Ecology: Lecture 16

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Ecology Lecture 16

• Parasitism and Mutualism

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Lecture overview

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

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Basics 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?

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Basics 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
• Hosts birth, death and growth rates
• Hosts mating success
• Hosts susceptibility to predation

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Characteristics of parasites

• 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)

British Broadcasting Company (BBC)
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Parasitoids

• Intermediate between predators and parasites
• Eventually kill their host, but it takes many to
  kill one (and only one)

Parasitoid wasps on caterpillar WSU Cooperative
Extension
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Hosts as habitatLocation as related to
needs/strategies

• Trematode parasites in snail gonads
• Lipid-rich environment!

Photos USGS Soundwave program
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Hosts as habitatLocation as related to
needs/strategies

• Tapeworms in digestive tract
• Absorbs ready-to-use nutrients!
• No digestive tract of its own

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Hosts as habitatLocation as related to
needs/strategies

• Schistosoma blood fluke in intestinal blood
  vessels
• Direct access to nutrients
• Location by intestine eggs released into
  intestine/exit with feces

University of York Department of Biology
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Host 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.
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Host entry

• Via burrowing
• Schistosoma mansoni burrows through feet or
  ankles of wading person

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Host entry

• Via insect bite
• Plasmodium, the malaria parasite, enters from the
  salivary glands of the mosquito into the
  bloodstream

www.solcomhouse.com
www.membranetransport.org
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Host entry Candiru

• Via body passageways

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Host exit

• Via the feces
• Schistosoma mansoni

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Host exit bot fly
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Host entry

• Via insect bite
• Malaria A few Plasmodium in, many Plasmodium
  out

www.solcomhouse.com
www.membranetransport.org
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Life cycle single hostDirect 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.

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Life cycle multiple hostsIndirect transmission

• Human
• Definitive host location of sexual reproduction
• Snail
• Intermediate host asexual reproduction only
• See outline and own notes for more detail

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Dynamics 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

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Dynamics of transmission

• Multiple host parasites Success linked to
• Effectiveness of transfer
• Availability of both species to complete life
  cycle

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Dynamics 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.

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Host response to parasitism

• 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)

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Galls on plants
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Host response to parasitism

• 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)

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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

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Obligate symbiotic mutualism coral reefs

• Reef and coral photos courtesy of NOAA

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Coral a closer look
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Mutualism

• 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