Predation,
Herbivory, and
Parasitism
Types of Species Interactions
 When
two species interact, the effects for
each species can be positive, negative,
or neutral.
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Competition -/Amensalism -/0
Commensalism +/0
Mutualism +/+
Exploitation +/-
Types of Species Interactions
 Exploitative
include:
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
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Predation
Herbivory
Parasitism
interactions
Parasitism

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
Parasites live in or on their host's body and
often spend most or all their lives eating tissues
or body fluids of just one host individual.
Sometimes multiple generations of parasites
live on the same host.
Because parasites depend on their hosts for
continued feeding, they do not generally kill
their hosts (at least not immediately).
Parasitism


Most parasites
associated with a single
host species have a
free-living life stage
during which they are
not attached to a host.
A great many other
parasites, though, have
multiple hosts with
different life stages
adapted to each host
(and possibly free-living
stages as well).
Endoparasites

The inside of an
organism is a much
more stable and
protected
environment than the
outside,
and endoparasites (e
ndo = inner) take
advantage of this by
living and feeding
inside their hosts
Ectoparasites
 Although
parasites
tend to be tiny and
hard to
see, ectoparasites
(ecto=outer), which
live on the outside
surface of their host,
are often easier to
observe.
Ectoparasites
 By
living outside their host, ectoparasites
avoid having to defend themselves
against the host's immune system.
 The trade-off, however, is that they are
exposed to predation and a sometimes
harsh exterior environment.
 Some species of predators specialize on
ectoparasites.
Ectoparasites
 The
cleaner
wrasse is a type of
fish that lives in
coral reefs.
 Other fish will wait
patiently while a
cleaner fish picks
off parasites from
their scales,
mouth, and gills.
Parasitoids and Hyperparasites



Most of the parasites discussed so far do not
directly kill their hosts but, parasitoids do.
Parasitoids develop inside their host and essentially
eat it from the inside out.
When the host is completely consumed, the
parasitoid transforms into an adult and crawls out
to find new hosts for its offspring.
Parasitoids and Hyperparasites
 Interestingly,
the parasitoid wasp has a
parasitoid of its own, known as a
secondary parasitoid, or hyperparasitoid.
 Hyperparasitoid wasps find aphids with
internal parasitoid larvae and lay eggs
inside the larvae.
Ecological Impacts of
Parasites


Parasites can have broad
ecological impacts.
These effects begin at the
individual level; because
parasites rob their hosts of
resources, host survival
and/or reproduction can
be reduced even when
hosts are not killed by
parasites directly.
Effects of Parasites on
Individual Hosts
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A protozoan causes rats to
become attracted to cats.
A nematode turns the bellies
of Amazonian ants red,
attracting berry-eating birds.
A fluke makes ants climb grass
stems so they will be eaten by
sheep.
A trematode causes killifish to
swim closer to the surface of
the water, making them easy
targets for birds.
Herbivores


Grazers are herbivores
that specialize on
herbaceous plants
(grasses, forbs, and herbs),
while browsers eat the
leaves, bark, and twigs of
woody plants.
Herbivores that specialize
on seeds are granivores,
while those specializing on
fruits are frugivores.
Plant Defenses Against
Herbivory
 Herbivory
is generally not a positive
experience for a plant, so plants have
evolved forms of self-defense.
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Mechanical - Developing structures like thorns
that make it harder for animals to eat them.
Chemical - Producing chemicals that are
noxious or poisonous to herbivores.
Nutritional - Growing structures that are less
nutritious for grazers (have less N and P).
Tolerance - Adaptions to regrow quickly after
being grazed.
Impacts of Herbivory on Plant
Communities
 Herbivory
can
reduce the overall
number of plants
and can also
have a profound
impact on the
composition of a
plant community.
Impacts of Herbivory on Plant
Communities
 When
plants have evolved without
selective pressure from herbivory, they
may not be very well-defended, and the
influence of herbivores can be even more
drastic.

