I. Types

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 it.

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?

A.  Laboratory Situations

1.  Prey 

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 a 'J' shaped 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.

 
2.  Predators

In the absence of enough food, the predator will starve and eventually die off.  As more prey becomes available, more predators can be supported.

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 birth rate.
 

 3. Interactions

Because of these factors, the predator-prey interaction can be divided into four different phases:

(a) 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 .

(b) 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.

 (c) 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 species.

(d) 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.
 

 B.  Real World Predator-Prey Interactions

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:

1.  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.

2. 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.

3.  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.