Dynamics of Predation

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Foraging continued Induced defenses of plants
to herbivory as opposed to constitutive
defenses Example Prickly Pear cactuses
introduced to islands with and without
cows Islands without cows prickly pear with no
spines Islands with cows prickly pear produce
spines in response
2
Optimal foraging theory in a nutshell (diet
selection)
1) All consumers are adapted to feed on certain
types of prey
2) All prey have certain benefits and costs
associated with feeding on them
Benefits energy, nutrients Costs energy to
digest, handling time, travel time, etc
3) Profitability of prey can be ranked in terms
of their relative costbenefit ratio
3
Predictions from optimal foraging theory (diet
selection)

• When preferred prey are abundant, predator diets
• are dominated by the most profitable prey

2) As preferred prey become scarce, predators
begin to include less profitable prey in their
diets
3) Diets are more diverse at low prey densities
4) Optimal versus opportunistic feeding? (e.g.,
fishes, wolves)
4
Risk sensitive foraging
Example 2 minnows foraging in safe and risky
habits (Gilliam and Fraser, 1987 Ecology)
refuge
2 or 3 predators
1 predator
no food
low food
higher food
How much more profitable does the dangerous
habitat have to be to lure the minnows into it?
5
Risk sensitive foraging
1 vs. 3 predators
1 vs. 2 predators
Line of no preference
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Dynamics of Predation

• Can predators reduce prey densities below
  carrying capacity?

2) Do predator-prey interactions cause
populations to oscillate?
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Consumers can limit their resources
Mites on strawberries
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Predators and prey often cycle
9
Predator-prey systems can be modeled with simple
math

• R is pRey popn P is Predator popn

dR/dt rR - cRP
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Predator-prey systems can be modeled with simple
math

• R is pRey popn P is Predator popn

dP/dt acRP - dP
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Lotka-Volterra predator-prey model at equilibrium
Predator decrease
Predator increase
Prey decline b/c too many predators
Prey increase b/c few predators
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pred ?
pred ?
prey ?
prey ?
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Predator-prey systems can be modeled with simple
math

• R is pRey popn P is Predator popn

dR/dt rR - cRP
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Predator functional response
Loss to predator population cRP
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3 possible forms of the Predator Functional
Response
Type I linear increase with increasing
prey assumed by Lotka-Volterra Type II
predator consumption saturates Type III
accelerating phase at low densities saturation
at high densities
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Exploring the Functional Response
1) Determine predator feeding rate at each prey
density. 2) Predator consumes as many prey as
they can in 20 seconds. 3) After completing all
trials for full range of prey densities for beans
(use randomized list), repeat with same predator
for the dried pasta as alternative prey. 4) Leave
yourselves at least 15 minutes to answer the
questions to hand in at the end of class.
Hint predation process includes the following
components 1) searching and encountering prey,
2) attacking prey, 3) handling and digesting
prey
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Numerical response

• Change in predator population size in response to
  predation
• Can be achieved through both population growth
  and immigration

19
Predator numerical response to prey
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Factors that stabilize predator-prey systems

• Predator inefficiency
• Density-dependence of predators or prey
• Alternative food sources for predator
• Refuges for prey
• Reduced time delays in predator response to prey
  dynamics