Exploitation

1
Exploitation
2
High
Parasitoids
Parasite
Intimacy
Low
Predator
Grazer
High
Lethality
Low
3
There are 4 general categories

• True predators
• Herbivores
• Grazers
• Browsers
• Granivores
• Frugivores
• Parasites
• Parasitoids

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True predators
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Herbivores

• Attack many prey items in a lifetime
• Consume only a bit of the victim
• Do not usually kill prey in the short term (but
  may do so in the long term)

• Grazer
• Browser
• Granivore
• Frugivore

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Parasites

• Consume part of their prey
• Do not usually kill their prey
• Attack one or very few prey items in their
  lifetime
• Parasitoids

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Parasites

• Parasitoids
• which straddle the parasite and true predator
  categories - they lay eggs inside their host
  which they eventually kill

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Predation is important because

1. It may restrict the distribution of, or reduce
   the abundance of the prey species.
2. Predation, along with competition, is a major
   type of interaction that can influence the
   organization of communities.
3. Predation is a major selective force, and many
   adaptations of organisms have their explanation
   in predator-prey coevolution.
4. Evolutionary arms race
5. Predation drives the movement of energy and
   nutrients in ecosystems.

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Vito Volterra(1860-1940)
Alfred Lotka(1880-1949)
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Predation
start

• Rate of increase of prey population
• dH/dt rH

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Predation

• Rate of increase of prey population
• dH/dt rH
• Predators eat prey
• dH/dt rH-a'HP
• a' capture coefficient
• H Prey pop size
• P Predator pop size

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Predation

• Rate of increase of predator populations
• dP/dt -qP
• If only predators exist, no prey, so predators die

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Predation

• Rate of increase of predator populations
• dP/dt -qP
• If only predators exist, no prey, so predators
  die
• dP/dt faHP-qP
• f is a predation constant
• Predators efficiency at turning food into
  predator offspring.
• a' capture coefficient
• q mortality rate

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Predation

• Equilibrium population sizes
• Predator
• dP/dt faHP -qP
• 0 faHP -qP
• faHP qP
• faH q
• H q/fa
• Prey
• dH/dt rH-aHP
• 0 rH-aHP
• rH aHP
• r aP
• P r/ a

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Predation

• Graphical Equilibrium
• Prey (H) equilibrium (dH/dt0) is determined by
  predator population size.
• If the predator population size is large the prey
  population will go extinct
• If the predator population is small the prey
  population size increases

Predator Pop size
dH/dt 0
r/a
Prey pop size
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Predation
dP/dt 0

• Graphical Equilibrium
• Predator (P) equilibrium (dP/dt0) is determined
  by prey population size.
• If the prey population size is large the predator
  population will increase
• If the prey population is small the predator
  population goes extinct

Predator Pop size
q/fa
Prey pop size
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Predation
dP/dt 0

• Predator-Prey interaction
• The stable dynamic of predators and prey is a
  cycle

Predator Pop size
r/a
q/fa
Prey pop size
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Do these models apply to natural populations?

• Lynx/Snowshoe Hare - Arctic system where there is
  one predator and one prey

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Lynx/Snowshoe Hare - Arctic system where there is
one predator and one preyAssumptions?
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Rosenzweig MacArthur (1963)

• Three possible outcomes of interactions
• The oscillations are stable (classical
  oscillations of Lotka-Volterra equations).
• The oscillations are damped (convergent
  oscillation).
• The oscillations are divergent and can lead to
  extinction.

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i) Prey iscoline
N
2
Predator density
Prey increase
K
N
Prey density
ii) Predator iscoline
1
1
N
2
K
2
Predator increases
Predator decreases
Predator density
N
Prey density
1
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Predator-Prey Models

• Superimpose prey and predator isoclines
• One stable point emerges the intersection of the
  lines
• Three general cases
• Inefficient predators require high densities of
  prey

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Predator-Prey Models

• Three general cases (cont.)
• A moderately efficient predator leads to stable
  oscillations of predator and prey populations

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Predator-Prey Models

• Three general cases (cont.)
• A highly efficient predator can exploit a prey
  nearly down to its limiting rareness

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All these models make a series of simplifying
assumptions

• A homogenous world in which there are no refuges
  for the prey or different habitats.
• There is one predator species eating one prey
  species and there are no other species involved
  in the dynamics of these two populations
• Relaxing these assumptions leads to more complex,
  but more realistic models.
• All predators respond to prey in the same fashion
  regardless of density
• Functional Response

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Conclusions form field studies

• There is not a clear relationship between
  predator abundance and prey population size.
• In some, but not all cases, the abundance of
  predators does influence the abundance of their
  prey in field populations.

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What makes predators effective in controlling
their prey?

• Foraging efficieny
• Within a patch, the searching efficiency of a
  predator becomes crucial to its success.
• But searching efficiency varies with abiotic
  factors and can also decrease at high predator
  densities because of interference of other
  predators.

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Predation

• Response of predator to prey density
• Numerical
• Aggregative
• Functional

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Types of functional responses

• Limited by handling time
• The rate of capture by predator
• Alters Behavior
• Type I
• Type II
• Type III

Eat all you want
Eat all you can
Eat all you can, if you can find it!
C. S. Holling(1930)
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Types of functional responses
Slide 25
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Not all predators are created equal
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Keystone predator

• Bob Paine at University of Washington

mussel is a competitive dominant in this system
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Keystone predator absent
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Keystone predator present
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Other examples of keystone predators
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Keystone predator absent
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Keystone predator present
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The effects of herbivory

• Individual plants are affected in the following
  areas
• plant defenses
• plant compensation
• plant growth
• plant fecundity

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Chemicals Defenses

• Quantitative Defenses
• Prevent digestion as they accumulate in the gut.
• Usually found in large quantities in the plant
  parts that are eaten.
• Most of these compounds are Carbon Rich
• Common defense of plants growing in nutrient poor
  soils (conifers).

• Qualitative Defenses
• Usually toxic in small quantities.
• Found in relatively small amounts in the portion
  of plants that is eaten (leaves).
• These compounds are Nitrogen Rich and therefor
  expensive to produce by the plant.
• More common in plants growing on nutrient rich
  soils.