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Proceedings of the First Symposium on Marsupials in New Zealand

Sample Strategy

Sample Strategy

Since di is a Poisson variable the lower the value of di the lower the variance and the smaller the sample required (Johnston & Kotz 1969). Now di is determined from a number of observations of baits taken (ni) and the larger the number taken at time (i) the better the estimate of di. These two variables can be controlled separately. The di is determined by animal numbers and may be varied by changing bait station design to make the station less conspicuous and possibly by varying bait type or the use of lures. The ni can be varied by altering the number of nights (i) the baits are set at each assessment and removing or leaving baits between assessments. Since much of the labour is in establishing bait lines it is usually more efficient to make several assessments on the same lines rather than to increase the number of bait stations. Samples of less than 100 stations however appear too small.

This requirement of 100 stations is difficult to satisfy in one line as at a spacing of 40 m between stations it represents 4 km. Such a long line of uniform habitat is usually difficult to achieve or traverse even in exotic forest. Often 1 km is a more practical distance and so sets of 20 or 25 stations are more realistic. In most areas lines will need to be grouped on the basis of uniform habitat or density. The value of this stratification is evident in the Western Bays data (Table 4) where in different habitats rates of kill and of subsequent re-invasion vary. The highest kill was obtained in the high density areas (as expected) but recolonization was also most rapid in such areas, presumably because of the high residual population in adjacent areas.