NOAA

Coral: a closer lookCoral: a closer look

MutualismMutualism

A brief survey of mutualistic relationships (cont.)

Obligate non-symbiotic mutualism: two organisms live physically separate lives, but cannot survive without each other Example: Pollination (some cases)

Non-obligatory (facultative) mutualism Example: Seed dispersal by animals

(sometimes) Defensive mutualism

Example: Alkaloid-producing fungus that lives within grass