Transcript Parasites

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Chapter 17: Predation and Herbivory
(and a bit of Chapter 20)
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
The rabbit/myxoma story
Interacting
populations
evolve in
response to
each other
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Evolution of Resistance in Rabbits
 Decline
in lethality of the myxoma virus in
Australia resulted from evolutionary
responses in both the rabbit and the virus
populations:
 genetic
factors conferring resistance to the
disease existed in the rabbit population prior to
introduction of the myxoma virus:
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the myxoma epidemic exerted strong selective pressure
for resistance
eventually most of the surviving rabbit population
consisted of resistant animals
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Evolution of Hypovirulence in
Myxoma Virus
 Decline
in lethality of the myxoma virus in
Australia resulted from evolutionary
responses in both the rabbit and the virus
populations:
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less virulent strains of virus became more prevalent
following initial introduction of the virus to Australia:
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virus strains that didn’t kill their hosts were more readily
dispersed to new hosts (mosquitoes bite only living rabbits)
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The Rabbit-Myxoma System Today
 Left
alone, the rabbit-myxoma system in
Australia would probably evolve to an
equilibrial state of benign, endemic
disease, as in South America:
 pest
management specialists continue to
introduce new, virulent strains to control the
rabbit population
 Contagious
diseases spread through the
atmosphere or water are less likely to
evolve hypovirulence, as they are not
dependent on their hosts for dispersal.
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RABBITS AND MYXOMA …
… is an example of a predator (the virus) and prey (the rabbits).
Prickly Pear cactus were also
introduced into Australia.
 Like
rabbits, they
quickly spread over
the continent.
A
predator of the
cactus was
introduced.
 The
cactus moth.
 The cactus only
survived in areas where
the moth was absent.
Comparing cactus before (a) and
after (b) the moth introduction.
The cactus is an example of
predator prey interactions.
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Do predators limit prey population growth?
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Do prey limit predator population growth?
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The balance between the two depends on their adaptations.
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Some adaptations were already found in species.
Some adaptations are a result of predator/prey interactions.
All life forms are both consumers
and victims of consumers.
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There are many consumer-resource interactions:
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Predator-prey
Herbivore-plant
Parasite-host
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Producers
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Consumers
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Predator; Parasite; Parasitoid: Herbivore; Detritivore
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Some Definitions
 Predators
catch individuals and consume them,
removing them from the prey population.
 Parasites
consume parts of a living prey
organism, or host:
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parasites may be external or internal
a parasite may negatively affect the host but does not
directly remove it from the population
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More Definitions
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Parasitoids consume the living tissues of their hosts,
eventually killing them:
 parasitoids
predators
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combine traits of parasites and
Herbivores eat whole plants or parts of plants:
 may
act as predators (eating whole plants) or as
parasites (eating parts of plants):
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grazers eat grasses and herbaceous vegetation
browsers eat woody vegetation
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Detritivores occupy a special
niche.
 Detritivores
consume dead organic material, the
wastes of other species:
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have no direct affect on populations that produce these
resources:
 do not affect the abundance of their food supplies
 do not influence the evolution of their resources
are important in the recycling of nutrients within
ecosystems
An example of a parasitoid wasp.
 This
was is laying its egg
in the caterpillar.
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The egg will develop into
larvae.
The larvae will consume the
caterpillar as it grows.
A
combination of
predation, and
parasitism.
Predators have adaptations for
exploiting their prey.
 This
lion has
adaptations to
capture fast prey.
 This
whale is a filter
feeder.
 Spiders
make webs to
subdue prey.
Even predator adaptations take
practice!
Predators and prey are different
sizes, and this can pose problems.
 If
a prey item is too small – it may be too hard
to handle.
 Imagine
trying to capture mice with your hands.
 If
a prey item is too large – the predator may
not be able to subdue.
 Imagine
 Blue
trying to tackle a elephant to eat.
whales weigh many tons, but eat tiny
shrimp (use of filters).
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Form and Function Match Diet
 Form
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and function of predators are closely tied to diet:
vertebrate teeth are adapted to dietary items:
 horses have upper and lower incisors used for cutting fibrous
stems of grasses, flat-surfaced molars for grinding
