I. Types
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.
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?
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.
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.
(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.
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.