ch 15 parasitism and mutualism
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Ch. 15 Parasitism and Mutualism
Coevolution
Coevolution is "the change of a biological object
triggered by the change of a related object”
Change in at least two species' genetic compositions
reciprocally affect each other's evolution
This is evident in the interactions between
parasites and their hosts
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Coevolution
Symbiosis – the intimate association between two
or more organisms of different species
The fate of individuals of one species depends on
their association with individuals of another
The result of the association may be positive,
negative, or benign
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Diverse effects of parasites in ecosystems:
linking interdependent processes
Parasites – usually regarded in terms of their
detrimental effects on the individuals they infect –
can also have positive impacts on other species
in the community… Parasites and pathogens act
as ecosystem engineers, alter energy budgets
and nutrient cycling, and influence biodiversity.
http://www.esajournals.org/doi/abs/10.1890/110016
Parasites Draw Resources from Hosts
In a parasitic relationship, one species
(parasite) benefits and the other species (host)
is harmed
Parasites increase their fitness by using the
host in a close, prolonged association for
food
habitat
dispersal
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Parasites Draw Resources from Hosts
Host fitness is often
decreased by the parasite
through
stunted growth
emaciation
behavior modification
sterility
The host may die from a
secondary infection
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https://www.youtube.com/watch?v=XuKjBIBBAL8
https://www.youtube.com/watch?v=Go_LIz7kTok
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Parasites generally
are much smaller than the host
are highly specialized
reproduce more quickly and in larger numbers
than the host
Parasites Draw Resources from Hosts Parasites Draw Resources from Hosts
A heavy load of parasites is an infection
The outcome of an infection is disease
Parasites can be categorized by size
Micro and Macro parasites
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Microparasites
Completes a full life cycle within one host
small size
short generation time
multiply rapidly within the host
associated with the term “disease”
the infection is short relative to the host’s
lifespan
transmitted directly from host to host or
through a carrier
Malaria, Giardia, Influenza, Ringworm
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Macroparasites
Macroparasite infections tend to be chronic and
accumulate slowly
larger size
longer generation time
usually do not complete their life cycle within a
single host
transmitted directly from host to host or through a
carrier or intermediate host
flatworms, flukes, roundworms, lice, fleas, ticks, fungi
(rusts and smuts)
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Parasitic plants
4000 species of parasitic plants that are parasites of
other plants
Hemiparasites are photosynthetic plants that obtain
water and nutrients from host xylem
Holoparasites are nonphotosynthetic plants that
function as heterotrophs, using the host’s phloem and
xylem to supply carbon, water, nutrients
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Figur.1
Mistletoe (Viscum album) in poplar tree. Mistletoes are
hemiparasites.
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Squawroot (Conopholis americana), a
member of the broomrape family, is a
holoparasite on the roots of oak.
Ectoparasites and Endosparasites
Ectoparasites live on the outside of the host
Ectoparasites of vertebrates live in skin, feathers,
scales, hair
Ectoparasites of insects live on the legs, upper and
lower body surfaces, mouthparts
Endoparasites live within the host
Some may burrow under the skin
Some live in the blood
Some live in organs, or within lining tissues, such as
the nasal tract
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Endoparasite Ectoparasites
How parasites get into their hosts
Parasites of animals can enter a host through the
mouth
nasal passages
skin
rectum
urogenital system
They can travel to their preferred habitat using the
pulmonary, circulatory, or digestive systems
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Direct Transmission
When a parasite moves from one host to another
without an intermediate organism
The most important debilitating external parasites
of birds and mammals spread by direct contact
Fleas, Lice, botfly larvae, mites
Direct transmission of parasites can occur through
dispersal by air, water, or other substrate
Microparasites are often transmitted directly
influenza virus – airborne
smallpox virus – direct contact
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Transmission Can Occur between Hosts
Many external macroparasites are spread
directly
lice
ticks
fleas
mange mites
Some lay their eggs directly on the host
Some lay their eggs in the environment and,
after the eggs hatch, jump onto nearby hosts
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Some parasitic plants use direct transmission
Some fungal parasites of plants spread through
root grafts, when the roots of one tree grow onto
the roots of a neighboring tree and attach
Transmission Can Occur between Hosts
Intermediate Vector
Some parasites are transmitted between hosts by
an intermediate vector often an arthropod
Each generation of tick must acquire a the infection anew.
