Exploitation
High
Low
Intimacy
Parasitoids
Parasite
Predator
Grazer
Low
Lethality
High
There are 4 general categories
• “True” predators
• Herbivores
–
–
–
–
Grazers
Browsers
Granivores
Frugivores
• Parasites
• Parasitoids
“True” predators
Herbivores
– Attack many prey
items in a lifetime
– Consume only a bit of
the victim
– Do not usually kill
prey in the short term
(but may do so in the
long term)
• Grazer
• Browser
• Granivore
• Frugivore
Parasites
• Consume part of their
prey
• Do not usually kill their
prey
• Attack one or very few
prey items in their
lifetime
• Parasitoids
Parasites
• Parasitoids
– which straddle the parasite
and true predator
categories - they lay eggs
inside their host which
they eventually kill
Predation is important because:
1. It may restrict the distribution of, or reduce the
abundance of the prey species.
2. Predation, along with competition, is a major
type of interaction that can influence the
organization of communities.
3. Predation is a major selective force, and many
adaptations of organisms have their explanation
in predator-prey coevolution.
1. Evolutionary arms race
4. Predation drives the movement of energy and
nutrients in ecosystems.
Alfred Lotka
(1880-1949)
Vito Volterra
(1860-1940)
Predation
start
• Rate of increase of prey
population
dH/dt = rH
population abundance
12000
10000
8000
6000
4000
2000
0
0
2
4
6
8
10
time
12
14
16
18
20
Predation
• Rate of increase of prey
population
– dH/dt = rH
– Predators eat prey
• dH/dt = rH-a'HP
– a' = capture coefficient
– H = Prey pop size
– P = Predator pop size
Predation
• Rate of increase of
predator populations
90
population abundance
dP/dt = -qP
– If only predators exist, no
prey, so predators die
100
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
time
60
70
80
90
100
Predation
• Rate of increase of
predator populations
dP/dt = -qP
– If only predators exist, no
prey, so predators die
• dP/dt = fa’HP-qP
– f = is a predation constant
» Predator’s efficiency
at turning food into
predator offspring.
– a' = capture coefficient
– q = mortality rate
Predation
• Equilibrium population
sizes
– Predator
•
•
•
•
•
dP/dt = fa’HP -qP
0= fa’HP -qP
fa’HP = qP
fa’H= q
H= q/fa’
– Prey
•
•
•
•
•
dH/dt = rH-a’HP
0= rH-a’HP
rH= a’HP
r = a’P
P = r/ a’
Predation
• Graphical Equilibrium
– Prey (H) equilibrium
(dH/dt=0) is determined
by predator population
size.
– If the predator
population size is large
the prey population will
go extinct
– If the predator
population is small the
prey population size
increases
Predator
Pop size
dH/dt =0
r/a’
Prey pop size
Predation
dP/dt =0
• Graphical Equilibrium
Predator
Pop size
q/fa’
Prey pop size
– Predator (P) equilibrium
(dP/dt=0) is determined by
prey population size.
– If the prey population size
is large the predator
population will increase
– If the prey population is
small the predator
population goes extinct
Predation
• Predator-Prey
interaction
– The stable dynamic of
predators and prey is a
cycle
dP/dt =0
Predator
Pop size
r/a’
q/fa’
Prey pop size
Do these models apply to
natural populations?
• Lynx/Snowshoe Hare - Arctic system where
there is one predator and one prey
Lynx/Snowshoe Hare Arctic system where
there is one predator
and one prey
Assumptions?
Rosenzweig & MacArthur (1963)
• Three possible outcomes of
interactions
• The oscillations are stable (classical
oscillations of Lotka-Volterra
equations).
• The oscillations are damped
(convergent oscillation).
• The oscillations are divergent and
can lead to extinction.
i) Prey iscoline
Predator density
N2
Prey increase
ii) Predator iscoline
Prey density
K1
N1
N2
Predator density
K2
Predator decreases Predator increases
Prey density
N1
Predator-Prey Models
– Superimpose prey and predator isoclines
• One stable point emerges: the intersection of the
lines
• Three general cases
Predator
isocline
a)
Population density
Predator Density
– Inefficient predators require high densities of prey
Damped oscillations
Prey
isocline
Prey Density
Time
Predator-Prey Models
• Three general cases (cont.)
Predator Density
b)
Predator
equilibrium
density
Prey Density
Population density
– A moderately efficient predator leads to stable oscillations
of predator and prey populations
Stable oscillations
Time
Predator-Prey Models
• Three general cases (cont.)
Population density
– A highly efficient predator can exploit a prey nearly down
to its limiting rareness
Prey Density
Increasing oscillations
Time
All these models make a series
of simplifying assumptions
• A homogenous world in which there are no
refuges for the prey or different habitats.
• There is one predator species eating one prey
species and there are no other species involved in
the dynamics of these two populations
• Relaxing these assumptions leads to more
complex, but more realistic models.
• All predators respond to prey in the same fashion
regardless of density
– Functional Response
Conclusions form field studies
• There is not a clear relationship between
predator abundance and prey population
size.
– In some, but not all cases, the abundance of
predators does influence the abundance of their
prey in field populations.
What makes predators effective
in controlling their prey?
• Foraging efficieny
– Within a patch, the searching efficiency of a
predator becomes crucial to its success.
– But searching efficiency varies with abiotic
factors and can also decrease at high predator
densities because of interference of other
predators.
Predation
• Response of predator to
prey density
– Numerical
– Aggregative
– Functional
Types of functional responses
•Limited by handling time
•The rate of capture by predator
•Alters Behavior
–Type I
–Type II
–Type III
Eat all you want
Eat all you can
Eat all you can, if
you can find it!
C. S. Holling
(1930–)
Types of functional responses
Slide 25
Not all predators are created equal
Keystone predator
• Bob Paine at
University of
Washington
mussel is a
competitive
dominant in
this system
Keystone predator absent
Keystone predator present
Other examples of keystone predators
Keystone predator absent
Keystone predator present
The effects of herbivory
• Individual plants are affected in the
following areas
– plant defenses
– plant compensation
• plant growth
• plant fecundity
Chemicals Defenses
• Quantitative Defenses:
• Qualitative Defenses:
• Prevent digestion as they
accumulate in the gut.
• Usually found in large
quantities in the plant parts that
are eaten.
• Most of these compounds are
“Carbon Rich”
• Common defense of plants
growing in nutrient poor soils
(conifers).
• Usually toxic in small
quantities.
• Found in relatively small
amounts in the portion of plants
that is eaten (leaves).
• These compounds are
“Nitrogen Rich” and therefor
expensive to produce by the
plant.
• More common in plants
growing on nutrient rich soils.