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EXPLOITATION
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Species 2
(predator P)
Species 1
(victim V)
-
Getting into the mind of the predator:
What constrains consumption?
Inside the predator’s daily planner:
6:00 am
8:00 am
12:00 pm
1:00 pm
3:00 pm
3:20 pm
6:00 pm
7:00 pm
7:10 pm
Get up and clean. Re-assert dominance in family group.
Begin searching for victims.
First victim spotted. Pursuit begins.
Abandon pursuit. Victim got away.
Second victim spotted.
Victim subdued and killed. Begin eating.
Victim eaten and sufficiently digested. Return home.
Arrive home. Try to mate.
Rest.
The point is:
1. Time is limited.
2. There are other things to do besides activities related to
eating (e.g. social interactions, mating rituals, grooming,
sleeping).
3. Eating-related activities consist of:
•
•
•
•
•
searching for prey
pursuing prey
subduing the prey
eating the prey
digesting (may not always exclude other activities)
The time constraints of foraging
other
essential
activities
foraging
Foraging time
The time constraints of foraging
Foraging time
other
essential
activities
Handling
time
search
handling
Search
time
The time constraints of foraging
Foraging time
other
essential
activities
Handling
time
eating
pursuing
&
subduing
Search
time
search
eating
pursuit &
time subdue time
Different species will allocate foraging time differently:
Filter feeder:
Sit & wait predator (spider)
eating
waiting
eating
subduing
Time allocation also depends on victim density and predator status:
Well-fed mammalian
predator:
Starving mammalian
Predator (victims at low dnsity):
eating
eating
pursuing
&
subduing
searching
pursuing searching
&
subduing
Building a theory of predation:
(After C.S. Holling)
ts
th
Total search time per day
Total handing time per day
tmax  t s  th
C.S. (Buzz) Holling
Total foraging time is fixed (or
cannot exceed a certain limit).
1) Define the per-predator capture rate as the number of
victims captured (n) per time spent searching (ts):
n
ts
2) Capture rate is a function of victim density (V). Define a
as capture efficiency.
n
 aV
ts
3) Every captured victim requires a certain time for
“processing”.
th  hn
t  t s  th
n
ts 
aV
th  hn
n
t
 hn
aV
n
aV

t 1  ahV
n
aV

t 1  ahV
0.1
Capture rate
0.09
0.08
0.07
0.06
0.05
0.04
Capture rate
limited by prey
density and
capture efficiency
0.03
0.02
0.01
0
0
50
Capture rate
limited by
predator’s
handling time.
100
Prey density (V)
150
Damselfly
nymph
(Thompson 1975)
Asymptote: 1/h
Decreasing prey size
The larger the prey, the greater the handling time.
(Thompson 1975)
Three Functional Responses
(of predators with respect to prey abundance):
Holling Type I:
Consumption per predator depends only on
capture efficiency: no handling time constraint.
Holling Type II:
Predator is constrained by handling time.
Holling Type III: Predator is constrained by handling time but
also changes foraging behavior when victim
density is low.
Per predator consumption rate
Type I (filter feeders)
Type II (predator with
significant handling
time limitations)
Type I:
n
 aV
t
Type II
n
aV

t 1  ahV
Type III (predator who
pays less attention to
victims at low density)
victim density
Type III
n
(aV )

2
t 1  (ahV )
2
Holling Type I functional response:
Type I functional response
Daphnia path
Daphnia
(Filter feeder on microscopic
freshwater organism)
Thin algae
suspension
culture
Thick algae
suspension
culture
Holling Type II functional response:
Slug eating grass
Cattle grazing in sagebrush grassland
Holling Type III functional response:
Paper wasp, a generalist predator, eating shield beetle larvae:
The wasp learns to hunt for other prey, when the beetle larvae becomes scarce.
We have seen that there are four possible
outcomes for two-species competition.
What are the possible outcomes for a
predator-prey system?
Gause’s Predation Experiments:
Didinium nasutum eats
Paramecium caudatum:
Gause’s Predation Experiments:
1) Paramecium in oat medium:
logistic growth.
2) Paramecium with Didinium in
oat medium: extinction of both.
3) Paramecium with Didinium in
oat medium with sediment:
extinction of Didinium.