Larval ticks feed on mice, squirrels and birds.
The infection is acquired by feeding on an infected reservoir animal.
Nymphs feed on a similar range of hosts to larvae; transmission of
spirochaetes to a competent reservoir for the next generation of larval ticks.
Adult ticks are not generally important for maintenance of B. burgdorferi in
the wild, as they feed predominantly on larger animals such as deer, which
are incompetent hosts.
However, deer are important for maintenance of the tick population
because adult ticks mate on them.
Although all three stages can feed on humans, nymphs are responsible for
the vast majority of disease transmission
It is unknown whether infected humans can transmit spirochaetes to
feeding larvae.
http://www.nature.com/nrmicro/journal/v10/n2/fig_tab/nrmicro2714_F1.html.
Intermediate Vector
Lyme disease is caused by a bacterial spirochete,
Borrelia burgdorferi
The hosts are vertebrates, most commonly birds,
mice, deer and humans
The vector is a black-legged tick (Ixodes scapularis)
The spirochete is dependent on the vector for
transmission between hosts
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Malaria is a disease caused by protist parasites
Different species infect specific host animals
Four species of Plasmodium cause malaria
The vector is a female mosquito
Mosquitoes transmit more than 50% of the
approximately 102 arboviruses (arthropod-borne
viruses), including dengue and yellow fever
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Intermediate Vector
Malaria is a recurring infection produced in humans by
protozoan parasites transmitted by the bite of an infected
female mosquito of the genus Anopheles. Insert shows Plasmodium falciparum parasite infecting two blood cells.
Malaria risk
No Malaria
Today, more than 40 percent of the world’s
population is at risk, and more than
1 million are killed each year by malaria
Dutch Elm disease has devastated elm tree (host)
populations in North America
The parasite is an ascomycete fungus
Vascular wilt disease
The insect vectors are elm bark beetles that carry
spores from one tree to another
Intermediate Vector
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Mistletoe is a plant parasite, taking water
and nutrients from the host plant
Birds are the vector, dispersing mistletoe
seeds
Birds feed on the mistletoe fruit
The sticky seeds pass through their gut
and are deposited with feces where they
perch
If the seed is deposited on a tree, the seed
germinates, sending out rootlets that cover
the limb and enter the sapwood
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Intermediate Vector Multiple Hosts and Stages of Transmission
Some parasites use multiple host species
Definitive host – the host in which the parasite
reaches maturity
Intermediate – harbors the host during some
developmental phase
The dynamics of a parasite population is tied to the
population dynamics, movement, and interaction of
various hosts
Why would a parasite evolve to use more than one
host to compete its life cycle?
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Complex Life Cycles: Why Refrain from Growth
before Reproduction in the Adult Niche?
Organisms with complex life cycles occupy
distinct habitats as larvae and adults. The ability
to efficiently exploit different resources throughout
ontogeny is thought to be a main reason for the
evolution of metamorphosis and complex life
cycles (Ebenman 1992; Moran 1994).