 deer lack upper incisors, simply grasping and tearing
vegetation, but also grinding it
 carnivores have well-developed canines and knifelike
premolars to secure and cut prey
+ A predator’s form and function are closely tied to its
diet. (a) upper incisors are used to cut plant
material; (b) flat-surfaced molars for grinding plant
material; (c) knifelike premolars secure prey and
tear flesh
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More Predator Adaptations
 The
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variety of predator adaptations is remarkable:
consider grasping and tearing functions:
 forelegs for many vertebrates
 feet and hooked bills in birds
 distensible jaws in snakes
digestive systems also reflect diet:
 plant eaters feature elongated digestive tracts with
fermentation chambers to digest long, fibrous molecules
comprising plant structural elements
+ Distensible jaws: shift the articulation
of the jaw with the skull from the
quadrate bone to the supratemporal
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Burmese python (3.9m) vs alligator
(1.8m) in Everglades National Park
(Florida)
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Digestive tracts of consumers are adapted to their
diets. Digestive organs of herbivores > carnivores
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Large carnivores tend to pursue large
prey.
Size of prey consumed is related to size of predator.
What about the prey?
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How much energy do you have available for growth?
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If you are predated upon, your growth rates are affected.
Prey have adaptations to avoid
being consumed.
 Hiding
 If
a predator can’t see you, it can’t eat you.
 Evolution of cryptic coloration.
 Escaping
 If
you can outrun your predator, it can’t eat you.
 Evolution of speed or maneuverability.
 Active
defense mechanisms
 Animals with poison glands.
 Plants with thorns, toxic substances.
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Crypsis and Warning Coloration
 Through
crypsis, animals blend with their
backgrounds; such animals:
 are
typically palatable or edible
 match color, texture of bark, twigs, or leaves
 are not concealed, but mistaken for inedible
objects by would-be predators
 Behaviors
of cryptic organisms must
correspond to their appearances.
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Cryptic appearances (a) mantid; (b) stick
insect; (c) lantern fly
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For more…See slideshow – posted
on the ecology site
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Chemical defenses.
 The
production of
chemicals which
repel potential
predators.
 Toxin
+ boiling temp
=>
 Notice
the colors of
this bombardier
beetle.
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Warning Coloration: aposematism
 Why
should a prey item evolve bright colors?
It definitely brings attention to you.
 Black and yellow are the most common colors.
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 Unpalatable
animals may acquire noxious chemicals from
food or manufacture these chemicals themselves:
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such animals often warn potential predators with warning coloration
or :
 certain aposematic colorations occur so widely that predators may
have evolved innate aversions
 If
an animal eats a brightly colored prey item:
It may get sick.
 It may die.
 If it lives, it will remember.
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Unpalatable organisms
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Why aren’t all prey unpalatable?
 Chemical
defenses are expensive, requiring large
investments of energy and nutrients.
 Some
noxious animals rely on host plants for their
noxious defensive chemicals:
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not all food plants contain such chemicals
animals using such chemicals must have their own means
to avoid toxic effects
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Batesian Mimicry
 Certain
palatable species mimic unpalatable
species (models), benefiting from learning
experiences of predators with the models.
 This
relationship has been named Batesian
mimicry in honor of discoverer Henry Bates.
 Experimental
studies have demonstrated
benefits to the mimic:
predators quickly learn to recognize color patterns of
unpalatable prey
 mimics are avoided by such predators
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+Harmless mantid (b) and moth (c) evolved to
resemble a wasp (a)
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Müllerian Mimicry
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Müllerian mimicry occurs among unpalatable species that
come to resemble one another:
 many
species may be involved
 each species is both model and mimic
 process is efficient because learning by
predator with any model benefits all other
members of the mimicry complex
 certain aposematic colors/patterns may be
widespread within a particular region
+ Costa Rican butterflies and moths
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Class petition…Any questions?
EXAM APRIL 22ND (EARTH
DAY): 2 TO 3.30 PM, EXAM
HALL
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Latest news…
MALARIA, MOSQUITOES,
EVOLUTION
For more…See slideshow – posted
on the ecology site
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Parasites!
 Parasites
have
adaptations to allow them
to live in the host.
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The host has adaptations to
fight off parasites.
 The
parasite does not
want to kill the host, but
disperse its offspring to
another host.
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Parasites have adaptations to
ensure their dispersal.
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Parasites are usually much smaller than their hosts and may
live either externally or internally:
 internal
