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Proceedings of the First Symposium on Marsupials in New Zealand

Fluctuations in Density Patterns of Possums Trichosurus Vulpecula Along the North Bank of the Taramakau Catchment Westland, New Zealand

page 183

Fluctuations in Density Patterns of Possums Trichosurus Vulpecula Along the North Bank of the Taramakau Catchment Westland, New Zealand


Fluctuations in density patterns of common brushtail possum populations were studied by faecal pellet counts along the North Bank of the Taramakau catchment from 1969 to 1975. The study area contained two major vegetation associations, rata/kamahi forest and red beech forest. Variations in density patterns over the years indicated that peak carrying capacities in the beech zone were approximately half those in the rata/kamahi zone.

The upper forest transitional zone above both major forest types reached similar peak densities. Canopy damage in this zone, which coincided with heavy use of the understorey by deer, is discussed on the basis of aerial photography runs flown over the area in 1960 and in 1973.

In the winter of 1974 the whole area was poisoned by air with 1080 impregnated carrot. Approximately 85% of the population was removed by this operation. The greatest decline in pellet densities was recorded in the lower and mid forest zones.

General Discussion

WODZICKI. How do you assess your populations and what were the possible errors in your estimates?

PEKELHARING. I was unable to give full details due to lack of time. Basically, populations were assessed using faecal pellet surveys in plots at 20 m intervals along lines up the hill; each line probably consisted of 80 such plots and we were just scoring presence or absence of possum pellets in those plots. Using frequency/density transformations we worked out a density index which was multiplied by the plot area to give the equivalent of density of possum pellets per hectare. I have not put confidence limits on these estimates because I consider that the changes were so drastic that I feel fairly confident about the results - pellet counts ranged from about 500/ha to 4,000/ha.

YOUNG. You seem to have touched upon a very fundamental point, simply that the factors controlling the population at one density level may be quite different from those allowing the population to develop again after control. Otherwise if you don't accept this, then you must say that the population level of 1974 would have fallen naturally down to its present 1977 level without control. There must be two sets of factors operating - one at the medium density levels of 1974 and the other at the very low level immediately after this.

PEKELHARING. I did not want to complicate matters in my paper, but part of this area was poisoned in 1970 as part of Dr Bamford's work. There was quite a high kill so the population did decline quite markedly. We got the same amount of drop outside the poison area so I concluded that the population was at peak and that what was removed in the one area by the poison, was removed in the other area by natural decline.

page 184

YOUNG. The critical drop was the next one from 1974 to 1977, following poisoning. Is there any indication that the population has tried to increase again, or has tried and not succeeded?

PEKELHARING. No. After 3 years I would not say with certainty that it has increased, rather I would say it was about the same level - at least it is nowhere near the level it was formerly. Fluctuations in pellet counts make a more precise opinion difficult.