Ch 15 Parasitism and Mutualism

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© 2015 Pearson Education, Inc.

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


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


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


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


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


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


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)


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


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


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


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


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


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


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?


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



First stage

Adult

Infective stage

First-stage larvae

Egg

http://www.jstor.org/stable/10.1086/668592#rf23

http://www.jstor.org/stable/10.1086/668592#rf49

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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


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


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



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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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


(a) Spruce cone gall (b) Oak succulent gall (c) Oak bullet gall (d) Goldenrod ball gall


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


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


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


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?


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



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 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

© 2015 Pearson Education, Inc. .

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?


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




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




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



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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



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


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



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.


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



Figure 15.10


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


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


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


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


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


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



Figure 15.16 Step 1 Slide 1

Truffle

(fruiting body

of ectomycorrhizae)



Truffle

(fruiting body

of ectomycorrhizae)

Vole



Truffle

(fruiting body

of ectomycorrhizae)

Vole

Fecal pellets

with spores



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


L-V model competing species

For species 1, N2 accounts for the competitive

effect of species 2- such that N2 is negative


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



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



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


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


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


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


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


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



Why does this situation leave

humans so vulnerable?

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Ch 15 Parasitism and Mutualism