Parasites

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Parasites

• Chapter 12

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Defining Parasites

• Parasite a predatory organism that feeds off
  another but generally does not kill it.
• Host prey of a parasite.

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Defining Parasites

• Parasitoid A specialized insect parasite that is
  usually fatal to its host and therefore might be
  considered a predator rather than a classical
  parasite.

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Defining Parasites

• Some parasites live with their host most of their
  lives (e.g., tapeworms).
• Some parasites drop off after prolonged periods
  of feeding (e.g., ticks, leeches).

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Defining Parasites

• Are mosquitoes and wildebeests parasites?

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Defining Parasites

• Some flowering plants are parasitic on other
  plants.
• Holoparasites lack chlorophyll, and are totally
  dependent on another plant for water and
  nutrients.

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Defining Parasites

• Hemiparasites photosynthesize, but do not have a
  root system, so they rely on the host for water
  nutrient uptake.
• Ex. Mistletoe.

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Defining Parasites

• Monophagous parasites that feed off one to three
  closely related species.
• Polyphagous parasites that feed off many host
  species.

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Defining Parasites

• Ectoparasites live on the outside of the host's
  body (e.g., fleas and ticks).
• Endoparasites live inside the host's body (e.g.,
  tapeworms and bacteria).

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Defining Parasites

• Haustoria plant parasite outgrowths that
  penetrate inside a host plant to tap into it's
  nutrient supply.
• Dodder

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Many parasites use multiple hosts fluke.
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• Parasites outnumber free-living species 4 to 1.

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Defense Against Parasites

• Cellular defense reactions.
• Eggs of parasitoids are rendered unviable by
  encapsulating them.

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Defense Against Parasites

• Immune responses in vertebrates
• Phagocytes may engulf and digest small alien
  bodies, and encapsulate and isolate larger ones.
• Hosts may develop a "'memory,"' that may make
  them immune to re-infection.

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Defense Against Parasites

• Defensive displays or maneuvers.
• Actions intended to deter parasites.
• Grooming and preening behavior.
• Behavior intended to remove ectoparasites.

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Modeling Parasitism

• Differ from models of predation and herbivory.
• Life cycles of many parasites involves
  intermediate hosts.
• Models of parasite population dynamics generally
  describe the population growth rate by the
  average number of new disease cases.

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Modeling Parasitism

• For microparasites, the number of infected hosts
  is the most important factor.

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Modeling Parasitism

• Parasites spread by a vector.
• Lifecycle of both parasite and vector become
  important in controlling diseases.
• Ex. Farmers use insecticides to kill aphids,
  which transmit viral diseases to crops (rather
  than chemicals to kill the parasite).
• Ex. Yellow fever was eradicated in the US by
  inoculation rather than eradication of all
  mosquitoes.

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Parasites Affect Host Populations

• Using biological control to study the effects of
  parasites on hosts.
• Ex. Hawkins 1999 Biological control of pests,
  especially by parasitoids, was greater in exotic,
  simplified, managed habitats than in natural
  habitats.

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Parasites Affect Host Populations

• Control is most often exerted by a single
  parasitoid species, in contrast to natural
  systems, which require a suite of generalized
  enemies.

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Parasites Affect Host Populations

• Thus, biological control projects can not provide
  rigorous evidence of the importance of parasites
  in natural systems.

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Parasites Affect Host Populations

• Effects of introduced parasites on natural
  systems.
• Chestnut blight in the Appalachian Mountains of
  North America.
• Virtually eliminated chestnut tree.
• Introduced in New York in 1904.

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Parasites Affect Host Populations

• In Britain, 25 million elm trees (out of 30
  million) were wiped out by Dutch elm disease
  between 1960s and the 1990s.

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Parasites Affect Host Populations

• Rinderpest,
• A virus with at least 47 natural artiodactyls
  hosts, most of which occur in Africa.
• The virus belongs to a class known as
  morbilliviruses, which includes measles and
  distemper.

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Parasites Affect Host Populations

• Spread by food and water contaminated by dung of
  sick animals.
• Can be fatal to certain animals (buffalo, eland,
  kudu, and warthogs).

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Parasites Affect Host Populations

• Major epidemic swept through Africa in the 1890s,
  leaving vast areas uninhabited by certain
  species.
• 80 of hoofed stock died. Disease traveled 5,000
  km in eight years.
• Brought under control in the 1960s, through the
  use of cattle vaccinations.
• Endangered species.

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Parasites Affect Host Populations

• Many endangered species are threatened by
  diseases from domestic animals.
• Ex. The demise of the marsupial wolf in Tasmania
  was because of a distemper-like disease obtained
  from domestic dogs.
• Some endangered species have been given
  vaccinations to protect them from disease.
• Mountain gorillas were vaccinated for measles.

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Parasites Affect Host Populations

• Natural systems
• Massive mortality of big horn sheep from
  infection by lungworms (Protostrongylus stilesi
  and P. rushi ).
• Predisposes animals to pathogens, which cause
  pneumonia.
• Infection rates of 91 and mortalities of 50-75
  have been reported.

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Parasites Affect Host Populations

• Colorado pine tree plantations and mistletoe.
  Mistletoe can cause 30 loss in extractable
  timber.

