Proceedings of the First Symposium on Marsupials in New Zealand

Results

Results

Table 1 summarises the age structure of samples collected in the Hokitika catchment (1970), Kapiti Island (1968), Waitotara (1970 and 1974), Hohonu Forest (1973-74), Tokoroa (1974), the Orongorongo Valley (1966-1974), Tennyson Inlet (1971-75), Wainuiomata Valley (1976), Ashley Forest (1975-76) and Copland Valley (1978). Table 2 summarises results obtained in the Taramakau Valley in 1970 and 1971 by Bamford (1972). Data from Tables 1 and 2, converted to percentage frequency of occurrence, are summarised in Figures 1–4.

Table 1. Age structure of possum populations. Data from the Hokitika River Catchment from Boersma (1974); from Tokoroa after Clout (1977); Tennyson Inlet from Dr R. Bray and G. Struik; Ashley Forest from Warburton (1977); the Copland Valley after Fraser (1979); Hohonu Forest from Dr B.R. Cook.

Table 1. Age structure of possum populations. Data from the Hokitika River Catchment from Boersma (1974); from Tokoroa after Clout (1977); Tennyson Inlet from Dr R. Bray and G. Struik; Ashley Forest from Warburton (1977); the Copland Valley after Fraser (1979); Hohonu Forest from Dr B.R. Cook.

Table 2. Age structure of seven possum samples from the Taramakau Valley, Westland, 1970 and 1971 From Bamford (1972).

Table 2. Age structure of seven possum samples from the Taramakau Valley, Westland, 1970 and 1971 From Bamford (1972).

The effect of the May birth pulse on age structure

Treating each sex as a separate subsample, six of the nine collections made between October and early February (at Hokitika, Tokoroa, Waitotara and Copland Valley) revealed a modal age of 0–1 year (Fig. 1a-1d). All but one of the 8 samples collected between late February and September revealed a modal age of 1–2 years - the exception being from the pine stand burned over at Tokoroa four years previously (Fig. 2a-d).

This change in the modal age of samples can be explained as the effect of the main birth pulse of May moving through the population. Animals born in May remain in the pouch until about September. Until that date they are classed as pouch-young and do not figure in these samples. From October to early February the 0–1 year-olds dominate most samples. In late February, a transformation apparently occurs as the first layer of dental cementum becomes distinguishable, that is, at the age of 10 or 11 months. From late February on, these over-ten-months-old animals, with their first distinguishable cementum line, are classed as 1–2 year-olds and their numbers dominate the age classes until September or October when a new crop of 0–1 year-olds displaces them as the modal age class.

Exceptions to this pattern (the 1974 male sample from Waitotara, the Sept. male sample from Tokoroa, and the female sample from the Copland Valley) which display unexpected peaks in the 2, 3 and 4 year-old age classes, are considered in the Discussion.

Life expectancy and survivorship

The 103 possums found dead or dying in the Orongorongo Valley between 1966 and 1974 form a useful mortality series. Converted to a life table (after Ilersic 1970) they reveal that, of 1000 animals which left the pouch, 47 could be expected to survive until their 13th year. The mean life expectancy on leaving the pouch was 6.2 years. At 3–4 years of age, the animals could expect to live another 5 years (See Table 5).

The mean annual mortality rate for all the animals which left the pouch is 14.9%. The mortality rate varied with age, however, 0–2 year-olds suffering a 11.1–12.6% loss; 2–4 year-olds a 3.9–5.0% loss; and animals over 4 years old losing 10.3–45.3% of their age class annually.