Density Dependence and the Stabilization of Animal Numbers 2. the Pine Looper

Abstract

To test the hypothesis that the density-dependent mortality of advanced larvae of the pine looper kept density within limits (i.e. regulated), the fluctuations of numbers in the field population studied by Klomp are compared with those in the null model in which, on average, this mortality of older larvae is kept at the field level, but the density dependence is removed. It is concluded that the density-dependent mortality of advanced larvae, whether or not in concert with the following density-dependent reduction of fecundity, did not regulate density, but rather had a somewhat destabilizing influence by increasing the chance that very low densities are reached. In an earlier paper the same effect was shown to occur in the winter moth population studied by Varley and Gradwell. It is suggested that this destabilizing tendency may result from the fact that the action of a significantly density-dependent factor can be frustrated by correlations with other, more independent and mutually non-randomly interrelated, mortality factors, by which the potentially stabilizing effect can get lost, and may even turn into the reverse.