BIOS 3010: Ecology
Lecture 8: Predator foraging & prey defense
•  1. Lecture Summary:
Bernard D Abrera at http://abacus.gene.ucl.ac.uk/jim/Mim2/dardanus.html
–  What is predation?
–  Predator diet breadth.
–  Preference & switching.
–  Optimal foraging.
–  Marginal value theorem.
–  Functional responses.
–  Interference.
–  Spatial distribution.
–  Ideal free distribution.
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Batesian mimicry in Papilio dardanus
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 1
2. Predation:
–  Predation is the ecological process that describes
the nature and dynamics of the interaction
between prey defense and predator foraging.
•  Defense against predators can also influence prey
foraging behavior as shown in Figs 9.18 & 9.19.
–  Here we will consider the nature of this
interaction behaviorally in an evolutionary context
and in the next lecture we will consider the
dynamics of prey-predator interactions
•  Remember Hutchinson's ecological theater and the
evolutionary play
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 2
3. Predator diet breadth and preference:
•  Food preference:
•  Monophagous:
–  Often specialists that feed on a single prey type
•  Oligophagous
•  Polyphagous:
–  Often generalists that attack many prey types.
–  Preference is defined as:
•  The proportion of a food type in an animal's diet being
higher than its proportion in the environment.
•  Table 9.1 and Fig. 9.14 - ranked profitability according
to energy return per unit handling time.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 3
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4. Switching diet:
Food items eaten disproportionately when common & ignored
when rare (Fig. 9.15). Predators also learn from experience.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 4
5. Optimal foraging:
•  To obtain food a predator must expend time and
energy to search for prey and then handle it:
–  Pursuing, subduing, accepting and consuming.
–  Optimal forager should balance costs and benefits of
searching and handling to maximize its overall rate of
intake.
•  Diets tend to be more generalist in unproductive
environments where prey items are relatively rare,
but specialist when prey are common and easy to
find (Fig. 9.17).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 5
6. The marginal value theorem:
–  Optimal patch residence time is defined in terms
of the rate of energy extraction at the moment of
departure (the marginal value of the patch)
see Fig. 9.22.
–  An optimal forager is assumed to maximize its
overall intake of a prey resource during foraging
bouts moving from patch to patch.
–  The currency to be maximized is energy and the
problem is how to do this without leaving a patch
too soon or staying too long.
•  See Fig. 9.23 for an example.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 6
2
7. Functional responses:
•  Relationship between predator consumption rate
and prey density:
–  Holling classified three basic types of functional response:
•  Type 1: linear rise of consumption rate with prey density to a
maximum level
–  e.g. filter feeders such as Daphnia (Fig. 10.8) or baleen whales.
•  Type 2: commonest; consumption rate rises at decelerating rate
with prey density to plateau (Fig. 10.9):
–  Search time decreases & handling time increases with
increasing prey density until all predator time is spent handling
prey which sets the level of the curve maximum.
•  Type 3: S-shaped (sigmoidal) with accelerating consumption at
low prey densities (Fig. 10.10):
–  Much like switching (shows the effect of learning, often through
search image formation).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 7
8. Mutual interference among consumers:
•  Increases in predator density lead to
intraspecific competition because of mutual
interference which depresses overall feeding
rates because searching efficiency is
reduced:
•  -m is the slope of searching efficiency that
decreases with increasing consumer density
and represents a coefficient of interference in
Fig. 9.10.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 8
9. Predators and patchily distributed prey:
•  Prey (food items) are usually patchily distributed (whether
random, aggregated or regular) and patches vary in quality or
quantity of food. So predators may show a density-dependent
aggregative response to prey density as in Fig. 9.11, thus:
–  1) Predators spend most time in patches with highest prey density
–  2) Most predators occur in patches with highest prey density
–  3) Prey in highest density patches are therefore most vulnerable to
predation and prey in low density patches are least vulnerable.
•  1) and 2) can be true, but some examples show inverse density
dependence or density independence as in Fig. 9.20.
•  3) may not be true because prey often aggregate for defense and so
predator aggregative responses may not increase the proportion of
prey attacked.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 9
3
10. Predators and patchily distributed prey:
•  Huffaker s oranges:
– Patchiness can also stabilize prey-predator
interactions as Huffaker showed in his
famous experiment with mites playing hideand-seek among oranges (Fig. 10.16) that
generated both spatial and temporal habitat
heterogeneity.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 10
11. The ideal free distribution :
•  Some conflict exists between the aggregative
response and mutual interference.
–  Initially high density food patches are attractive
but as more predators aggregate they interfere
more and so the patch is less profitable.
–  Hence the ideal free distribution - consumers
are ideal in their assessment of patch
profitability and they are free to move from
patch to patch according to profitability
(Fig. 9.27a) and patches should eventually be
equally profitable if consumers are equal (9.27b).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 11
Figure 9.18: Seasonal changes in predicted habitat
profitability & actual foraging by bluegill sunfish.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 12
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Figure 9.19: Effect of largemouth bass on
bluegill foraging location
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 13
BIOS 3010: Ecology
Lecture 8: slide 14
Table 9.1 (3rd ed.):
Dr. S. Malcolm
Figure 9.14: Prey profitability and
selection by crabs and wagtails
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 15
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Figure 9.17: Optimal diet choice in bluegills & great tits with
predictions of Charnov s (1976) optimal diet model.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 16
Figure 10.8: Type 1 functional response
Daphnia filtering yeast.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 17
Figure 10.9: Type 2 functional response - damselflies
eating Daphnia & bank voles eating willow shoots
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 18
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Figure 10.10:
Type 3 functional
responses for (a)
shrews & mice eating
sawfly cocoons, (b, d)
flies eating sugar
drops, (c, e) wasps
attacking aphids.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 19
Figure 9.10 (3rd ed.): Mutual interference among (a)
Encarsia wasps and (b) predatory mites.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 20
Figure 9.11 (3rd ed.): Aggregative responses in (a) parasitoid
wasp, (b) coccinellid larvae, (c) redshank, (d) woodpigeons.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 21
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Figure 9.20: Percent parasitism in the field showing
(a, c) direct density dependence, (b) inverse density
dependence, (d, e) density independence
Dr. S. Malcolm
Figure 10.16:
BIOS 3010: Ecology
Lecture 8: slide 22
Eotetranychus
(a) orangefeeding mite
dynamics
Eotetranychus,
(b) single
oscillations
caused by
predatory
mite, (c)
oscillations
sustained by
habitat
patchiness
Dr. S. Malcolm
Typhlodromus
Eotetranychus
BIOS 3010: Ecology
Lecture 8: slide 23
BIOS 3010: Ecology
Lecture 8: slide 24
Figure 9.22:
The marginal
value theorem.
Staying time
predictions based
on rates of
energy extraction
and cumulative
energy extraction
with time spent in
patches of food.
Dr. S. Malcolm
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Figure 9.23: Experimental test of the marginal
value theorem with caged great tits.
+energy
costs
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 25
Figure 9.27: Ideal free distribution in 33
ducks fed bread at 2:1 profitability
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 8: slide 26
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