Biology of Predation

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Biology of Predation

• Reading Smith and Smith, Chapters 15-16

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• The term predation has come to encompass a
  range of interspecific interactions where one
  species obtains its energy and nutrients by
  consuming another living organism.
• These include
• predation-strict sense
• this can include filter and suspension feeding
• parasitism
• parasitoids
• herbivory

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Predators, in the strictest sense, are animals
that hunt down other animals and eat them. In
general, predators are larger than their
prey, and consume many prey over the course of
their lifetimes. Some predators are specialists
to some extent, but many are generalists that
will consume all prey of a certain size. This is
an alligator lizard, Lacerta vivipara, it is a
generalist though primarily an insectivore.
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Not all organisms are strict predators- many
unicellular algae are facultative predators or
mixotrophs. -facultative predators, such as these
haptophytes photosynthesize, but also ingest
small protozoans that they catch via a
structure called a haptonema.
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-A large number of oceanic animals (and a few
protozoans) are filter feeders- they consume
prey suspended in the water column. -Filter
feeders take prey in a certain range of sizes- no
filter can catch prey of every size without
being tremendously inefficient. -Blue mussels
Mytillus edulis filter oceanic plankton from
moving waves-this can be either plant, animal,
or protist.
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Parasitoids hunt down animal hosts and use them
as food for their larvae. -ectoparasitoids
lay their eggs on the outside. -endoparasitoids
lay their eggs inside the host. -koinobionts
allow the host to develop for a while before the
larvae kill it and pupate. -idiobionts
stun-paralyze it right away. Either way, the
larvae kill the host and disperse
This is an ichneumenoid wasp attacking a sawfly
larva.
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Parasites encompass a wide range of organisms
that live on or within the host, and consume the
host without killing it right away. -ectoparasites
, such as this leech, attack the host from the
outside -endoparasites attack the host from
within. Microscopic parasites are frequently
called pathogens.
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Herbivores are animals that eat plants. Some,
such as these Asian antelope, are grazers-they
chop away the leaves of grass and
herbaceous plants without killing
them. Browsers-selectively eat parts of
woody vegetation. -in so doing, they resemble
parasites more than predators
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These aphids suck juices from plants, they are
essentially plant ectoparasites. Some
herbivores, notably bison, sheep, and elephants,
consume the entire plant, and thus resemble
predators
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Some Possible Defenses Against Predators

• Defensive behavior
• Toxins
• Sheer size
• Armor
• Speed
• Crypsis
• Mimicry

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Defensive Behavior

• Fighting back-is sometimes, but not usually, an
  option.
• For herbivores,specialization to the lifestyle
  usually precludes being able to fight off a
  sophisticated carnivore.
• For example teeth adapted for grinding plant
  material are useless as weapons.
• Generalist predators usually avoid prey that are
  able to hold their own, one strategy is to fool
  the predator into thinking you are tougher than
  you are-this is called a threat display.

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For example, this hognose snake, Heterodon
platirhinos has an impressive threat behavior
designed to deter predators. In fact, it is
nonvenemous Threat behaviors scare off some
generalists, like coyotes, but specialists, and
predators that have learned to ignore the
behavior, catch them anyway.
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Vigilance

• Vigilance is a very common, and presumably a very
  effective, antipredator behavior.
• Seeing the predator first confers options to the
  potential prey, such as
• running away
• hiding
• clustering into a dense mass
• warning ones relatives
• As we will see, vigilance can be costly, and the
  presence of predators can affect the growth of
  prey through fear and caution alone

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For example, Beldings ground squirrel, Spermophi
lis beldingi, is known for vigilance. Members of
a kin group forage together-at least one keeping
watch at any given time (they pay attention to
each others activity) When a predator is
sighted, the vigilant individual gives an alarm
call that potentially gives others time to
escape to their burrows.
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• Clustering together can be an effective defense
  against some predators, because groups of
  individuals under attack can make it very
  difficult for a predator to select a single
  individual as a target.

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• Sheer size works well. Elephants and giraffes
  have essentially no nonhuman predators as adults.
  
• The problem is growing that large, young
  individuals can be very vulnerable.
• Large size carries other costs and benefits as
  well, and it is not an option for many organisms
  (arthropods).

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Toxins

• Some animals, such as the poison arrow frog
  Dendrobates auratus manufacture powerful toxins.
• This defense is often coupled with bright color,
  which is thought to warn potential predators of
  the risk of attacking these animals

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• For instance, Milkweeds produce toxic compounds
  called cardiac glycosides, which can kill
  mammals.
• Monarch larvae Danaus plexippus have evolved to
  eat milkweeds and sequester the toxins for
  protection against predators.

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Crypsis

• Crypsis is the evolutionary modification of an
  organisms morphology, color, smell, or behavior,
  to avoid being detected.
• Predators can also be cryptic to avoid being
  detected by potential prey.

