BIOL 4120: Principles of Ecology
Lecture 13: Predation
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Outline (chapter 14)
14.1 Forms of Predation
14.2 Mathematic model of predation
14.3 Model suggests mutual population regulation
14.4 Functional responses relate prey consumed to
prey density
14.5 Numerical response of predator to prey density
14.6 Foraging decision making
14.7 Foragers seek productive food patches
14.8 Risk of predation can influence foraging behavior
14.9 Coevolution can occur between predator and prey
14.10 Animal prey have evolved defenses against
predators
14.11 Predators have evolved hunting tactics
14.12 Plants and herbivores, carnivores interact
Predation
Consumption of all or part of one living organism
by another
Prey  Predator
Functions:
1. Energy transfer
2. Predators are agents of mortality and feed on
living organisms rather than scavengers or
decomposers
3. Shape the community structure and evolution.
14.1 Forms of Predation

Types of predation
• Carnivory

Direct taking of animal prey for immediate consumption
• Hawk taking a mouse
• Herbivory

Consumption of plant material when plant is killed
• Consumption of nuts and seeds
• Parasitoidism

Predator lives in or on a host and eventually kills to provide
a food source
• Parasitic wasps
• Parasitism

Predator lives in or on a host and consumes, but does not
usually kill the host
• Ticks on mammals
• Cannibalism

Predation on same species (a special form)
• Tadpoles in a pond
14.2 Mathematical model for
predation
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Lotka and Volterra equation for predation
Prey dN prey
dt
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
 rN prey  cN prey N pred
Where cNpredNprey is mortality of prey due to predator.
c is per capita capture rate, and Npred, Nprey are the
number of predators and prey, respectively.
Predator
dN pred
dt

 b(cN prey N pred )  dN pred
Where b is efficiency of conversion of prey consumed
(cNpredNprey) and d is death rate of predators
Solving the equations

For predator density (dNPrey/dt=0)
• Npred = r/c



Growth rate of prey population is zero when density
of predators equals per capita growth rate of prey
divided by per capita capture rate of predators.
Any increase in predator density will result in
negative growth in prey population
For prey density (dNPred/dt=0)
• Nprey = d/bc


Growth rate of predator population is zero when rate
of increase of prey is equal to rate of mortality
divided by the product of b and c.
Thus the two equations interact and this
can be done graphically
Pred


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There is a cyclical
rise and fall in
both the predator
and prey
populations with
time
Density of
predators lags
behind density of
prey
Feast and Famine
scenario
Prey and
predators are
never quite driven
to extinction
Mutual population
regulation
14.3 Model suggests mutual
population regulation

cN_preyN_pred:
• For prey population, this term serves to
regulate population growth through mortality
• For predator population, it serves to regulate
population growth through two distinct
responses:
Predator’s Functional responses: the great the number
of prey, the more the predator eats. The relationship
between per capita rate of consumption and the
number of prey (cNpreyNpred).
Predator’s Numerical response: an increase in
consumption of prey results in an increase in
predator reproduction (b(cNpreyNpred).


The Lotka–Volterra model is widely criticized for
overemphasizing the mutual regulation of
predator and prey populations
• Still a valuable model
Additional factors that influence predator–prey
interactions
• Cover or refuges for the prey
• Difficulty of locating prey as it becomes scarcer
• Choice among multiple prey species
• Coevolution
14.4 Functional Responses Relate Prey
Consumed to Prey Density


The functional response is the
relationship between the per capita
predation rate (number of prey consumed
per unit time) and prey population size
• This idea was introduced by M.E.
Solomon in 1949
Three types of functional response (I, II,
and III)
• Developed by C.S. Holling
14.4 Functional responses related prey
consumed to prey density

Functional response
• Ne=cNpreyTs
• Ts: period of search time
• Ne: per capita rate of predation, i.e., # of
prey eaten during a given period of search
time, Ts.
• Type I functional response
• Ne=c NpreyTs
• Passive predator such as spider or the prey
is less sufficiently abundant (e.g., kestrels
and voles)
• All time allocated to feeding is searching.

