Environmental factors are classified as reactive if they change quantitatively in response to changes in the density of the subject population and if that change is sufficient to impact the birth and death rates of individuals in the population. Reactive factors are usually living organisms used as food, or enemies that use the subject population as food. Of course populations of food organisms change quantitatively because they are consumed and, thereby, removed permanently from the environment by the feeding organisms. Enemy populations change quantitatively because they obtain more food and thereby reproduce more offspring as the density of the prey population rises. If these changes in the reactive factors are sufficient to induce changes in the per-capita birth or death rates of the subject population, then a circular causal pathway (feedback) from the population back to itself is created and the 4^th principle will be evoked. In other words, reactive environmental factors are part of the endogenous feedback structure of the population system. When this feedback is negative, the population will tend to be stabilized at or around an equilibrium density but the second order dynamics in a variable environment are often cyclical.

Circular causal pathways can also be created if the resource changes qualitatively in response to changes in population density. Examples can be found in caterpillars that induce changes in foliage quality of their deciduous host trees depending on the intensity of feeding in preceding years (Baltensweiler and Fischlin 1988, Haukioja et al. 1988).

It is also possible, but not common, for physical factors to be affected by population density and to become part of circular causal pathways. For example, air and water pollution caused by expanding human populations may feed back to increase the rate of human mortality from cancer and other diseases.

The main dynamic consequence of the 4^th principle operating in a noisy environment is that the population will often exhibit regular cycles of abundance with fairly long period (often between 6 and 11 years) and high amplitude. An example is the larch budmoth in the Swiss Alps. We should be aware, however, that there are other possible explanations for the causes of regular population cycles, but all involve delayed - feedback in one form or another.

References

Baltensweiler, W. and A. Fischlin. 1988. The larch budmoth in the Alps. Pages 331-351 in Dynamics of Forest Insect Populations: Patterns, Causes, Implications (Berryman, A. A., ed.), Plenum Press, New York.

Haukioja, E., S. Neuvonen, S. Hanhimaki and P. Niemela. 1988. The autumnal moth in Fennoscandia. Pages 163-178 in Dynamics of Forest Insect Populations: Patterns, Causes, Implications (Berryman, A. A., ed.), Plenum Press, New York.

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