Transcript Chapter 12. Parasitism

Chapter 14. Parasitism
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What’s a parasite? – hard to define
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Intimate contact (feed off host)
Usually do not kill host (parasitoids do)
Herbivores(?)
Parasitic Plants
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Holoparasites (lack chlorophyll) – Rafflesia (biggest flower)
Hemiparasites (photosynthesize) – Mistletoe
– Microparasites – reproduce inside host
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Bacteria, viruses
– Macroparasites – release juvenile outside
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E.g. trematodes
– Ectoparasites vs. endoparasites
“Weird” Parasites
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Nest Parasites
– Brownheaded Cowbird
– European Cuckoo
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Sexual Parasites
– Gynogenetic fishes
Amazon molly
 Resided/Finescale Dace hybrid
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Parasitism Common
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Possibly more parasites than anything
else
– 50% of insects parasitic
– Potentially 4:1 parasites:free-living forms
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Often complex life cycles
– E.g. lancet fluke, other trematodes
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Several intermediate hosts
Modeling Parasitism
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Complex because of intermediate hosts, and infection
rate
– Not usually sensitive to “actual” r for parasite (this is gigantically
high)
– Important variables:
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Rp – number of infected hosts
If Rp > 1 then parasite spreads
– For microparasites
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Rp = NBL
N – density of susceptible hosts
B – transmission rate of parasite
L – length of time host is infectious
– Nt (host pop. size) = 1/BL (if Rp = 1)
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Critical host density (upshot is disease cycles as Nt reached by
recruitment)
Effects on natural populations
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Introduced parasites – large effect
– Chestnut blight, Dutch elm etc.
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Natural systems
– Dodder (Cuscuta) – plant parasite – may act to
maintain diversity
– Fuller and Blaustein – deer mice
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Found infected had lower overwinter survival
– Hurtrez-Bousses – microwaved blue tit nests
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Found higher size at fledging and lower failure rate
– Red Grouse
Community Effects
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Brainworm – host is white-tailed deer
– Not much effect
– All other cervids and pronghorns
susceptible
– “apparent competition” – as white-tailed
deer expand range, other species affected
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Other examples of effects
– Flour beetles, Anolis lizards
Biocontrol
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Some success (about 16%)
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E.g. myxoma and rabbits in Australia
Evolution of reduced virulence
– How much of the rest deleterious uncertain
– Pesticides degrade in environment
– Introduced parasites remain
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Switch hosts?? Cause other problems?
Some advocate shotgun approach
Some advocate “targeted” approach
– I think – last-ditch effort (and maybe not even
then)
Mutualism
Both species benefit
 Plant-pollinator
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– Often tightly coevolved relationships
E.g. figs and fig wasps – 900 species of figs,
each with its own pollinating wasp
 Yucca plants and yucca moths
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– Perhaps each trying to “cheat”?
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Reciprocal parasitism?
Seed Dispersal
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Fruits attract dispersers
– Color, smell, abundance etc.
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Hypotheses for seed dispersal
– Reduced competition
– Colonization hypothesis
– Directed dispersal hypothesis (ants)
– Predator escape hypothesis
Variety of Mutualisms
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Resources
– Leaf cutting ants/fungus
– Nitrogen fixing bacteria / plants
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Protection
– Cleaner fish and “customers”
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Some are mimics (cheaters)
– Ants and aphids
– Ants and acacia trees (herbivory)
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Obligate mutualisms
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Lichens (algae and fungus)
Ruminants/bacteria
Deep sea fishes/luminescent bacteria
Corals/zooxanthellae
Endosymbiont theory
Modeling Mutualism
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Similar to Lotka-Volterra comp. eqns.
– Replace negative effect with positive
– Change K to X (carrying capacity is raised)
Can become weird (unstable) or can
become stable when facultative
 Obligate mutualisms even more
unstable (though obviously there are
stable areas)
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Indirect effects on community
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Mycorrhizal fungi / plants
– Reduce herbivory
Increased vigor
 Increased antiherbivore defenses
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– Increased mycorrhizal diversity can be
positive for community
– Or…introduced mutualists can outcompete (endophytes in Indiana)
Commensalisms
Cattle egrets/cattle
 Clinging seeds and hosts
 Flower mites and hummingbird nostrils
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