Population Dynamics of the Australian Plague Locust, Chortoicetes Terminifera (Walker) in Central Western New South Wales Iii. Analysis of Population Processes.

Abstract

An analysis of population processes in the Australian plague locust showed that the key factor determining local population fluctuations was net migration. The density-disturbing effects of migration were over-compensated by the delayed effects of natural enemies on immature survival. There was no evidence that the amplitude of fluctuations was controlled at any stage by density-dependent processes, even at the highest densities encountered. Gregarization, although density-dependent and leading to daytime emigration: rarely stabilized population peaks because swarming populations remained relatively static. Densities never reached the carrying capacity of the habitats, even locally, because of the frequency of emigration at night. Local population extinctions due to emigration and low productivity were compensated by eventual immigration at night in favourable weather. In neither situation was the size of night migration shown to be density-dependent, although a spurious effect was detected. Variations in natality and survival had little influence on local population trends but, when synchronized over large areas by unusually favourable weather, influenced the course of large-scale fluctuations. Regional populations were more stable than local ones, due to spatial and temporal differences in reproductive success in a heterogeneous environment, and to peripheral emigration losses. Variations in the synoptic circulation and rainfall at the times of migration and reproduction cause stochastic processes to exert a major control of regional population fluctuations in the Australian plague locust.