When discussing the first three principles of population dynamics we assumed that the environment affected, but was not affected by, the population. Thus, although population density influenced the degree of intraspecific competition and cooperation between individuals, it had no effect on the environment and, in particular, had n impact on the abundance of resources or enemies. Hence, environmental factors acted as one-way inputs into the population system (see the top figure). The fourth principle of population dynamics recognizes that populations can affect the properties of their environments and, when this happens, feedback loops can be created between the population and its resources, enemies, or any other environmental component (see bottom figure).

Populations under the influence of the 4^th principle tend to "cycle" around their equilibrium levels, much like the larch budmoth. The reason for this cyclic behavior, rather than the sharp "saw-toothed" oscillations seen when the 3^rd principle operates, is that the –feedback between a population and its environment creates time delays in the feedback structure. Think of it this way: Suppose a population increases for some reason or another. This will make more food available for its natural enemies, which will then produce more offspring. This larger enemy population will then eat more prey, and thereby reduce the density of the prey population. This process however takes time -- time for the enemies to capture, digest, mate, and give birth -- and it is this time lag that induces the cyclical dynamics.

<Use BACK key to return to Sessions>

© 1998 Alan A. Berryman