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Parasites Affect Host Populations

• Saline marshes in North America and the plant
  parasite, marsh dodder, Cuscuta salina.

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Parasites Affect Host Populations

• Infects the most common plant in California
  marshes, Salicornia virginica thus promoting the
  growth of two other species, Limonium and
  Frankenia.

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Parasite Removal Experiments

• Fuller and Blaustein (1996) compared the
  survivorship of parasite infected and uninfected
  free-living deer mice.
• Conducted in large outdoor enclosures.
• Decreased over-winter survivorship for those deer
  mice infected with the protozoan Eimeria
  arizonensis.
• Contamination spread through the digestion of
  contaminated feces.

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Parasite Removal Experiments

• Hurtrez-Bousses et al. (1997) reduced the number
  of blowfly larvae parasites in young blue tits in
  Corsica.
• Blowfly larvae suck blood from chicks, causing
  anemia and high mortality.
• Removal was accomplished by removing nests from
  nest boxes, and microwaving the nests to kill the
  parasites, and then returning the nests and
  chicks.

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Parasite Removal Experiments

• Chicks from microwaved nests were found to have
  greater body weight at fledging.

Success!
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Parasite Removal Experiments

• Stiling and Rossi (1997) manipulated parasitic
  infection levels of a gall-making fly on a
  coastal plant, Borrichia frutescens, on isolated
  islands off the coast of Florida.

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Parasite Removal Experiments

• Low rates of parasitism treatment.
• Allowed potted plants on one island to be
  colonized by gallflies.
• Plants were removed before parasitoids could find
  them.
• High rates of parasitism treatment.
• They left plants on the island longer, to allow
  the parasites to colonize the galls.

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Parasite Removal Experiments

• Results High degree of parasitism of gallflies
  resulted in a significant reduction in the number
  of new galls.

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Parasites Affect Host Populations

• The degree of mortality the parasite inflicts on
  the host depends on the age of the association
  between the two.
• Older associations are thought to cause less harm
  and may even evolve toward commensalisms or
  mutualisms.

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Parasites Affect Communities

• Parasites affect the presence or absence of
  various species in a community.

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Parasites Affect Communities

• The meningeal brainworm Parelaphostrongylus
  tenuis.
• Usual host is the white-tailed deer, which is
  tolerant of the infection.
• All other cervids and the pronghorn antelope are
  potential hosts.
• Worm causes severe neurological damage.

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Parasites Affect Communities

• The worm makes the white-tailed deer a potential
  competitor with other cervids, because they can
  not survive in the same area as the white-tailed
  deer. This phenomenon is known as apparent
  competition.

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Parasites Biological Control

• Not all parasites are detrimental to humans.
• Many are used to protect crops from pests
  Biological control.
• Only 16 of classical biological control would
  qualify as economic successes.

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Parasites Biological Control

• Organisms used in biological control, are
  released in a 'hit or miss' technique Just
  release a bunch of parasites and predators, and
  hope that one of them does the job.
• New techniques Ex. novel parasite-host
  associations.
• Review of 548 control projects the more
  parasites that were released, the lower the rate
  of establishment.

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Parasites Biological Control

• Necessary attributes of a good agent of
  biological control (Huffaker and Kennett 1969).
• General adaptability to the environment and host.
• High search capacity.

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Parasites Biological Control

• High rate of increase relative to the host's.
• General mobility adequate for dispersal.
• Minimal time lag effects in responding to changes
  in host numbers.

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Parasites Biological Control

• Methods affecting the success of biological
  control (Stiling 1990).
• Factor of greatest importance climatic match
  between the control agent's locality of origin
  and the region in which it will be released.

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Parasites Biological Control

• Importance of climatic variation was underscored
  by another review of biological failures (Stiling
  1993).
• Climate accounted for 34.5 of the failures.

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Parasites Biological Control

• Risks of biological control.
• Reduction in native Hawaiian butterflies moths
  was partly due to wasp species introduced for
  biological control of lepidopteran crop pests.
• Wasps attacked target exotic species but also
  non-target native species.
• Stresses the importance of more narrowly focused
  release, rather than the traditional "hit and
  miss" technique.

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Parasites Biological Control

• Risks of biological control.
• Problems with crops.
• While it is of interest to ensure that the agent
  does not adversely affect the crop, however,
  non-crop plants are not as vigorously tested.

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Summary

• The true definition of parasite is problematic.
  Parasites may include many species that feed on
  plants, plus more traditional parasites such as
  tapeworms, leeches, bacteria, viruses and
  parasitoids that attack animals. 80 of all life
  forms are considered parasitic.

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Summary

• The presence of various parasite defense
  mechanisms is testament to the importance of
  parasitism in nature.

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Summary

• Mathematical models suggest that effective
  parasites will keep their hosts alive as long as
  possible, thereby facilitating the transmission
  of parasites to additional hosts.

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Summary

• The huge influence of introduced diseases, such
  as chestnut blight and Dutch elm disease, provide
  evidence for the severe effects that parasites
  can have on a host's population and host density.

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Summary

• Parasites of insects can often be used as control
  mechanisms against crop and forest pests. This
  technique is termed biological control. Finding
  the attributes of successful biological control
  agents is valuable. Unfortunately, biological
  control agents can have a significant impact on
  non-target natural populations.