Predation
Hypotheses for Patterns of
Diversity
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Evolutionary Time
Ecological Time
Primary Production
Stability of Primary Production
Structural (Habitat) Diversity
Climatic Stability
Competition
Predation
Predation
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In an ecological sense, predation is not
just carnivores like wolves eating musk
oxen, or coyotes eating mice.
In fact, deer are acting as predators on
plants, parasites act as predators on
their hosts, and mice act as predators
on the seeds they eat.
The difference here is in the extent of
the effect.
Predation
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Carnivory – capture, kill, and consume an
animal.
Herbivory – consumption of plant material by
an animal.
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Grazer/folivore consumes leafy material
Browser consumes woody material and bark.
Granivore consumes seeds
Frugivore consumes fruit
Exudivore consumes exudates like sap.
Predation
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Parasitism – association w/ host.
Objective is to keep host alive.
Generally, parasite stays with same
host throughout its life.
Parasitoids – parasitic activities limited
to larval stages.
Predation
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Carnivorous predation:
– predator must locate, capture, and
consume prey.
– Mammalian predators employ a diversity of
morphological, physiological, and
behavioral techniques to to this.
– Reptilian predators do this as well.
Compare a monitor lizard with a snake.
Predation
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Prey detection and recognition
– Search image.
– Smell - chemoreception
– Sound - bats and marine carnivores.
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Prey capture
– Stalk and ambush
– Finess.
– Pure power.
Prey Adaptations
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Avoiding detection
– Crypsis
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Avoiding capture
– Herd behavior in ungulates = safety in
numbers and increased vigilance.
– Detection of predator as in kangaroo rats.
– High speed locomotion, or use of refugium.
– Display as in baboons.
– Chemical defense as in skunks and toads.
– Body armor as in turtles.
Herbivorous Predation
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Herbivores use a variety of devices to
improve efficiency:
– Pectinate teeth in dermopterans.
– Thumb in giant panda
– Elongated intestines and ceacum and/or
ruminant stomach.
Plant Adaptations to Herbivores
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Chemical defenses such as tanins
– Grey and fox squirrels and red and black
oak acorns.
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Synchronous flowering or seed set
‘swamps’ potential herbivores – safety
in numbers.
Structural adaptations – spines in cacti
and euphorbs.
Effects of Herbivory
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For the most part, herbivory is not good
for the plant. However,
Grazing may increase production in
some cases.
Optimal Foraging
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Predators are under intense selection
pressure to find and consume prey.
We expect that organisms should
forage in a way that optimizes their
inclusive fitness.
How can this be done?
2 ways: Energy Maximizers and Time
Minimizers.
Optimal Foraging
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Energy maximizers:
Get maximum
possible energy
return under a given
set of foraging
conditions – EMs
get the maximum
amount of energy
possible.
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Time minimizers:
Get maximum
possible energy
return under a given
set of foraging
conditions – TMs
obtain a given
amount of energy in
the min. amt. of
time.
Optimal Diet
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There is a trade-off between a
specialized diet and a generalized one:
– Specialized diet: food items are of high
value, but may require extensive search
energy or search time. These items may
also require extensive handling.
– Generalized diet: food items may be more
abundant, but will not be of equal value.
Optimal Foraging
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Each item consumed contributes to the
average energy input. The better diet is
the one that increases the average
energy input.
The question becomes, should the
organism broaden its diet or narrow its
diet?
Optimal Foraging
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Energy input per item can be written as:
Ei
hi
Optimal Foraging
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In this formulation, we compare the
caloric content of each item, to the
handling time (or energy) required to
capture, subdue, and consume that
item.
Lets create a model that will allow us to
predict what an organism should do.
Optimal Foraging
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Define the amount of time spent
searching for prey as Ts seconds.
Our predator encounters 2 types of prey
at rates 1 and 2 prey per second.
These prey items contain E1 and E2
calories, and take h1 and h2 seconds to
handle.
Optimal Foraging
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If the predator spends Ts seconds
searching for prey, it will encounter:
n1 = Ts 1
type 1 prey
n2 = Ts 2
type 2 prey
Optimal Foraging
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The total energetic return, E, will be
equal to the number of encounters
times their respective energetic contents.
E = n1E1 + n2E2
Optimal Foraging
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The total time spent handling these prey
items will be:
Th = n1h1 + n2h2
Optimal Foraging
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Substituting for n1 and n2, we get:
E  Ts  1 E1  Ts  2 E2
E  Ts  1 E1   2 E2 
Optimal Foraging
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The total time spent handling prey is
given by:
Th  Ts  1h1  Ts  2 h2
Th  Ts ( 1h1   2 h2 )
Optimal Foraging
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So, the total time spent searching for
and handling prey will be:
T  Ts  Th
T  Ts  Ts  1h1   2 h2 
Optimal Foraging
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And the energetic return per unit time
spent searching for and handling prey
becomes:
T
(

E


E
)
s
1
1
2
2
E 
T T  T  h   h 
s
s
1 1
2 2
Optimal Foraging
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This simplifies to:
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E

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E
1
1
2
2
E 
T 1   h   h 
1 1
2 2
Optimal Foraging
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Try an example. Suppose our optimal
forager has 100 seconds to search for
prey. It encounters prey type 1 at a rate
of 0.10/s, and prey type 2 at 0.01/s.
Thus,
1 = 0.1
2 = 0.01
Optimal Foraging
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Also, prey type 1 contains 10 calories
and takes 5 seconds to handle, while
prey type 2 contains 10 calories and
takes 10 seconds to handle.
E1 = 10
E2 = 10
h1 = 5
h2 = 10
Should our predator be a generalist or a
specialist?
Marginal Value Theorem
Marginal Value Theorem