Download Chapter 12. Parasitism

Chapter 14. Parasitism

What’s a parasite? – hard to define
–
–
–
–
Intimate contact (feed off host)
Usually do not kill host (parasitoids do)
Herbivores(?)
Parasitic Plants


Holoparasites (lack chlorophyll) – Rafflesia (biggest flower)
Hemiparasites (photosynthesize) – Mistletoe
– Microparasites – reproduce inside host

Bacteria, viruses
– Macroparasites – release juvenile outside

E.g. trematodes
– Ectoparasites vs. endoparasites
“Weird” Parasites

Nest Parasites
– Brownheaded Cowbird
– European Cuckoo

Sexual Parasites
– Gynogenetic fishes
Amazon molly
 Resided/Finescale Dace hybrid

Parasitism Common

Possibly more parasites than anything
else
– 50% of insects parasitic
– Potentially 4:1 parasites:free-living forms

Often complex life cycles
– E.g. lancet fluke, other trematodes

Several intermediate hosts
Modeling Parasitism

Complex because of intermediate hosts, and infection
rate
– Not usually sensitive to “actual” r for parasite (this is gigantically
high)
– Important variables:


Rp – number of infected hosts
If Rp > 1 then parasite spreads
– For microparasites




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)

Critical host density (upshot is disease cycles as Nt reached by
recruitment)
Effects on natural populations

Introduced parasites – large effect
– Chestnut blight, Dutch elm etc.

Natural systems
– Dodder (Cuscuta) – plant parasite – may act to
maintain diversity
– Fuller and Blaustein – deer mice

Found infected had lower overwinter survival
– Hurtrez-Bousses – microwaved blue tit nests

Found higher size at fledging and lower failure rate
– Red Grouse
Community Effects

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

Other examples of effects
– Flour beetles, Anolis lizards
Biocontrol

Some success (about 16%)


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



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

– 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

– Perhaps each trying to “cheat”?

Reciprocal parasitism?
Seed Dispersal

Fruits attract dispersers
– Color, smell, abundance etc.

Hypotheses for seed dispersal
– Reduced competition
– Colonization hypothesis
– Directed dispersal hypothesis (ants)
– Predator escape hypothesis
Variety of Mutualisms

Resources
– Leaf cutting ants/fungus
– Nitrogen fixing bacteria / plants

Protection
– Cleaner fish and “customers”

Some are mimics (cheaters)
– Ants and aphids
– Ants and acacia trees (herbivory)

Obligate mutualisms
–
–
–
–
–
Lichens (algae and fungus)
Ruminants/bacteria
Deep sea fishes/luminescent bacteria
Corals/zooxanthellae
Endosymbiont theory
Modeling Mutualism

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)

Indirect effects on community

Mycorrhizal fungi / plants
– Reduce herbivory
Increased vigor
 Increased antiherbivore defenses

– 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
