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Faecal Pellet Counts

Pellet counts have been used in many areas to estimate possum densities but, in general, the technique is most suited to bush areas where access is limited. Their most extensive application has been in the Haupiri research area where monthly assessments have been made over the period of a year.

An estimate of possum numbers is obtained by calculating the number of pellets accumulating each day in a given area and dividing this by the average number of pellets voided each day by one animal. Experience in the Haupiri indicates that to obtain a meaningful estimate, a large sample (in excess of 500 1-metre-radius plots) is necessary. Two such samples, normally one to two weeks apart, are required for a single estimate of possum density. In the first, pellet density is measured and pellets are identified in some way, such that when the second sample is run, the density of those which remain can be measured. The number of new pellets recruited into the population is basically the difference between the number of survivors and the total density in the second sample.

Although animal numbers may remain fairly static, pellet densities may fluctuate considerably. For example in the Haupiri, consecutive pellet densities fluctuated, on average by ± 42%. Wind and rain which caused, on the one hand a high rate of pellet disappearance through litter fall and mechanical breakdown, and on the other, lowered activity levels and therefore a reduction in the number of pellets being deposited, probably accounted for most of this variation.

Results from a 'one-hit' sample which do not include an estimate of pellet disappearance rate can only be interpreted in very broad terms. The animal controller who wishes to use pellet counts to measure the effectiveness of an operation is faced with a before and after survey consisting, in each case, of two pellet density estimates and a disappearance rate estimate: something like 30 man-days of effort.