This is a lappet moth Phyllodesema americana
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Mimicry

• Mimicry is a widespread evolutionary adaptation
  to resemble species that predators are likely to
  recognize as poisonous.
• Batesean Mimicry-is deceptive, mimic is harmless
• Mullerian Mimicry-mimic is harmful, advantage is
  that predators are more likely to recognize
  potential trouble

Eumenid wasp
Syrphid fly
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Defenses Against Herbivores

• Passive defenses-these are always present
• Toxins
• Spines
• Silica-erodes mammal teeth
• Tannins-impedes edibility, digestibility,
  nutrition
• Induced defenses-these are present when the plant
  is under attack
• Toxins
• Tolerate herbivory

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Toxins

• Toxins are very common among vascular plants,
  particularly angiosperms.
• Toxins can provide very good defense against
  herbivory
• drawbacks toxic compounds are thought to be
  expensive to produce
• hervivores, especially insect specialists, may
  evolve resistance or even sequester them.

This is Jimson weed-Datura stramonium, it
produces a toxin deadly to mammals
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Tolerate Herbivory

• Many plants have evolved to simply tolerate
  certain kinds of herbivory.
• It is possible that certain types of plants
  actually benefit from some grazing.
• Meristems, undifferentiated tissue used to
  produces new shoots, stay under the ground or in
  sheltered locations-this allows rapid re-growth
  and prevents the herbivore from destroying the
  plant entirely.

Many grassland species are notably tolerant of
grazing.
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Predator-Prey Population Dynamics

• Real populations of prey, and their predators,
  tend to exhibit cycles in abundance in which the
  peak in prey abundance precedes a peak in
  predator abundance.
• The following is the classic (Elton, 1928) study
  of the Canada Lynx and the snowshoe hare. This
  figure is based on historical data using the
  numbers of hare and lynx pelts sold to the Hudson
  Bay company.
• Note the ten year cycles-originally attributed to
  sunspots by some.
• Note some of the drawbacks in using historical
  data such as this. Does this trend reflect
  economics?
• Subsequent studies have supported the notion that
  lynx predation is an important factor driving the
  cycle, partially because fear of lynx predation
  impairs the foraging efficiency of hares

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• The Lotka-Volterra model
• Alfred Lotka and Vito Voterra independently came
  up with a simple mathematical model of
  predator-prey population dynamics.
• For the prey population, it starts with the
  exponential model of population growth, and
  invokes a term for prey killed by a predator,
  thus
• dR/dtrR - cRP
• where R is the prey population,
• dR/dt is the growth rate of the prey population,
• r is the preys intrinsic rate of natural
  increase,
• c is a constant representing the efficiency of
  the predator, and
• P is the predator population

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• The predator population depends upon the prey
  population as well. Thus
• dP/dtacRP-dP
• dP/dt is the growth rate of the predator
  population
• where P is the population of the predator
• c is a constant reflecting the efficiency of the
  predator
• R is the prey population
• d is the per capita death rate of the predator
  population
• a is a constant reflecting the efficiency with
  which captured prey are converted into new
  predators.

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This model has some interesting properties

• There is an equilibrium point at
• Rd/ac Pr/c
• this equilibrium is neutral
• this model produces predator-prey oscillations
  that are neutrally stable-further pushing from
  the equilibrium produces cycles of greater
  amplitude.

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In each zone, you can draw a vector representing
the change in the population of predators, and
prey respectively -predatorup-down preyside-side

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• The original model is interesting in that it
  predicts the predator-prey oscillations
  frequently observed in nature.
• An increase in the birth rate of the prey
  increases the equilibrium density of the
  predator, but not the prey
• this prediction is borne out in simple
  bacteria-bacteriophage experiments by Bohannan
  and Lenski.
• It has several weaknesses, however
• completely neutral oscillations are not observed
  in nature-they are an artifact of the simplicity
  of the model
• model assumes efficiency of prey capture is
  independent of prey density-prey cannot be
  satiated
• model assumes no density-dependence on prey

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Density-dependence can slow recruitment of prey
populations as they reach carrying
capacity -This effect tends to dampen
predator-prey oscillations and make the
equilibrium point stable.
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MacArthur and Rozenswig argued that the shape of
the prey isocline should be a hill, because
recruitment falls off at low densities near zero,
and at very high densities near carrying capacity.
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Some predators tend to compete with each other at
high densities. This would change the shape of
the predator isocline. -This effect also tends
to increase the stability of the system, and make
the equilibrium point stable.
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• The functional response of a predator describes
  how its ability to impact the prey population is
  affected by prey density.
• Type I-density has no effect, the rate of prey
  consumption is directly proportional to density
• A type I functional response is implicit in the
  simple Lotka-Volterra model.
• Type II-prey consumption decreases at high prey
  densities because predators become satiated or
  because prey defend themselves as a group.
• Type III-predators become more efficient as prey
  become more common-this may trail off as they
  become satiated.

