Long-Term Trends in Possum Numbers at Pararaki—Evidence of an Irruptive Fluctuation

We examined possum trapping data collected from 1945 to 1989 in the Pararaki catchment to assess whether there was any evidence for a major natural decline in possum numbers several decades after colonisation and whether the population has subsequently shown any long-term trend in abundance. The catchment was probably colonised by possums around 1915-20. We found evidence for a major decline (c. 80%) in possum numbers between 1945 and 1965. There was no significant trend in our trap catches from 1965 to 1976, but in 1977 there was a further abrupt decline. The next decade was characterised by a progressive increase in possum numbers, which by 1989 reached a level slightly below that during the stable period 1965-76. We suggest that the population now experiences irregular fluctuations around a more-or-less stable equilibrium abundance. Our data thus support cautious application of the irruptive fluctuation model (sensu Riney, 1964) to possums, which have some of the demographic characteristics of larger ungulate herbivores.