http://www.jstor.org/stable/10.1086/668592
Meningeal worm Slide 1
First stage
Adult
Infective stage
First-stage larvae
Egg
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Meningeal worm Slide 2
First stage
Adult
Infective stage
First-stage larvae
Egg
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Meningeal worm Slide 3
First stage
Adult
Infective stage
First-stage larvae
Egg
Infective stage
The meningeal worm (parasite) has a complex life
cycle that requires indirect transmission and
multiple hosts
The white-tailed deer is the definitive host
Snails and slugs in grass are intermediate hosts
Deer eats an infected snail or slug while grazing
Larvae leave the snail in the deer’s stomach
Larvae puncture the stomach wall, entering the
abdominal membranes
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Multiple Hosts and Stages of Transmission
Travel via spinal cord to spaces around the
brain where, now adults, they mate and
produce eggs
Eggs move through bloodstream to lungs,
where the larvae break into the air sacs to enter
the lungs
They are coughed up, swallowed, and passed
through the digestive tract, exiting with feces
Snails and slugs come into contact with deer
feces on the ground and acquire the first-stage
larvae
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Multiple Hosts and Stages of Transmission Hosts Respond to Parasitic Invasions
Behavioral defenses may help a host to avoid
infection
grooming – birds and mammals remove
ectoparasites from their bodies
can remove both adult and juvenile parasites
location – can move to an area with fewer parasites
Deer use dense, shaded places to avoid deerflies
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Hosts Respond to Parasitic Invasions
If a parasite has infected a host, defenses include
Inflammatory response – injured host cells trigger
the release of histamines that act as a chemical alarm
Blood flow to the site increases
White blood cells and other cells attack infection
Scab forms to reduce further entry
Internal cysts – reactions can produce a hard cyst in
the muscle or skin that encases the parasite
Pigs form cysts around roundworms (Trichinella) that
infect their muscles
Trees form cankers
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Hosts Respond to Parasitic Invasions
Plant responses to parasites include
cyst or scab formation in roots and fruits in response
to bacterial and fungal infections
Isolates parasite – no contact with healthy tissue
insect attacks on leaves, stems, fruits, seeds by
forming abnormal structures (galls) unique to the
particular type of insect
This may expose the insect larvae to predation
Downy woodpeckers excavate goldenrod ball galls
containing insect larvae
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(a) Spruce cone gall (b) Oak succulent gall (c) Oak bullet gall (d) Goldenrod ball gall
Hosts Respond to Parasitic Invasions
A second line of defense is the immune response
Antigen - a toxin or other foreign substance that
induces an immune response in the body
Antibodies are produced
Antibodies target specific chemicals on or released by
the parasite
Antibodies affect the ability of the parasite to feed,
spread, and reproduce, but do not have to kill it
Immune system remembers past antigens and
responds quickly if a parasite re-infects a host
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Parasites can circumvent the immune system
Some vary their antigens almost continuously
Stay ahead of the host’s ability to respond
Infection by the parasite becomes chronic
Antibodies are made mostly of protein
If an animal has a protein deficiency, antibody
production is inhibited
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Hosts Respond to Parasitic Invasions Effects on Host Survival and Reproduction
Western fence lizards can be infected by malaria
Infected females produce clutches of eggs that are
15% smaller
Store less fat during summer
Infected males have fewer courtship and territorial
behaviors, altered coloration, and smaller testes
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Effects on Host Survival and Reproduction
Parasites can adversely affect the ability of males to
attract mates
Female mate choice often is based on male
secondary sexual characteristics
Male zebra finches have a bright red beak
Color depends on level of carotenoid pigments
These pigments also stimulate antibody production
The birds obtain these pigments only through food
Only male finches with few parasites and diseases
have enough pigment to make a red beak
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A parasite can alter the behavior of a host, making
it more susceptible to predation
Rabbits (host) with the bacterial parasitic disease
tularemia are sluggish and more vulnerable to
predators
This bacterial infection is transmitted by the rabbit
tick
Effects on Host Behavior
Killifish (host) with a parasitic trematode- flat worm
infection display abnormal behaviors such as
surfacing and jerking
The more intense the parasitism, the more abnormal
behaviors are seen – fish are more susceptible to be
eaten by birds
The birds is the trematode definitive host and ensures
the completion of life cycle
Effects on Host Behavior
The parasite’s eggs are released in the droppings of shorebirds. Horn snails
consume the droppings and become sterile. Once the parasite has lived in the
snail a couple of generations, the disk-shaped larva swim out into the marsh.
Then they latch onto the gills of killifish and make there way to the brain cavity.
15
10
5
20
25
0
35
30
Intensity of infection
The frequency of conspicuous behaviour was
positively correlated to the intensity of parasitism
0 500 1000 1500 2000 2500
Uninfected
Infected
Co
ns
pic
uo
us
be
ha
vio
rs
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Uninfected
0.2
0.4
0.6
0
1.0
0.8
Infected
< 1400 cysts
Infected
> 1400 cysts
Pro
po
rtio
n e
ate
n b
y b
ird
s
A comparison of the proportion of fish eaten by birds after
20 days (y-axis), showing that heavily parasitized fish
were preyed on more frequently than unparasitized fish.
