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Chapter 12
Parasitism
© 2002 by Prentice Hall, Inc.
Upper Saddle River, NJ 07458
Outline
• Parasites feed on a host, but
generally do not kill it
• Hosts have evolved many defenses
(e.g., immune responses) against
parasites
• Models show that the rate of
spread of diseases is govern by the
density of susceptibles in the
population, the transmission rate
of the disease and the length of life
of the infected host
Outline
• Parasites can substantially
decrease host population size
• Parasites can affect the
structure of host communities
• Parasitoids help in biological
control by reducing the density
of pests
Defining Parasites
• Parasite: a predatory organism
that feeds off another but
generally does not kill it
• Host: prey of a parasite
• Parasitoid: Cases where the
host does not survive but one
host is insufficient for the
development of the parasitoid
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)
Defining Parasites
• Are mosquitoes and Wilde beasts
parasites?
• Some parasites are parasitic on
other plants
– Holoparasites: lack chlorophyll, and are
totally dependent on another plant for
water and nutrients
– Hemiparasites: photosynthesize, but do
not have a root system, so they rely on
the host for this function
• Ex. mistletoe
Defining Parasites
• Monophagous: parasites that feed off
one to three closely related species
• Polyphagous: parasites that feed off
many host species
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)
Defining Parasites
• Haustoria: plant parasite outgrowths
that penetrate inside a host plant to tap
into it's nutrient supply
• Use of multiple hosts: fluke肝蛭 (Figure
12.3)
Adult flukes produce eggs inside a cow.
The eggs are passed in the cow’s feces.
Adult lancet fluke
Life cycle of lancet fluke,
Dicrocoelium Dendriticum
Ants eat the “slime balls.”
Some of the flukes migrate into
the ant’s brain, causing it to climb
to the tip of a blade of grass where
it can be eaten by a cow.
Snails eat the fluke eggs; later the
eggs hatch in the snail’s intestine.
The eggs hatch and asexually
produce offspring.
The offspring are passed from the
snail in “slime balls.”
Defining Parasites
• Parasites outnumber free-living species 4 to 1
(Figure 12.4)
Average number of parasite species per host
0
2
4
6
8
10
12 14 16
Fish
Birds
Mammals
True bugs
Beetles
Flies
Wasps
Butterflies and moths
Trees
(95)
Defense Against Parasites
• Cellular defense reactions
– Eggs of parasatoids are rendered
inviable by encapsulating them
• 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 then immune to reinfection
Defense Against Parasites
• Defensive displays or
maneuvers
– Actions intended to deter
parasites
• Grooming and preening
behavior
– Behavior intended to remove
parasites
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
Modeling Parasitism
• For microparasites, the number of infected
hosts is the most important factor
• Rp = NBL
– Rp = number of infected hosts, with p for
parasite and R for net reproductive rate
•
•
•
•
Transmission threshold; Rp = 1
For disease to spread; Rp > 1
For disease to die out; Rp < 1
Microparasites are transferred from host to host
– N = density of susceptible hosts in population
– B = transmission rate of disease
– L = average period over which the infected
host remains infectious
Modeling Parasitism
– Generalizations (cont.).
• As L increases so does Rp
• If diseases are highly infectious, Rp
increases
• Large populations of susceptible
hosts promotes the spread of
diseases
Modeling Parasitism
• Critical threshold NT where Rp = 1
– NT is an estimate of the number of
susceptible hosts needed to maintain the
parasite population at a constant size
– NT = 1 / BL
» If B is large, N is small
» If B or L are small, the disease can
only persist only in a large population
Modeling Parasitism
• Many diseases undergo periodic cycles
Number of cases per 3-month interval
(thousands)
– Ex. Measles麻疹 in England (Figure 12.5)
• Peaks occur because host immunity is developed
• New births lead to new susceptible hosts, and
cycle repeats
400
300
200
100
0
1948 1952
1956 1960 1964
Year
1968 1972
1976 1980
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
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
– Control is most often exerted by a single
parasitoid species, in contrast to natural
systems, which require a suite of generalized
enemies
– Thus, biological control projects can not
providerigorous evidence of the importance
of parasites in natural systems
Parasites Affect Host
Populations
• Effects of introduced parasites on
natural systems
– Chestnut blight in the Appalachian
Mountains of North America
Density of stems per hectare
• Virtually eliminated chestnut tree (Figure
12.6)
200
160
120
80
40
0
1934
1941
Year
1953
Parasites Affect Host
Populations
– Chestnut blight in the Appalachian
Mountains of North America
• Introduced in New York in 1904
• In Britain, 25 million elm trees (out of
30 million) were wiped out by the
disease between 1960s and the
1990s (Figure 12.7)
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
asmorbilliviruses, which includes
measles and distemper
• Spread by food and water
contaminated by dung of sick
animals
• Can be fatal to certain animals
(buffalo, eland, kudu, and warthogs)
Parasites Affect Host
Populations
– Rinderpest, (cont.).
• 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
Parasites Affect Host
Populations
– Endangered species
• 1. Many endangered species are threatened by
diseases from domestic animals (Table 12.2)
Parasites Affect Host
Populations
– Endangered species (cont.).
• 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
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
Parasites Affect Host
Populations
– Massive mortality of big horn
sheep from infection by
lungworms (Protostrongylus
stilesi and P. rushi ) (cont.).
• Infection rates of 91% and
mortalities of 50-75% have been
reported
Parasites Affect Host
Populations
– Colorado pine tree plantations and
mistletoe. Mistletoe can cause
30% loss in extractable timber
– Saline marshes in North America
and the plant parasite, Cuscuta
salina (Figure 12.8a)
Parasites Affect Host
Populations
– Saline marshes in North America and the plant
parasite, Cuscuta salina (cont.)