This is a problem when new herbivores are
introduced.
Predation
 Lynx
are fast, but lack
endurance, so they
don't chase hares
over long distances.
 Instead,
they stalk hares,
hiding behind trees
and brush until they
can get close enough
to pounce on a hare.
Predation
 Stalking
is one of a variety of strategies
used by predators for catching mobile
prey.
 Others include pursuit, where the
predator chases prey over a
distance; ambush, where the predator
hides and waits in one spot until prey
comes along; and random encounter,
where the predator and prey meet by
chance.
Animal Defenses Against
Predation
 Just
as plants have evolved defense
mechanisms to combat herbivory,
animals have evolved ways to defend
themselves from predation.



Chemical - Producing chemicals that are
noxious or poisonous to predators.
Physical - Developing physical barriers to
predation (e.g., shells).
Behavioral - Behaving in ways that minimize
risk from predation.
Animal Defenses Against
Predation



Aposematism - Warning
colors, sounds, or other
characteristics to alert
predators that this prey will
not be tasty.
Crypsis - Camouflaged
colors, shapes, and other
ways of hiding from
predators.
Mimicry - Looking, sounding,
or in other ways mimicking a
species that predators
avoid.
Predator-Prey Dynamics

Lynx & Snowshoe hare
population size records
available back to 1800s.



Fur traders
Populations of lynx and
hare seemed to follow an
interesting cyclic pattern.
Both species peak and
then sharply decline about
every ten years, at slightly
non-overlapping intervals.
Prey Dynamics: Modeling Hare
 Scientists
often use computer models to
investigate how different underlying
mechanisms might produce particular
dynamics in systems.
Cycling and Extinction


As the prey population size drops from predation, the
predators have less and less food to eat, causing the
predator population size to drop.
With fewer predators, the prey population starts
growing again, and the cycle repeats.
Interactions among Trophic
Levels
 Predators
do not act
alone in determining
whether prey
populations can survive.
 Prey can also be strongly
affected by the
availability and quality
of their food.
Deterministic vs. Stochastic
 The
deterministic Lotka-Volterra predatorprey model predicts regular, even cycles
of predator and prey populations, with
predator cycles following prey cycles.
 More realistic models that include chance
(stochastic) events show variation in both
the period and amplitude of predatorprey cycles.
Predator-Prey Dynamics in a
Complex World
 Large
predator populations cannot be
supported by small prey populations.
 There is a feedback loop: when prey
populations are small, predator
populations shrink, and when predator
populations shrink, prey populations grow.
Predator-Prey Dynamics in a
Complex World
 The
equations in the Lotka-Volterra
predator-prey model predict stable
population cycling.
 However, because this is a deterministic
model, there are no fluctuations in the
cycles.
 Each cycle is exactly the same as the one
before it.
Predator-Prey Dynamics in a
Complex World
 By
simulating a simple predator-prey
system that included some stochasticity
(randomness), we see that irregular
population cycling can lead to extinction.
Predator-Prey Dynamics in a
Complex World
 Predator-prey
systems in nature involve
many complicated dynamics that are
thought to contribute to irregular patterns
of cycling.
Predator-Prey Dynamics in a
Complex World
 Researchers
studying
lemmings (prey) and
stoats (predators) in
Greenland believe that
the irregularities of cycling
in their data is due in part
to the fact that predators
besides stoats prey on
lemmings (e.g., arctic fox,
snowy owl, and longtailed skua).
Unrealistic Assumptions of
Lotka-Volterra
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There are no effects of crowding for either the prey
or predator (i.e., there is no density dependence in
growth rates).
All predator and prey individuals are equally likely to
meet any of the others (i.e., the populations are
evenly spread out and well-mixed).
The prey species is the only food source for the
predator.
The predator is the only significant cause of death for
the prey.
Each predator individual can catch and eat prey
individuals instantaneously (there is no handling
time).
There is no immigration or emigration of either prey or
predators.
Density Dependence
 Density-dependence
prey and predator
populations tends to
stabilize the
populations and
reduce cycling.
in
Refuges
 Refuges
and
metapopulations
tend to stabilize
predator-prey
systems and
reduce cycling.
Functional Responses to
Exploitation
 The
functional response, describes how
an individual predator's feeding rate
depends on prey density.
 The question is, given a certain density of
prey, how fast will a predator eat them?
Refuges & Metapopulations
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
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Patchiness in populations can promote
stability in predator-prey systems and help
avoid extinctions.
This was shown in a famous laboratory
experiment done by Carl Huffaker using two
mite species, one of which was a predator on
the other.
The prey mites can live on oranges, so
Huffaker set up a variety of trays with different
arrays of oranges.
Refuges & Metapopulations