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parasites exist in a benign environment:
both food and stable conditions are provided by host
 parasites
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must deal with a number of challenges:
host organisms have mechanisms to detect and destroy
parasites
parasites must disperse through hostile environments,
often via complicated life cycles with multiple hosts, as
seen in Plasmodium, the parasite that causes malaria
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Parasite-Host Systems: A
Balancing Act
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The parasite-host interaction represents a balance between
parasite virulence and host defenses:
 immune
system of host can recognize and
disable parasites
 but parasites may multiply rapidly before an
immune response can be deployed
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Parasites may defeat a host’s
immune response.
 Circumventing
the host’s immune system is a
common parasite strategy:
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some parasites suppress the host’s immune system (AIDS
virus)
other parasites coat themselves with proteins that mimic
the host’s own proteins (Schistosoma)
some parasites continually coat their surfaces with novel
proteins (trypanosomes)
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Cross-Resistance
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Some parasites elicit an immune response from the host,
then coat themselves with host proteins before the immune
response is fully mobilized:
 initial
immune response by host may benefit the
host later when challenged by related parasites
in a phenomenon known as cross-resistance
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Once an immune response has been elicited, antibodies can
persist for a long time, preventing reinfection.
Many parasites have complex life
cycles.
Malaria (Plasmodium) parasitic life cycle.
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Plants have antiherbivore
defenses.
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Plant-herbivore “warfare” is waged primarily through
biochemical means.
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Full spectrum of plant defenses includes:
 low
nutritional content of plant tissues
 toxic compounds synthesized by the plants
 structural defenses:
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spines and hairs
tough seed coats
sticky gums and resins
Plant adaptations against predation.
 Nutritional
value?
 It
could be as simple as
a spine.
 “Ouchy
bush!”
 It
could be as
complicated as
chemicals.
 Tannins.
 Secondary
compounds.
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Spines protect the stems and leaves
(a) cholla cactus and (b) prickly pear cactus
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Digestibility
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Animals typically select plant food according to its nutrient
content:
 especially
important to young animals, which
have high demands for protein
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Some plants deploy compounds that limit the digestibility of
their tissues:
 tannins
produced by oaks and other plants
interfere with the digestion of proteins
 some animals can overcome the effect of tannins
through production of digestive dispersal agents
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Secondary Compounds
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Secondary compounds are produced by plants for
purposes (typically defensive) other than metabolism.
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Such compounds can be divided into three major classes:
 nitrogen
compounds (lignin, alkaloids,
nonprotein amino acids, cyanogenic glycosides)
 terpenoids (essential oils, latex, plant resins)
 phenolics (simple phenols)
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Induced and Constitutive Defenses
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Constitutive chemical defenses are maintained at high levels in
the plant at all times.
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Induced chemical defenses increase dramatically following an
attack:
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suggests that some chemicals are too expensive to maintain under light
grazing pressure
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plant responses to herbivory can reduce subsequent herbivory
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Herbivores control some plant
populations.
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Examples of control of introduced plant pests by herbivores
provides evidence that herbivory can limit plant
populations:
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prickly pear cactus in Australia
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controlled by introduction of a moth, Cactoblastis
Klamath weed in California
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controlled by introduction of a beetle, Chrysolina
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Effects of Grazers and Browsers on
Vegetation
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Herbivores consume 30-60% of aboveground vegetation in
grasslands:
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demonstrated by use of exclosures limiting access to vegetation
by herbivores
Occasional outbreaks of tent caterpillars, gypsy moths, and
other insects can result in complete defoliation of forest
trees.
Imagine a plant being eaten, which
stimulates plant or chemical
production.
Mite growth is inhibited if the plant was
previously eaten.
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Look at the impact of herbivores.
Outbreaks of herbivorous insects can
defoliate forests.
Tell me please – do not put your name
on the paper
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
3 issues you want to discuss in the remaining class period

