Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay
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Types of Consumption
Remember, predation differs from competition in that in predation, one
organism directly kills and consumes another.
Parasitism differs from predation in that parasites take their food from
the body of their host, usually live in or on the host but do not kill
I. Predator - Prey interactions
For a long time, natural oscillations in the numbers of a predator species
and its prey have been noted in the laboratory. These sorts of
oscillations have not been easily identifiable with wild populations.
One of the more important ecological questions asked by ecologists in the 20th
Century was why this discrepancy exists.
Let's start out simple, and allow there to only be two species in a system,
one a predator and the other its prey. How would we expect these two
species to interact?
First, let's allow lots of resources for the prey, so that they will
not reach their carrying capacity in the laboratory. In the absence
of the predator, we would expect that this population will grow rapidly in
an exponential manner.
If the predator is present, however, prey will be eaten and removed
from the population. The number eaten will be related to the number
of predators present: the more predators, the more eaten.
We can imagine that at some number of predators just as many prey are
eaten as are born.
In the absence of enough food, the predator will starve and eventually
die off. As more prey becomes available, more predators can be
To keep the predator population stable, the number of prey must be high
enough so that the the death rate of the predator does not go above the
Because of these factors, the predator-prey interaction can be divided
into four different phases:
- Initially, prey populations are low, keeping predator numbers down
as few prey are available to eat. Since the number of predators is
low, prey births will be more than deaths, allowing the prey population to
grow. As the numbers of prey increase, the predator population
(which had been falling from too little food) will stabalize .
- As predator populations are still low, the prey population
continues to increase. Enough prey are now present in the system to
allow for rapid growth in the predator population as well.
- The predator populations eventually become large enough that
they begin to eat more prey than can be born. At this point, the
prey population begins to decrease in size. However, the numbers of
prey are still large enough to allow for continued growth of the prey
- The number of predators in the system are still large enough to
make prey death rate exceed birth rate, so prey populations continue to
fall. However, the number of prey has now fallen so much that not
enough are left to maintain the predator population, and predators begin
to starve. The predator population begins to fall as well. We then
return to stage 1, where predators are so few they cannot keep prey
populations from rising.
Real World Predator-Prey
A number of assumptions built into this simple model are violated in the
real world, which leads to the rarity of predator-prey oscillations outside
of the laboratory. Some of these are:
- Prey populations cannot grow forever in size. Eventually
they will run out of resources (independent of the number of predators), and
their populations will stop growing. This limits the maximum number of
prey during times of low predators, and will also limit the number of
predators that population can support. This will tend to dampen the
oscillation, and the population sizes will not become exceedingly large.
- Predators will eat more than a single prey. When the numbers of
one prey become low, and they are not worth hunting, predators will switch
to something else. This will tend to keep predator populations more
constant in size.
- If prey become so uncommon that predators switch to other food
sources, the prey population may begin growing again. Because of this,
prey levels may not fall as low as they would if they were the only source
of food for the predator, again damping the oscillation.
All of these factors lead to more stable populations of
predators and prey, and make their stable coexistence possible.
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Created 2 September 2011, Last Update
02 September 2011
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