Type II response
• Total time use include prey search and prey
handling
• T=Ts+(Th*Ne)
• Replace Ts with T-Th*Ne and rearrange the
equation, get:
• Ne=c NPreyT/(1+c NpreyTh)

Type II response
• Ne=c NPreyT/(1+c NpreyTh)
As prey # increase, Th*Ne increase, less time
for prey search, decrease mortality rate of prey.
this is the most commonly reported responses.

Type III functional response
• Sigmoid (S-shaped) response
• At high prey density, the response is the same as
type II response; however, the rate of prey
consumed is low when the prey density is low at
first, increasing in a S-shaped fashion.
•
•
•
•
Factors caused the S-shape response
1. availability of cover to escape the predators
2. predator’s search image
3. Prey switching. Switch to other preys (more
abundant)
Functional responses related prey consumed to prey
density

Functional response
• As prey increases,
predators take more prey
• But how
 Linear
• Rate of predation is
constant
 Decreasing rate to
maximum
• Rate of predation
decline
 Sigmoidal
• Reaches maximum
then declines
(Right panel is expressed as
proportion of prey density, #
prey consumed divided by
prey density)

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Linear Type 1 (European kestrel to vole)
 Mortality of prey simply density
dependent
 No limits on system
Decreasing Type 2 (weasel on rodent)
 Predators can only eat so much –
satiation
 Time needed to kill and eat prey
becomes limiting
Sigmoid Type 3 (warbler on budworm
larvae)
 Capture rate is density dependent
 Availability of cover
 Alternative prey when preferred is
rare (prey switching)
 Prey not part of predators search
image, not a desirable food source
Model of prey switching

Prey switching
• Palatable versus
less palatable
• Better return per
kill
• Less energy
needed to find
and kill an
abundant prey
14.5 Predators respond numerically to
changing prey density

Aggregative response in the
redshank
Numerical response
• Predators reproduce
more
 However
reproduction
usually slower than
prey
• Movement into high
prey density areas
 This aggregative
response is very
important as it
rapidly increases
predator density

Another example of
numerical response
• Bay-breasted Warbler
• Spruce budworms

Other numerical response as increased reproductive effort
•
•
•
•
Weasels as predators
Rodents as prey
Predators followed prey in reproduction
Increase of rodent was due to good harvest in 1990
14.6 Foraging involves decisions regarding
the allocation of time and energy
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Optimal foraging theory
Hypothesis: natural selection should favor
efficient foragers, those individuals that
maximize their energy or nutrient intake
per unit of effort.
Foraging: what food to eat; where and
how long to search; how to search.
Costs and benefits
• Cost: time and efforts on foraging
• Benefit: survive and reproduce more, fitness
A simple model

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Two kinds of prey: P1 and P2
Energy yields:
E1 and E2
Time required:
Th1 and Th2
Profitability
E1/Th1 and E2/Th2
If E1/Th1 > E2/Th2, then P1 is more
profitable:
When a predator searches for P1, but
finds a P2, should it eat it?
What is a P1 is nearby?
Optimal choice
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Determined by search time
Search time:
Ts1 and Ts2
Eat P2, profitability is E2/Th2
Search and eat P1, E1/(Ts1+Th1)
If E2/Th2 is larger than
E1/(Ts1+Th1), then the predator
should eat P2

Predators show prey
preference
• Optimum size for
prey of wagtail is the
middle prey length
• Small one easy to
handle, but E/Th is
small.
Wagtail
14.8 Risk of predation can influence
foraging behavior

Predator can be prey to other
• Insects—birds—owls


Balance energy gained against being
eaten
Avoid predator while foraging
• Change foraging behavior (e.g., time)
• Change foraging site

from most profitable, but predator-prone site to a
less profitable, but more secure part
14.9 Coevolution can occur between
predator and prey

