Predation as an important process
• predation on a population may restrict distribution or reduce abundance of a prey
Useful to man if the prey is a pest, undesirable if the prey is a valuable resource
• Predation like competition can influence the organization of communities
• predation is a major selective force, and many adaptations such as warning coloration and
mimicry have arisen
What effect has the predator on its prey population?
This depends on
• the density of the prey and the predator
• the efficiency of the predator to catch the prey and the efficiency of the prey to escape
been caught.
Graphical models to explain fluctuations in predator-prey populations (Chapter 7).
Model based on the Lotka-Volterra equations:
For the prey population, it is assumed that
• in the absence of predators, the prey population increases exponentially
• the rate of removal of prey increases with an increase in encounter rate by predators
• the efficiency of the predators to catch prey is denoted (a)
• the rate of removal of prey is dependent on
o the number of predators C, and
o the number of prey N.
• the number of prey consumed by predators is thus = aCN
• in the presence of predators subtract from equation (1) the amount of individuals eaten by
predators
For the predator population
• the reproductive rate of the predators depends on the number of prey available.
• in the absence of prey predator numbers decline exponentially through starvation thus
• the rate of growth of the predator is dependent on:
• the rate at which food is consumed aCN
• he predators efficiency of turning the food eaten into offspring f
Thus the birth rate of the predator is faCN
• The Lotka Volterra equation for the predator is thus
The prey zero isocline
• At equilibrium, the predator or prey population must exist somewhere on the zero
isoclines.
• a) the prey zero isocline
• rN = aCN or C = r/a
• prey abundance increases at low predator abundance and decreases at high predator
abundance.
The predator zero isocline
C
• faCN = qC or N = q/fa
C
• predator abundance increases at high prey densities and
decreases at low prey densities
• c) when the predator and prey zero isoclines are combined we
obtain the behavior of joint populations in which we
r/a
have regular oscillations of predator and prey, this
occurs indefinitely.
Weak points and limitations of the Lotka-Volterra model
• the model is to simple
q/fa
NN
•
•
assumes homogeneity in the environment and among the organisms themselves.
the predator zero isocline in the Lotka-Volterra is vertical which means that the same
number of prey (q/fa) is sufficient to maintain any number of predators. Unlikely!!
The predator zero isocline
• More predators need more prey
• Predator zero isocline
• larger populations of predators must require larger populations of prey
• at high densities, even in the presence of excess food, most predator populations will be
limited by availability of some other resource
• thus the predator zero isocline is represented as a curve
As predator density increases, interference among predators increases, more fighting instead
of feeding, thus more prey is required
• At high densities of prey the rate of growth of predators will be limited by other factors,
nesting sites, safe refuges etc.
A
B
C
C
The prey zero isocline in the Lotka-Volterra is horizontal unlikely!!!
• The prey zero isocline is best represented as a curve
• At low prey densities there is no intraspecific competition
• As density increases intraspecific competition increases, the isocline decreases and
reaches the prey axis at the carrying capacity
• at the carrying capacity the prey population can only just maintain itself
A more realistic situation is obtained by superimposing both isoclines
Predator population high
• has the most predators and the least number of prey at the equilibrium point
• the predators are relatively efficient
• the least stable (most persistent oscillations)
• extinction of both the predator and prey could occur
Predator population intermediate
• At the equilibrium point a large number of prey are needed to maintain a small
population of predators
• predators are less efficient
• there are less persistent oscillations.
Predator population low
• eliminates oscillations altogether
• Most stable, strong predation limitation
• At the equilibrium point C* the number of predators is low and N* is close to K, the
carrying capacity.
• Introducing heterogeneity into the model
Presence of a refuge
• predator feeds on surplus prey which due to some reason or other are unable to find a
refuge.
• if the prey is provided with a refuge in which predation could be avoided the cycle
becomes stable without any further change.
• Example Macro-algae as a refuge from predation for recruits of the mussel Chloromytilus
chorus. JEMBE 191. 181-193 (1995)
•
Testing the assumptions of predator-prey models
 Laboratory studies
o Gause's experiments: i) Paramecium (prey) ii) Didinium (predator)
o when predator and prey are reared in oatmeal medium, the predator eats all the
prey and dies of starvation.
b) Gause provides a refuge for the prey =sediment; Paramecium in the sediment were safe
from the predator. The predator ate all Paramecium found above the sediment but could not
get to Paramecium in sediment-predators die of starvation.
o added immigrants into the experiment to maintain predator-prey interaction.
Conclusion

predators and prey can coexist, if there is enough diversity to allow for a refuge for the
prey
Introduced Predators
1. Dingo are predators on kangaroos in Australia
2. European foxes and feral cats in Australia damage domestic livestock
Functional response
 How an individual predator responds to prey density can affect how predators interact
with prey
The three types of functional response
 Type I: Individuals consume more prey as prey density increases (linear)
 Type II: Predators can become satiated and stop feeding, or limited by handling time.
 Type III: Feeding rate is similar to logistic curve; low at low prey densities, but increases
quickly at high densities (sigmoid)
•