When all oranges were easily accessible from
their neighbors, the predator mites quickly ate
all the prey mites, and one or both
populations went extinct.
But when Huffaker isolated the oranges by
adding vaseline "walls" throughout the tray
that made it harder for mites—especially
predator mites—to move from orange to
orange, he isolated groups of prey from their
predators.
In essence, he created refuges for prey, and
the populations started cycling.
Refuges & Metapopulations



Some of the prey mites arrived at an orange
without any predators, and their population
would increase.
Eventually, the predators would find the
orange, but by that point some of the prey
mites had moved on to another orange.
In this way, although individual populations
did not survive for long on any one orange,
they did persist as a metapopulation across
the whole landscape of oranges.
Predicting Value of Models
 Models
allow ecologists to predict which
factors might be important in determining
predator-prey dynamics.
Functional Response



In a type I functional response,
predation rate increases linearly
with increasing prey density.
In a type II functional response,
predation rate increases with prey
density, but the rate of increase
slows down as prey density gets
higher, and the predation rate
eventually levels off.
A type III functional response shows
an S-shaped curve, with predation
rate increasing more slowly at low
prey density, then more quickly at
high prey density. Like a type II
response, it gradually levels off at a
maximum predation rate.
Functional Response
 The
type of functional
response and the
shape of the
functional response
curve depend on
characteristics of the
predator, the prey,
and their environment.
The Evolutionary Arms Race
 Environments
are dynamic. When a
population's environment changes,
different characteristics in that population
become more or less advantageous.
 A population's environment includes not
only physical factors, but also biological
ones, including interactions with other
species.
 Exploitation can favor genetic diversity in
the species involved.
The Evolutionary Arms Race
 When
one species exploits another, traits
that increase the success of the exploiter
will be selectively favored, while traits that
decrease the vulnerability of the exploited
species will also be favored.
 Exploitation can thus create an
"evolutionary arms race" between two
species.

Coevolution
Predator-Prey Coevolution


Newts produce a
highly poisonous
substance known as
tetrodotoxin, or TTX.
Small amounts of this
toxin can kill most
potential predators,
but garter snakes
have evolved ways to
neutralize the toxin,
allowing them to eat
newts.
Predator-Prey Coevolution


As snakes become
resistant to the newt toxin,
this selectively pressures
the newts to elevate
production of the toxin,
which again selects for
snakes with higher
resistance.
As the graph at right
shows, this reciprocal
selection creates a tight
match between newt
toxicity and snake
resistance for co-occuring
populations.
Trade-offs
 Coevolution
may come with a price. All
traits, including those under selective
pressure from coevolution, generally
involve trade-offs.
 Some alleles that benefit an individual's
ability to exploit (or avoid exploitation)
may reduce growth, survival, or
reproduction, and the allele's
advantageousness may be offset by
other disadvantages.
Red Queen Hypothesis



If one species evolves much slower than the
other, the slower evolver suffers a
disadvantage and would likely go extinct.
Species that evolve fast enough to keep up
with (or outpace) evolution in their enemies
will generally persist longer than those that
evolve more slowly.
Like the Red Queen, a species and its
enemies continually evolve "to keep in the
same place."
Red Queen Hypothesis
 Sexual
reproduction is energetically
costly.
 Sexual reproduction, through
recombination, speeds up the rate of
evolution, improving its practitioners'
ability to survive in a Red-Queen world.
 The Red Queen hypothesis is a likely
explanation for the evolution of sexual
reproduction.