• Infects the most common plant in California
marshes, Salicorniavirginica thus promoting the
growth of two other species, Limonium and
Frankenia (Figure 12.8b)
(b)
Plant mass (g)
Salicornia
Limonium
Frankenia
25
20
15
10
5
0
Uninfected
Infected
With
Cuscuta
Parasites Affect Host
Populations
– 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
Parasites Affect Host
Populations
• 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
Parasites Affect Host
Populations
• Hurtrez-Bousses et al. (1997) reduced the
number of blowfly larvae parasites in young
blue tits in Corsica (cont.).
– Chicks from microwaved nests were found to have
greater body weight at fledging (Figure 12.9)
60
10
30
9
8
0
% nest failure
Mass at fledging (g)
11
Parasites Affect Host
Populations
• Stiling and Rossi (1997) manipulated
parasitic infection levels of a gallmaking fly on a coastal plant,
Borrichia frutescens, on isolated
islands off the coast of Florida
– Low rates of parasitism treatment
» Allowed potted plants on one island to
be colonized by gallflies
Parasites Affect Host
Populations
– Low rates of parasitism treatment (cont.).
» 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
Percentage parasitism
Galls per 200 terminals
Parasites Affect Host
Populations
– Results: High degree of parasitism of gallflies resulted in
a significant reduction in the number of new galls (Figure
12.10)
35
30
25
20
15
10
5
0
May June
(a)
July
High parasitoids
Low parasitoids
100
90
80
70
60
50
40
30
20
May June
Aug
1995
Sept Oct
Nov
Sept
Nov
(b)
July
Aug
1995
Oct
Parasites Affect Communities
• Parasites affect the presence
or absence of various species
in a community
Parasites Affect Communities
• The meningeal brainworm
Parelaphostrongylus tenuis
– Usual host is the white-tailed
deer, which is tolerant of the
infection
Parasites Affect Communities
• The meningeal brainworm
Parelaphostrongylus tenuis
(cont.).
– All other cervids and the
pronghorn antelope are potential
hosts
Parasites Affect Communities
– All other cervids and the
pronghorn antelope are potential
hosts (cont.).
• Worm causes severe neurological
damage
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
Parasites and Biological Control
• Not all parasites are
detrimental to humans
• Many are used to protect crops
from pests: Biological control
Parasites and Biological Control
• Many are used to protect crops
from pests: Biological control
(cont.).
– Only 16% of classical biological
control would qualify as economic
successes
Parasites and Biological Control
• Many are used to protect
crops(cont.).
– 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.
Parasites and Biological Control
• Many are used to protect
crops(cont.).
– New techniques: Ex. novel
parasite-host associations
Parasites and Biological Control
• Many are used to protect
crops(cont.).
– Review of 548 control projects:
the more parasites that were
releaseds, the lower the rate of
establishment
Parasites and 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
Parasites and Biological Control
• Necessary attributes of a good
agent of biological control
cont.)
– High rate of increase relative to
the host's
– General mobility adequate for
dispersal
Parasites and Biological Control
• Necessary attributes of a good
agent of biological control
cont.)
– Minimal time lag effects in
responding to changes in host
numbers
Parasites and 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
Parasites and Biological Control
• Methods affecting the success
of biological control (Stiling
1990)(cont.).
– Importance of climatic variation
was underscored by another
review of biological failures
(Stiling 1993)
Parasites and Biological Control
– Importance of climatic variation was
underscored by another review of biological
failures (Stiling 1993)(cont.).
• Climate accounted for 34.5 % of the failures
(Figure 12.11)
Death by natural enemies
Lack of alternative hosts
Prey has refuge
Wrong strain of host
Wrong climate
Other reasons
40
30
20
10
% reasons in failure in biocontrol
0
Parasites and Biological Control
• Risks of biological control
– Reduction in native Hawaiian
lepidopterans was partly due to
wasp species introduced for
biological control of lepidopteran
crop pests
Parasites and Biological Control
• Risks of biological control
– Reduction in native Hawaiian
lepidopterans (cont.)
• Wasps attacked target exotic
species but also non-target native
species
Parasites and Biological Control
– Reduction in native Hawaiian
lepidopterans (cont.)
• Stresses the importance of more
narrowly focused release, rather
than the traditional "hit and miss"
technique
Parasites and Biological Control
• Risks of biological control
– Problems with crops
• While it is of interest to insure that
the agent does not adversely affect
the crop, however, non-crop plants
are not as vigorously tested
Applied Ecology
• Importance of parasites to
plants and animals
• Five main categories of
disease-causing organisms
that most plants and animals
are susceptible to (Table 1)
Applied Ecology
• Leading causes of human
death by disease, worldwide
(Table 2)
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
Summary
• The presence of various
parasite defense mechanisms
is testament to the importance
of parasitism in nature
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.
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
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
Discussion Question #1
• Discuss some of the main
differences between
microparasites, macroparasites,
parasitoids, predators, and
herbivores in terms of their life
history strategies.
Discussion Question #2
• Why can we eradicate some
diseases, such as yellow fever,
through vaccinations, while we
have not been able to eradicate
diseases such as malaria?
Discussion Question #3
• Can we ever expect chemical
pesticides to be replaced
entirely by biological control
agents? Explain.
Discussion Question #4
• Parasites are usually very small
organisms, capable of passing
through screen enclosures. With
this in mind, how might you design
an experiment to remove parasites
from a population and compare that
population's survival rates with
those of an unmanipulated control
population?
Discussion Question #5
• Do you think that biological
control would be more
successful on islands or on
mainland continental areas?
Against native or exotic pests?