Vertical lines in bars represent 95 percent confidence
intervals
(Lafferty and Morris 1996).
Why do you think parasites exist?
What role do parasites play in the ecosystem?
Parasites May Regulate Host Populations
For a parasite and host to coexist, the host needs
to resist infection or eliminate or minimize the
effect
The parasite needs to have an effect that allows
it to survive (reproduce and be transmitted) but
should not kill the host
Do you think natural selection would favor a more
peaceful coexistence of hosts and parasite?
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Maximum fitness for the parasite, just as for the
host, involves trade-offs
Virulence vs transmissibility
High virulence parasites are often deadly
Low virulence parasites may be more gentle
The host’s condition is important to a parasite only
as it relates to its ability to reproduce and transmit
If a parasite is deadly it needs to be highly
contagious or it will die out with its host
Ex. Ebola
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Parasites May Regulate Host Populations
Vertical transmission is the transmission of
parasites from mother to offspring just before
or just after birth
Parasites with this mode of transmission are
generally less virulent
Recipient host (offspring) must survive to
reproduce in order to pass on the parasite
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Parasites May Regulate Host Populations
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Parasites May Regulate Host Populations
Parasites may be density-dependent regulators
of host populations
Most common with directly transmitted native
parasites that are kept in the population through a
small pool of infected carriers
Outbreaks occur when the host population
density is high and can sharply reduce the host
population
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90
60
30
0
Parasites per host
2 10 4 6 8
Nu
mb
er
of
ho
sts
Clumped distribution of
the shrew tick on a
population of the
European field mouse.
Most individuals in the
host population carry no
ticks. A few individuals
carry most of the
parasite load.
(Adapted from Randolph 1975.)
Parasitism Can Evolve into a Mutually
Beneficial Relationship
Parasitism is a symbiotic relationship
Parasite benefits at the expense of the host
Hosts evolve defenses to reduce the parasite’s
negative effect
Sometimes the host can adapt to completely
counter the negative impact
When can host-parasite relationship become
beneficial to both species through coevolution?
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If a host has completely countered the negative
effects of the parasite, the relationship could
become commensalism: one species benefits
while the other is largely unaffected
If the relationship becomes beneficial to both host
and parasite, it would be mutualism
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Parasitism Can Evolve into a Mutually
Beneficial Relationship
Rats (host) infected with the intermediate stages of
a tapeworm (parasite) grow larger than uninfected
rats
The tapeworm (Spirometra) produces a vertebrate
growth hormone
Mollusks (host) infected with
intermediate stages of
digenetic flukes (parasite)
develop to be thicker, heavier
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Parasitism Can Evolve into a Mutually
Beneficial Relationship
Most probable examples of evolution from
parasite to mutualist are parasites with vertical
transmission (mother to offspring)
Selection acts on parasite to increase host
survival and fitness because it benefits the
parasite and host
Wollbachia (bacterial parasites) infect the
reproductive tissues of some arthropods
In the host wasp Nasonia
vitripennis, infected females
produce more offspring
Parasitism Can Evolve into a Mutually
Beneficial Relationship
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Mutualistic relationships
Many mutualistic relationships are reciprocal
exploitation rather than cooperation
The benefits of the interaction for one or both
species may depend on the environment
Trees and their mycorhizzal fungi
The trees benefit in nutrient-poor soils but fungus
appears to be more parasitic in nutrient rich soil
Mutualism is a positive interaction between two
species that can be characterized by a number of
benefits
The variety of benefits received:
provision of essential resources (food, shelter)
protection from predators, herbivores, parasites
reduction of competition with a third species
enhanced reproduction
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Mutualistic relationships
Obligate mutualists cannot survive or reproduce
without the interaction
Facultative mutualists can survive and reproduce
without the interaction
The degree of specificity of a mutualism can vary
Specialist mutualists have a one-to-one species
specific association
Generalist mutualists associate with a diversity of
partners of different species
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Mutualistic relationships Degrees of intimacy
Some mutualists are non symbiotic, free-living
individuals
Some mutualists are symbiotic, coexisting, and
often in an obligate relationship
At least one member of the pair is totally dependent
on the other
Some associations are so permanent and obligatory
that the distinction between the two organisms blurs
Reef-forming corals are symbiotic with algae
Individual coral animals, polyps, live in small cups
within the skeleton that forms the reef
Zooxanthellae live within the coral animal’s tissues
A coral obtains about 10% of its energy from filter
feeding on zooplankton and about 90% from the
carbon produced by the photosynthetic algae
The corals cannot survive in their nutrient-poor
environment without the algae
The algae obtain shelter and nutrients, mainly
nitrogen
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Mutualistic relationships
Epidermis
Tentacles
Mouth
Epidermis
Gullet
Gastrodermis
Skeleton
Corallite cup
Sclerosepta
in the cup
Zooanthellae
in gastrodermal
layer
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Warmer water temperatures can result in coral bleaching. When water is too warm, corals will expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white. This is called coral bleaching.
