The Queensland Koala Phascolarctos cinereus and possum harvests were regulated from 1906-1927 and 1906-1936, respectively. Before that, there was an uncontrolled harvest. Historical data from the harvest period were analysed to gain information on P. cinereus and possum (mainly Trichosurus vulpecula) population ecology and status. P. cinereusi> numbers peaked in southern Queensland around the turn of the century or in the first decade of the 20th century. In central Queensland, they peaked later, probably in the 1920s, and in north Queensland there does not appear to have been any pronounced fluctuation in numbers. P. cinereus populations experienced diseases similar to those occurring today. Chlamydial diseases were common and occurred in most parts of the State, with cystitis being more common. Some diseases were not clearly identified. In contrast to current populations, P. cinereus populations then had many more episodes of high mortality, population size appears markedly more volatile and overall abundance was much higher. Southern Queensland populations declined greatly at the time of the harvests, but the central Queensland population expanded after the last harvest and harvest mortality does not provide an adequate explanation of population decline. Although it is sometimes said, particularly with regard to island populations, that Queensland (and New South Wales) P. cinereus populations differ from those of Victoria in that they do not exhibit local irruptions and excessively high abundance, the population phenomena appear similar to those occurring in Victorian populations. Queensland populations achieved very high growth rates that resulted in unsustainable population densities and, eventually, mass mortality. Possum populations were subject to very high harvests that were apparently sustainable, at least in the short term, and showed fluctuations that were partly similar to those of P. cinereus. The P. cinereus, and probably also the possum population fluctuations, are mostly consistent with models of herbivore irruptions. The koala population fluctuations are interpreted in terms of herbivore irruption theory. We suggest that koala population expansion was a response to an increase in food availability (possibly due to the development of regrowth vegetation and/or waves of dieback) resulting from land development patterns. A subsequent decline in food availability, accompanied by severe outbreaks of chlamydial disease (probably due to poor nutrition), led to severe declines in koala population size, often to levels well below the apparent carrying capacity. Populations of at least some possum species (T. vulpecula and possibly P. peregrinus) also appear to have experienced irruptions and declines, but the fluctuations do not correlate as closely with major aspects of the development of land for pastoralism and agriculture as do the changes in P. cinereus populations. Specific factors responsible for the expansion and decline of T. vulpecula and P. peregrinus populations were not identified. Very high harvests in 1919 and 1920 also seemed to affect possum numbers.
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