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Note that these are not isoclines
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Predator efficiency can have major effects on a
predator-prey system A-Less efficient predators
are only able to reproduce effectively when their
prey are near carrying capacity. More
stable C-Very efficient predators can easily
drive their prey extinct, and going extinct
themselves. Less stable
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Predator-Prey Coexistence

• In natural systems, some predators tend to drive
  their prey locally extinct, others do not.
• Some predator-prey systems are stable.
• Predators and prey may attain a stable,
  oscillating cycle in which predator abundance
  tracks prey abundance.
• Predators may tend to regulate prey
  populations, keeping them at a stable equilibrium
  below carrying capacity.

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• Some predator-prey systems are unstable.
• When driven locally extinct, the predator may
  either go locally extinct itself, or it may
  switch to another prey species. If the predator
  switches, to another prey, it then permanently
  drives the prey out of the habitat. The prey can
  only exist where the predator cannot or does not
  live.
• If predator and prey both go locally extinct,
  that may free the habitat to be recolonized by
  the prey. Ultimately, the predator may show up
  and the cycle may repeat itself.
• Such a system might be locally unstable, but the
  metapopulation might be stable.

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Stability vs. Instability

• Factors that promote stability
• inefficient predators
• density-dependence of either predators or prey
• predator switches to alternative food before prey
  go entirely extinct
• low time lags in predator response to prey
  density
• prey refuges

• Factors that promote instability
• very efficient predators
• inverse density dependence of predators or prey
• high time lag in predator response to prey
  density
• simple environments, no refuges

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Example, fish and Daphnia

• Freshwater fish, such as yellow perch and
  bluegills, are incredibly efficient predators of
  small crustaceans.
• Daphnia sp. are small, filter feeding, freshwater
  crustaceans with an enormous potential for
  reproduction, but with no defenses or
  antipredator behavior.
• When introduced to a lake, freshwater fish will
  drive vulnerable species such as Daphnia extinct,
  and then switch to other prey (usually insect
  larvae).
• Generally, Daphnia only persist in lakes with no
  fish.

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Yellow perch-Perca flavescens
Bluegill Lepornis macrochirus
Water flea-Daphnia pulex
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Example, Dieratiella vs. Aphids

• The parasitoid wasp, Dieratiella rapae, is an
  incredibly efficient predator-many factors make
  the system locally unstable
• Females lay one egg inside each aphid-one female
  can lay hundreds of eggs - thus their potential
  rate of increase is enormous.
• Females are incredibly effective in searching for
  aphids-they cue in on chemicals the plants emit
  to lure parasitoids
• There is a time lag between oviposition, and the
  death of the aphid-the aphid grows to adulthood
  and then is suddenly eviscerated by the
  parasitoid.
• Dieratiella quickly drives aphids extinct from a
  patch of host plants, and then disperses to find
  other hosts
• Once both species are gone, host plants may be
  recolonized by aphids

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Experiments

• In laboratory experiments, predator-prey systems
  in simple environments often result in the
  extinction of the prey, followed by the
  extinction of the predator.
• In an experiment, C.F. Gausse added the predatory
  protist, Didinium sp. to an already established
  colony of Paramecium sp.. The result was the
  extinction of the prey, followed by the
  extinction of the predator.

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• Refuges
• In a second experiment, Gausse added a glass
  sediment to the cultures. This provided hiding
  places for Paramecium.
• The result was the extinction of Didinium,
  followed by a rebound by the prey.
• Extinction-Recolonization
• In a third experiment, Gausse repeatedly
  inoculated the system above with Didinium.
• The result, was a cycling of predator and prey
  abundance.

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The Huffaker Mite Experiments

• C. B. Huffaker studied a predator prey system
  involving two species of mites.
• The six-spotted mite, Eotetrancyus sexmaculatus
  is a common mite that eats oranges.
• Typhlodromas occidentalis is a predator of the
  six-spotted mite.
• Huffaker sought to create an artificial system
  that would exhibit the population fluctuations
  found in real-world systems.

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Huffaker worked on an experimental array of
oranges, they were covered in such a way as to
enable him to control the surface area of the
system He also used rubber balls the same size
as oranges to add areas of unsuitable habitat
through which mites might need to pass to get to
better areas
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• Result-in simple systems, such as a single
  orange, or an array of oranges clustered
  together, predators quickly drove their prey to
  extinction, and went extinct themselves.
• In more complex systems, such as arrays of
  oranges at random locations, this process took
  much longer.
• Huffaker was finally able to achieve (temporary)
  population cycling, by adding Vasaline barriers
  to predator dispersal, and sticks to serve as
  launching pads for prey dispersal.

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