Mutualistic relationships
Lichens exemplify a symbiotic relationship between
a fungus and a photosynthetic algae or
cyanobacteria
They are fused into a single body, a thallus
The fungus
protects the algae from harmful light intensity
produces a substance that speeds up photosynthesis
provides water and nutrients for both organisms
The photosynthetic partner supplies food for both
organisms
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Mutualistic relationships
Figure 15.10
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Cortical layer
Algal layer
Medulla
(a)
(b)
Non-symbiotic Mutualisms
Two organisms do not physically coexist
Depend on each other for some essential function
Most are not obligatory but are facultative,
generally not confined to two species
pollination in flowering plants
seed dispersal
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Nitrogen fixing bacteria
The plant receives
nitrogen from bacteria
The bacteria receive
carbon and other
resources from the plant
Too much fertilizer in
fields will inhibit this
process and make the
soil less fertile
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Some Mutualisms Are Defensive
Rye and tall fescue infected by
endophytic fungi living within the plant
tissues
Fungus benefits by feeding from host
Host benefits from presence of
alkaloid compounds produced by
the fungus
Compound makes grass taste bitter
to grazers
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Some Mutualisms Are Defensive
Protection of acacia trees by ants in Central America
Ants benefit through shelter and food provided by
the trees
Plants benefit from defensive behaviors of the ant
will swarm herbivores, emitting strong odors and
attacking them directly
will remove competing plants living around the
acacia, cutting off leaves with their jaws
If you feed the ants too much nitrogen they lose their
aggressive behavior and thus studies indicate the
plant feeds them nitrogen-poor food
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Cleaning of many fish species by cleaner shrimp
and cleaner fishes
Fish benefit by removal of ectoparasites and dead
tissue
Cleaners feed on ectoparasites and dead tissue
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Mutualisms
Bluehead wrasse
participating in
cleaning symbiosis
with a moray eel.
The cleaner fish
obtains food by
cleaning
ectoparasites from
the host fish.
The red-billed oxpecker of Africa feeds almost
exclusively by gleaning ticks and other parasites from
the skin of large mammals such as the impala shown
here.
