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Proceedings of the First Symposium on Marsupials in New Zealand

Measurement of Damage

Measurement of Damage

The previous investigations have approached the problem of measuring possum damage in two ways: (i) by questionnaire surveys, and (ii) by analysis of stomach or faecal contents coupled with estimates of the density of possums. Both approaches are indirect and both contain errors. The results of questionnaires are possibly biassed from a variety of sources of error (Deming 1944, Oppenheim 1966). For example, farmers are not always aware of, or concerned about, possum damage on their properties, but this may not be apparent from their response to questionnaires. However, provided its limitations are recognised, a questionnaire survey can give a general appraisal of the situation which might serve as a guide to policy. Quantitative stomach analyses are also subject to several sources of error, including the difficulty of estimating the extent of root and tuber consumption (Vorhies and Taylor 1933), and faecal analyses require accurate determination of the differential break-down rates of plant materials (e.g., Dunnet et al. 1973, Fitzgerald 1976). However, the most serious limitations in extrapolating from analysis of diet to loss of production are: (1) the strong dependence on accurate estimates of the density of possums feeding on an area of farm land, (2) no estimate is made of any compensatory growth by plants in response to browsing or grazing, and (3) no estimate is made of the amount destroyed but not eaten. In such cases, reliable estimates of losses to the farmer must be based on measurements of yield (various references in Chiarappa 1971).

Methods of assessing reduction in yield caused by pests have many similarities to agronomic and horticultural yield assessments, such as described by Boswell (1974).

We adapted one of these methods to estimate losses to possums, as an initial step in a cost/benefit analysis, by placing wire-netting cages in unstocked paddocks to exclude possums from small replicated plots of about 3 square metres. Each protected plot was paired with an unprotected plot (separated by only 1 m to minimise site differences), and a paired comparison t-test (Sokal & Rohlf 1969) was used to compare yield. In only one trial out of six was there a significant reduction of yield definitely attributable to page 200 possums. This was a 26% reduction in a chou moellier/swede crop, but because of the small area involved (0.4 ha), the crop lost to possums in 3 months would have supported only 336 ewe equivalents for 1 day. It was felt that these results reflect the limitations of the trials rather than a lack of significant possum damage.

The method we used has the following disadvantages:

1.It can only be used over a small area at a time.
2.It is costly, time and labour-consuming.
3.The protected (enclosed) plots probably have a modified micro-climate which may influence yield (see Lynch 1966), although if this is predictable it may be possible to correct for it.

However, it is possibly the only method suitable for use on pasture.

For crops it would be desirable to develop an alternative method to give a measure of the loss after the damage has occurred. This could perhaps be achieved by scoring the frequency of damage, and determining the dry weight yield of damaged and undamaged plants at points along a transect. Having established a quantitative relationship between the amount of damage and the resulting loss of production, it should be possible to survey the amount of damage to estimate losses on a regional basis.