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Chapter 13

chapter 13

Biological Sciences
Course Code
Marc Cadotte

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Chapter 13 Ecology
Case study: Hapless cricket: walks to the edge of a body of water, jumps in and drowns,
after drowning a hairworm parasite begins to emerge from the body of the cricket. The
larva enters the crickets body and feeds on its tissues, growing from microscopic size into
an adult that fills all of the crickets’ body cavity except its head and legs. When the larvae
are fully grown, they must return to water, where adult males and females cluster in tight
masses to mate. After mating, the next generation of larvae are released to the water where
they will die unless they’re ingested by a terrestrial arthropod host. Enslaver parasites:
fungal species that alter the perching behaviour of their fly hosts such in a way that their
spores can be dispersed more easily. Rats typically engage in predator avoidance behaviours
in areas that show signs of cats, but infected rats with the protozoan parasite: toxoplasma
gondii behave abnormally towards cats (they do not avoid cats, and in some cases they are
attracted to cats). This is a fatal attraction for the rat, but it benefits the parasite because it
increases the chance that the parasite will be transmitted to the next host in its complex life
cycle (the cat). Wasp (hymenoepicmecis argyraphaga manipulates its host, the orb-weaving
spider (Plesiometa argyra) so precisely that shortly before killing the spider, the wasp larva
induces the spider to make a special cocoon web This cocoon web looks dramatically
different from the spiders web, and the larva kills the spider and eats it. The larva then
spins a cocoon in which itll complete its development and hangs the cocoon from the web it
induced the spider to make. The cocoon web serves as a strong support that protects the
larva from being swept away by torrential rains.
(Definitions of symbionts, parasite, host, and pathogens are in lecture slides).
Parasites which constitute roughly of 50% of the species on Earth, typically feed on only one
or a few host species.
Pathogens are parasites. Parasites have negative effects: they harm but do not immediately
kill their host (unlike predators).
Our faces: home to mites that feed on exudates from the pores of our skin and secretions at
the base of our eyelashes.
Athletes foot: fungus.
Leishmania trpoica can cause disfigurement.
Yersinia pestis: plague.
(Macroparasites andmicroparasites are in lecture)
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Parasites include herbivores (aphids or nematodes w) that feed on one or a few host plants,
as well as parasitoids (whose larvae feed on a single host (usually killing it).
Most species are attacked by more than one parasite, even parasites have parasites.
Ectoparasites (defined in lecture) are dodder plants which grow on a host plant and obtain
water and food by using modified roots called haustoria to penetrate the tissues of the host.
Dodder cannot photosynthesize and hence depends on its host offer both mineral nutrients
and carbohydrates.
Cicadas are found on roots of trees and animals like these eat plants and live on their outer
Ectoparasites are herbivores and parasites. Trichophyton rubrum (athletes foot) feed on
the tissues or blood of their hosts. Some f these parasites transmit diseases to their hosts
(fleas that spread the plague and ticks that spread Lyme disease).
Fig 13.6: A.) Tapeworm Taneia taenilformis uses the suckers and hooks to catch the
intestinal wall of its mammalian host (rodent, rabbit or cat usually) and can grow up to 5m
in length. B.) Mycrobacterium tuberculosis causes the lung disease tuberculosis which kills
2-3M people a year. C.) Section of a cactus that shows th the destruction wrought by
Erwinia carotovora, bacterium that causes soft rot. Affected areas become soft with decay
and develop a distinctive foul odour.
Hosts have adaptations for defending themselves against parasites, and parasites have
adaptations for overcoming host defences. Hosts immune systems and biochemical defences
can protect against parasites. An exoskeleton prevents the ectoparasite from piercing its
body or make it difficult for endoparasites to enter. Endoparasites that do manage to enter
the hosts body are often killed or rendered less effective by the hosts immune system or
biochemical defences.
Immune systems: includes specialized cells that allow the host to recognize microparasites
to which it has been previously exposed (memory cells of immune system are so effective
that the host has lifelong immunity against future attack by the same microparasite).
Biochemical defences: hosts regulate their biochemistry to limit parasite growth. Secondary
defence compounds: they eat specific plants in order to treat or prevent parasite infections.
Ex. Woolly bear caterpillars switch from their usual food plant (lupines) to a diet of
poisonous hemlock. The new diet doesnt kill the parasites, but it increases the chance that
the caterpillar will survive the attack and metamorphose into an adult tiger moth. Ex.
Chimpanzees infected with nematodes specifically seek out and eat a bitter plant which
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