Lecture 11: Predation
Recall: Over half the species on Earth obtain energy by feeding on other organisms, in a variety
of types of interactions. All are exploitation – a relationship in which one organism benefits by
feeding on, and thus directly harming, another.
Figure 12.5 – guppies experiment; when given 2 kinds of prey , the guppies ate
disproportionate amounts of whichever prey was most abundant.
Switching may occur because the predator forms a search image of the most common
prey type and hence tends to orient towards that prey, or because learning enables it to
become increasingly efficient at capturing the most common prey type.
Classic Model equation: ratio of prey 1 eaten relative to prey 2 = preference for prey 1
times ratio of prey 1 in habitat, relative to prey 2.
Population cycles can be caused by feeding relations, such as a
threeway interaction between predators, herbivores, and
A specific effect of exploitation can be population
Lotka and Volterra evaluated these effects
Prey equation N is the number of prey. P is the
number of predators, r is the population growth rate
and a is the capture efficiency
When P=0, the prey
population grows exponentially. When prey doesn’t = 0 the rate of prey capture depends
on how frequently they encounter each other (NP) and efficiency of prey capture (a).
The overall rate of prey removal is aNP.
Predator equation: N is the number of prey P is the number of predators, m is mortality
rate a is capture efficiency and b is the efficiency with which prey are converted to
If N=0, predator population decreases exponentially at death rate m. When prey are
present N doesn’t = 0, individuals are added to the predator population according to
number of prey killed (aNP), and the feeding efficiency with which prey are converted to
predator offspring (b).
Prey pop. Decreases if P>r/a and increases if Pm/ba
Zero population growth isoclines can be used to determine what happens to predator and
prey populations over long periods of time
Combining these reveals that predator and prey populations tend to cycle.
A considering the prey pop first, the abundance of prey
does not change when dN/dt=0, which occurs when
B considering the predator pop, the abundance of
predators does not change when dP/dt=0, whichi occurs
C by plotting the abundance of predators versus
abundance of prey. They have the inherent tendency to D By plotting the abundance of both predators and prey
vs time; the four inset diagrams plot the abundance of
predators vs the abundance of prey.
Note that the predator abundance curve is shifted ¼ of a
cycle behind the prey abundance curve.
The LotkaVolterra predatorprey model suggests that
predator and prey populations have an inherent tendency
It also has an unrealistic property: the amplitude of the
cycle depends on the initial numbers of predators and
Why don’t predators cause prey extinction? prey
In natural systems, predators rarely cause More complex models don’t show this dependence on
1. Habitat complexity
• Population cycles are difficult to
achieve in the laboratory.
• In Huffaker’s (1958) experiments with a
predatory mite that eats the herbivorous
sixspotted mite, both populations went
• When Prey is easy for predators to find,
predators typically drive prey to
extinction, then go extinct themselves.
• Figure 12.21B when a predatory mite was
introduced into this simple environment, it drove
prey to extinction, causing its own population to go extinct as well.
Figure 12.23 – Results 2. Spatial refuges Figure 12.22 –
from further experiments 3. Prey switching
indicated that these 4. Prey prey and
unusual population cycles predators both
were driven by persisted,
evolutionary changes in illustrating a form
of “hide and
the prey population. seek”
5. Evolution • In experiments with rotifer predato