Mutualisms Are Involved in Seed Dispersal
Some plants enclose their seeds in a nutritious fruit
that attracts fruit-eating animals, frugivores
These animals are not seed predators because they
eat only the fruit surrounding the seed
Plants must attract frugivores when the seeds are
mature, not before
Unripe fruit contains immature seeds and is often
green (cryptic)
It may have a hard outer coat or unpalatable texture
to discourage consumption
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Figure 15.15
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Mutualism with a Middle Man
The vole feeds on truffles, the reproductive
structure of the fungus, and disperses fungal
spores in their feces
These spores germinate and infect the roots of
other conifers
Voles eat truffles - spores become concentrated
in fecal pellets - voles disperse the spores to
locations where they can infect new host plants
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Figure 15.16 Step 1 Slide 1
Truffle
(fruiting body
of ectomycorrhizae)
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Figure 15.16 Step 2 Slide 2
Truffle
(fruiting body
of ectomycorrhizae)
Vole
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Figure 15.16 Step 3 Slide 3
Truffle
(fruiting body
of ectomycorrhizae)
Vole
Fecal pellets
with spores
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Figure 15.16 Step 4 Slide 4
Truffle
(fruiting body
of ectomycorrhizae)
Vole
Fecal pellets
with spores
Rootlet with
mycorrhizae
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Mutualism Can Influence Population
Dynamics
The effect of mutualism can be modeled in a
way similar to competition
If the growth rate of each species increases
with the increasing density of the other
species
This interaction allows both populations to exist
in higher numbers than either could alone
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L-V model competing species
For species 1, N2 accounts for the competitive
effect of species 2- such that N2 is negative
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A Model of Mutualistic Interactions
The competition coefficients are replaced by
positive interaction coefficients
Sp.1, α21N2 accounts for the positive per capita
effect of sp. 2 on sp. 1
dN1/dt = r1N1(K1 - N1 + α21N2)/ K1
Similarly Sp. 2
dN2/dt = r2N2(K2- N2 + α12N1)/ K2
The presence of the other species increases the
carrying capacity
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A Model of Mutualistic Interactions
Facultative interaction where carrying capacities of
both species are positive, and each species can
grow in the absence of the other
If:
r1 = 3.22, K1 = 1000, α21 = 0.5
r2 = 3.22, K2 = 1000, α12 = 0.6
Calculate the zero-growth isocline for each species
The zero-growth isoclines extend beyond K for both
species because the presence of the mutualism
benefits both species
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0 0
4000
3000
2000
1000
1000 2000 3000 4000
N1
K1
K2
N2
dN1
dt
= 0
= 0 dN2
dt
E stable equilibrium
Both species
A Model of Mutualistic Interactions
Using the equations, we can predict the
density of each population through time
Each reaches a higher density in the
presence of the other species
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Quantifying Ecology Box Figure 2
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Species 2
0
500
1000
1500
0
2500
2000
Time (t)
50 100 150 200
0
500
1000
1500
0
2500
2000
Time (t)
50 100 150 200
Species 1
Without species 2
With species 2
Without species 1
With species 1
Ab
un
da
nc
e (
N1)
Ab
un
da
nc
e (
N2)
Ecological Issues
How do land-use changes affect the abundance
and dispersal of pathogens and parasites?
The impact of forest clearing on the spread of
Lyme disease has been well-documented
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The number of reported cases in North America has
dramatically increased
About 300,000 people contract the disease annually
Parasite – bacterium (Borrelia burgdorferi)
Vector – blacklegged tick (Ixodes scapularis)
Tick life cycle has four stages: egg, larva, nymph,
adult
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Ecological Issues Blacklegged Tick (Ixodes scapularis)
adult
female
adult
male nymph larva
Life stages of blacklegged tick: egg, larva, nymph, and
adult (female and male). United States dime shown for
comparison of size/
Larval ticks are not infected when they hatch
Blood feeders are infected by feeding on host
animal (mammals, birds, reptiles)
Some host species are more likely to transmit the
bacterium to the feeding tick
White-footed mouse (Peromyscus leucopus) infects
40 to 90% of feeding larvae
Human activity has led to forest fragmentation
White-footed mice do well in fragmented forests
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Hypothesis: Small forest patches less than 2
hectares (ha) have a higher density of infected tick
nymphs than larger patches (2 to 8 ha)
Sampled tick density and bacterial infection
incidence in patches from 0.7 to 7.6 ha
This would be a terrible job!!!
Nymph density shows an exponential decline as
patch size increases
Infected nymphs increase as patch size decreased
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0 8 7 6 5 4 3 2 1
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.00
Area (ha)
De
ns
ity
of
ny
mp
hs
(ny
mp
hs
/m2)
White-tailed deer are the primary host species
for adult blacklegged ticks
Adult ticks feed on deer; after feeding, the female
tick drops her eggs to the ground and the life cycle
begins again
Forest clearing and fragmentation have increased
the population of both deer and mice, resulting in
an increase in the population of ticks and the
transmission rate of the Lyme disease bacterium
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Why does this situation leave
humans so vulnerable?