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.
page 68Table 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.
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.
Sex-ratios in increasing age classes
Males outnumbered females in 10 of the 14 samples listed in Table 1 but their contribution to different age classes varied greatly. In 9 of the 12 samples, males dominated the 0–2 year age class.
Table 3 is based on nine pooled samples and reveals the males dominated the 0–1 year class by 100 females: 135 males. The ratio rises to 100:141 if the large Hokitika and Hohonu samples are added. The 1–2 year-old class is also dominated by males (100 females: 124 males). From the age of 2–9 years the sex-ratio remained about equal but, over the age of 9 years, females outnumbered males by 38:14, or 19:3 over the age of 10 years.
page 71 page 72 page 73 page 74Age class (years) | Females | Males | Ratio Females/Males | Statistical Significance |
---|---|---|---|---|
0- 1 | 154 | 209 | 1:1.35) | X2 = 7.1 |
) | ||||
1- 2 | 234 | 292 | 1:1.24) | p < 0.01 |
2- 3 | 141 | 131 | 1:0.92 | n.s. |
3- 4 | 90 | 84 | 1:0.93 | n.s. |
4- 5 | 86 | 65 | 1:0.76 | n.s. |
5- 6 | 50 | 44 | 1:0.88 | n.s. |
6- 7 | 38 | 35 | 1:0.92 | n.s. |
7- 8 | 34 | 27 | 1:0.79) | |
) | ||||
8- 9 | 19 | 15 | 1:0.79) | |
) | ||||
9–10 | 19 | 11 | 1:0.57) | |
) | X2 = 4.166 | |||
10–11 | 7 | 1 | 1:0.14) | |
) | p < 0.05 | |||
11–12 | 9 | 2 | 1:0.22) | |
) | ||||
12–13 | 1 | 0 | 1:0) | |
) | ||||
13–14 | 2 | 0 | 2:0) | |
Total | 884 | 916 |
Age structure of samples in 'better' and 'poorer' condition
The large sample of female possums taken up the Hokitika River by Boersma (1974) came from 19 catchments. Boersma listed the age structure of each of the 19 samples and ranked each sample according to the average condition of the animals (based on fat reserves, asymptotic size, k, and fecundity). We have regrouped his data into two classes - those with a condition ranking above the average 9.6 and those below this average, and have calculated the age structure of each group (Table 4). The age structure of 'better' and 'poorer' samples showed no significant differences (X2 (13) = 19.62; P > 0.05).
page 75Condition of possums | Years old | ||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
0–1 | 1–2 | 2–3 | 3–4 | 4–5 | 5–6 | 6–7 | 7–8 | 8–9 | 9–10 | 10–11 | 11–12 | 12–13 | 13–14 | Total | |
Animals in 'poorer' condition | 95 | 35 | 51 | 45 | 50 | 32 | 24 | 14 | 7 | 6 | 5 | 2 | 1 | 1 | 368 |
Animals in 'better' condition | 140 | 107 | 82 | 76 | 63 | 58 | 37 | 22 | 15 | 17 | 7 | 8 | 0 | 0 | 632 |
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.
Annual and seasonal differences in mortality
Those resident adult possums known to have died or disappeared from the 14 ha study area of the Orongorongo Valley between 1966 and 1973 are listed in Table 6. Deaths and disappearances occurred more frequently in 1967 and 1968 than in the other years while very few animals died or disappeared during the warm winter of 1971. The number of resident animals found dead or disappearing from the study area fluctuated 15-fold (from 4 to 62) between the best and worst years.
page 76 page 77Table 6. Possum mortality. Seasonal and annual distribution of possums found dead or dying and resident animals known to have disappeared permanently from trapping grids in the Orongorongo Valley, Wellington.
Over the eight-year period, the greatest number of deaths occurred in July and fewest in March. Seasonally, 44.5% of deaths and disappearances occurred in winter, 20% in spring, 19% in summer and 16.5% in autumn.