Please include at least one topic relevant to Lebanon or the region

Now – take 5 minutes
Also tell me please
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The good

The bad
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The anything else (beautiful or ugly)
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Chapter 18: Dynamics of Predation
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
Population Cycles of Canadian
Hare and Lynx
 Charles
Elton’s seminal paper focused on
fluctuations of mammals in the Canadian boreal
forests.
 Elton’s
analyses were based on trapping records
maintained by the Hudson’s Bay Company
 of special interest in these records are the regular
and closely linked fluctuations in populations of the
lynx and its principal prey, the snowshoe hare
 What
causes these cycles?
Some Fundamental Questions
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The basic question of population biology is:
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what factors influence the size and stability of populations?
Because most species are both consumers and resources for
other consumers, this basic question may be refocused:
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are populations limited primarily by what they eat or by what eats
them?
More Questions
 Do
predators reduce the size of prey
populations substantially below the
carrying capacity set by resources for the
prey?
 this
question is prompted by interests in
management of crop pests, game populations,
and endangered species
 Do
the dynamics of predator-prey
interactions cause populations to oscillate?
 this
question is prompted by observations of
predator-prey cycles in nature, such as Elton’s
lynx and hare
Consumers can limit resource
populations.
 An
example: populations of cyclamen mites, a pest
of strawberry crops in California, can be regulated
by a predatory mite:
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cyclamen mites typically invade strawberry crops soon
after planting and build to damaging levels in the second
year
predatory mites invade these fields in the second year and
keep cyclamen mites in check
 Experimental
plots in which predatory mites were
controlled by pesticide had cyclamen mite
populations 25 times larger than untreated plots.
What makes an effective predator?
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Predatory mites control populations of cyclamen mites in
strawberry plantings because, like other effective predators:
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they have a high reproductive capacity relative to that of their
prey
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they have excellent dispersal powers
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they can switch to alternate food resources when their primary
prey are unavailable
Consumer Control in Aquatic
Ecosystems
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An example: sea urchins exert strong control on populations
of algae in rocky shore communities:
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in urchin removal experiments, the biomass of algae quickly
increases:
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in the absence of predation, the composition of the algal
community also shifts:
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large brown algae replace coralline and small green algae
that can persist in the presence of predation
Predator and prey populations often
cycle.
 Population
cycles observed in Canada are
present in many species:
 large
herbivores (snowshoe hares, muskrat,
ruffed grouse, ptarmigan) have cycles of 9-10
years:

predators of these species (red foxes, lynx, marten,
mink, goshawks, owls) have similar cycles
 small
herbivores (voles and lemmings) have
cycles of 4 years:

predators of these species (arctic foxes, rough-legged
hawks, snowy owls) also have similar cycles
 cycles
are longer in forest, shorter in tundra
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Predator-Prey Cycles: A Simple
Explanation

Population cycles of predators lag slightly behind population
cycles of their prey:

predators eat prey and reduce their numbers

predators go hungry and their numbers drop

with fewer predators, the remaining prey survive better and prey
numbers build

with increasing numbers of prey, the predator populations also
build, completing the cycle
Time Lags in Predator-Prey
Systems
 Delays
in responses of births and deaths to an
environmental change produce population
cycles:
 predator-prey
interactions have time lags associated
with the time required to produce offspring
 4-year and 9- or 10-year cycles in Canadian tundra or
forests suggest that time lags should be 1 or 2 years,
respectively:


these could be typical lengths of time between birth and
sexual maturity
the influence of conditions in one year might not be felt until
young born in that year are old enough to reproduce
Time Lags in Pathogen-Host
Systems
 Immune
responses can create cycles of
infection in certain diseases:
 measles
produced epidemics with a 2-year cycle
in pre-vaccine human populations:

two years were required for a sufficiently large
population of newly susceptible infants to accumulate
Time Lags in Pathogen-Host
Systems
 other
pathogens cycle because they kill sufficient
hosts to reduce host density below the level where
the pathogens can spread in the population:
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such cycling is evident in polyhedrosis virus in tent
caterpillars
In many regions, tent caterpillar infestations last about 2
years before the virus brings its host population under
control
In other regions, infestations may last up to 9 years
Forest fragmentation – which creates abundant forest edge
– tends to prolong outbreaks of the tent caterpillar

Why?

Increased forest edge exposes caterpillars to more intense sunlight 
inactivates the virus  thus, habitat manipulation here has secondary
effects