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Predators exert a selective pressure on prey —
any characteristic that enables individual prey to
avoid being detected and captured by a predator
will increase its fitness
Natural selection should
• Function to preserve “smarter,” more evasive
prey
• Produce “smarter,” more skilled predators
Coevolution: as prey species evolve ways to
avoid being caught, predators evolve more
effective means to capture them
14.10 Animal prey have evolved defenses
against predators

Predator defenses: characteristics that function to avoid detection,
selection, and capture by predators
• Chemical defenses
 Pheromones to warn related species of attack
• Fish
 Poisons
• Arthropods and fungi, snakes: example, stinkbug
• Cryptic coloration (colors and patterns, object resemblance))
 Hide in normal environment
• Moths on trees. Flounder
• Flashing coloration
 Distraction
• Deer and rabbits, butterfly, grasshopper
• Warning coloration or aposematism (bold colors with patterns that
serve as warning to would-be predators)
 Learnt behavior due to bad experience
• Bees and wasps
• Snake
• Skunk (black and white stripes)

Predator defenses (cont.)
• Mimicry
 Copy coloration of toxic species
• Batesian mimicry of tropical butterflies
 Edible species mimic inedible species, nonvenomous mimic venomous species
• Mullerian mimicry
 Unrelated species have a shared color patterns
that function to keep predators away
 Snakes, social wasps
• Armor
 Difficult to kill
• Clams, hedgehogs, armadillos
• Behavorial defense
 Grouping together
• More difficult to attack a large herd, see African
antelope
• Timing of reproduction—predator satiation
(cicadas)
Predator Defenses
Cryptic coloration, warning coloration,
Mullerian mimicry, Batesian mimicry,
behavior defense

Predator defenses fall into two classes:

Constitutive defenses
• Fixed features of the organisms


Object resemblance and warning coloration
Induced defenses
• Defenses are brought about or induced by the
presence or action of predators
 Chemical defense
 Behavior defense
14.11 Predators have evolved efficient
hunting tactics

Hunting tactics
• Ambush

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Low success rate
Low energy consumption
Crocodiles, frogs, etc
• Stalking
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Long search time
Short pursuit time
Cats
• Extreme example is cheetah
• Pursuit
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Know where prey is present so there is a short search time
Long pursuit time
Wolves, lions, hawks
• Note this is a simplification
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Stalking can involve ambush at water hole
Pursuit can involve stalking if there is a large herd
Cats can use ambush
• Leopards up trees

Predators may use cryptic coloration to blend into
background and use deception by resemble the
prey
14.12 Herbivores prey on plants
Amount of biomass eaten by herbivores:
6-10% forest,
30-50% grassland
Outbreak of grasshopper, gypsy moths etc can kill
http://www.tnstate.edu/biology, chalktalk materials (Dr. Sam
McNaughton)
14.13 Plants defend themselves from
herbivores

Plants defend themselves
• Chemical

Qualitative inhibitors
• Poisons
 Fungi
Serve as call for helpers (corn, caterpillar,wasp)

Quantitative
• Tannins reduce protein availability
 Bushes in deserts
• Structural

Thorns, spines, etc
• Roses and Acacia

Note also carnivorous plants
• Ambush strategy
• Attractants
14.14 Plants, herbivores, and carnivores
interact

Complete
interaction
between plants,
herbivores and
carnivores
End
14.7 Foragers seek productive food
patches
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
Marginal value theorem:
Predict the length of
time an individual should
stay in a resource patch
before leaving and
seeking another
Length of stay is related
to the richness of food
patch (prey density),
time require to travel
(travel time t), and the
time required to extract
the resource.
Optimal foraging theory
predicts: predators
should abandon the
patch when the rate of
return is at its maximum
value, after which the
rate of return begins to
decline