New Zealanders have been fascinated by how many possums there are in New Zealand ever since the estimates of 60-70 million were generated in the 1980s. Northland Regional Council and the Animal Health Board asked Landcare Research to review the 1980s estimates of the number of possums in New Zealand taking into account the impact of control operations by the Animal Health Board, Department of Conservation and regional councils. This review indicates current expenditure is having a significant impact on possum numbers both locally at those sites where it is important to do so, and nationally.

Bruce Warburton delivered the results of this research to the National Possum Control Agencies' conference today. He said that, based on the carrying capacity of the 36 vegetation classes analysed, New Zealand could support an estimated 48 million possums in New Zealand if there was no control. This is considerable fewer than the 70 million widely quoted for last two decades and is a result of better habitat maps now being available and differences in carrying capacity values used.

However, when they took possum control efforts into account, the researchers estimated there would be "only" about 30 million possums now, an overall reduction of about 36%. (This reduction is conservative because it excludes possums taken by commercial fur hunters, possum control in sanctuaries, and other private conservation initiatives.)

Possum control is carried out over a total area of about 13.3 million ha, about half of the total vegetated area of New Zealand. During 2008/09, about 9.8 million ha had control related to managing bovine TB; 2 million ha were controlled by DOC for conservation purposes; and 1.5 million ha was controlled by councils for production and conservation purposes. About 8.3 million ha (62%) of the total area controlled is in the South Island.

On a regional basis Wellington had the greatest percentage reduction (87%). Hawkes Bay, Manawatu/Wanganui and West Coast also had reduction of greater than 50%.

The new estimates have been feasible because of the common use of Geographic Information Systems (GIS) by researchers and agency staff. Bruce Warburton's colleague, James Shepherd, was able to use a Land Cover Database (LCBD2) and EcoSat indigenous forest layers to obtain more accurate vegetation classification than what was available in the 1980s, and agencies including AHB, DOC and regional councils were able to provide spatial information for their possum control operations. The integration of these data enable the updated estimates to be made.

However, the scientists give several cautions. One, the figures are estimates only - there are many variables affecting possum numbers and densities over time and between locations. Two, the estimates of carrying capacity for many vegetation classes are not very robust. Third, and most important, controlling possums is not done for the sole purpose of reducing possum numbers per se; but to achieve a certain outcome (i.e. either to reduce the number of infected herds that have bovine TB or to protect conservation values).

So although the number of possums might be of general interest, it is critical that the focus of possum control remains on achieving the desired outcomes and that numbers killed or overall percentage reduction is not used to measure control success. Such general metrics have no relevance to effective management of possum in New Zealand.