Proceedings of the First Symposium on Marsupials in New Zealand
Age Structure and Mortality of Possum Trichosurus Vulpecula Populations From New Zealand
The age structure of samples of possums from 11 areas in New Zealand varied seasonally, mainly in response to the major birth pulse in May. Between May and September, 1–2 year-olds predominated in trapped or poisoned samples. Between October and early January, 6–12 months-old animals usually predominated. Animals under 2 years old provided the bulk of most samples. Of 2492 females aged by counting annual cementum layers on teeth, 4 reached the age of 14 years. On leaving the pouch, possums in the Orongorongo Valley, Wellington, had a life expectancy of about 6 years, mortality being lightest in the animals 2–4 years old. The average annual mortality rate of adult and subadult animals in the Orongorongo Valley was about 15%. Males outnumbered females in the 0–2 year age classes in 9 of 12 samples analysed and, in pooled samples, 0–1 year-old males outnumbered females by 135–100. This disparity is attributed to a greater proportion of males in the pouch and a trapping and poisoning bias in favour of males. Females increasingly outnumbered males in the over-7-year age classes. In the Orongorongo Valley 44% of natural deaths of adults and subadults occurred between June and August and the number of animals found dead or dying ranged from 4 to 62 in their best and worst years.
Tyndale-Biscoe (1955) first investigated the age structure of common brushtail possum Trichosurus vulpecula populations. Using epiphysial fusion of the tibiae as a criterion he divided 120 Banks Peninsula animals into three age classes, the youngest of which were sexually immature and probably under a year old. Between May and August the proportion of 0–1 year olds fell from 48% to 23%. In one area with a history of severe trapping, young animals comprised one third of the sample whereas in another undisturbed area they comprised only 8% and those over 4 years comprised 55%. This implies that adult animals lived on average for more than six years (Tyndale-Biscoe 1975).
In 1953, 1449 possums were taken from the Orongorongo Valley, Wellington and, using tooth wear and skull sutures as an aging technique Kean (1975) estimated that 52.9% were over 4½ years old.page 64
Pekelharing (1970) developed a reliable method for allotting possums to annual age classes based on the deposition of cementum layers of the molars. Earlier techniques could be misleading (B.D. Bell, A.J. White, unpubl.).
Throughout this paper, animals without an inner cementum layer on the lower first molar are classed as 0–1 year olds. More precisely their age lies between about 6 months, when they first leave the pouch (and are first liable to be poisoned, trapped or shot) until they lay down their first layer of cementum. This is customarily thought to occur at 12 months, but the first layer may be deposited from about 10 months onward (See Discussion). Summarising:
No cementum layer =0–1 year old
1 cementum layer = 1–2 years old
2 cementum layers =2–3 years old
This definition is necessary because some authors group results on another basis, e.g. Bamford (1972) divided his samples into "pivotal" age classes - a pivotal age of 1 including animals between six and eighteen months, a pivotal age of 2 including animals between eighteen and thirty months old, etc. Because of this, Bamford's results cannot always be incorporated into our analyses.
Since Pekelharing (1970) first developed his aging technique, several workers have sampled possum populations and analysed them according to this method. The samples are very uneven, having been gathered for various purposes. Some were gathered within a day or two, other samples accumulated over several years. Some came from a small area, others over large areas. Most were poisoned, trapped or shot; one sample was found dead or dying. Fragments of history or recent reproductive performance are known for some samples. Little of the information has been published, most of it appearing in internal government reports or student theses. Enough analyses have now been made to look for useful generalisations and to try to explain differences and similarities between populations.
Bamford (1972) aged 1356 possums from an area stretching 19 km along the north bank of the Taramakau River, Westland. 686 were taken in 1970 from 3 areas where he thought the possums were on the increase, static or in decline. He may have been right about the status of these populations but the age structure of the three populations is statistically homogeneous (chi square page 65 test) and affords no evidence of differences (Fig. 4a). An additional sample of 237 animals was gathered over the same ground a year later after it had been poisoned with aerially distributed chopped carrot impregnated with 1080 poison and another sample of 196 from an unpoisoned area. Bamford showed that one-year-old animals were significantly under represented in the poisoned sample (X2 1 = 4.8, p < 0.02). Bamford suggested that, as the poison was laid when the young were still in the pouch (August), the sucklings may have succumbed to doses which left their mothers unaffected but passed to them in their mother's milk; or at weaning, the young may be possibly more susceptible to traces of 1080 than adults. Another possibility is that sublethal doses of 1080 stopped the flow of milk and the young starved to death.
Boersma (1974) poisoned, trapped or shot 1001 female and 1176 male possums over a large area of the Hokitika River catchment. Only the 213 males without cementum layers were aged. The females were divided into annual age classes. He found mortality was high in the first year of life and again increasingly so after the fifth year. Between 2 and 4 years his females enjoyed a low death rate. Earlier control programmes appeared to have left no mark on the age structure of the Hokitika animals.
Cook (1975) investigating the incidence of bovine tuberculosis among possums of the Hohonu Forest, Westland, estimated the ages of 1203 animals by dental cementum layers. He interpreted an excessive number of 7–8 year old animals as reflecting the introduction of tuberculosis to the area 7 or 8 years earlier.
Clout (1977) studied possums in young and old pine forest near Tokoroa and estimated the age of 244 animals excluding pouch-young. He interpreted the large proportion of 3–4 year-old males in one sample as reflecting the clearing and burning of the site 4 years earlier.
Crawley (1970) reported that the oldest of 218 possums, tagged and recaptured in the Orongorongo Valley between 1956 and 1970, was in her twelfth year.
Fraser (1979) working in the Copland Valley, South Westland, found that males dominated the younger age classes, females the older age classes.
The following samples of possums have been included in this survey:
|1.||Hokitika River catchment, October-December 1970. After Boersma (1974).|
|2.||Kapiti Island, June 1968. 56 animals were gin-trapped by commercial hunters in the bushed central part of the island along the Trig and McKenzie tracks, leading from Rangatira flats to the summit. These animals had been subject to moderate annual trapping for many years.|
|4.||Hohonu State Forest, Westland. Between January 1973 and February 1974, Dr B.R. Cook, livestock officer of the Ministry of Agriculture and Fisheries and Dr J. Coleman of the N.Z. Forest Service collected 2269 possums at two-monthly intervals. 1203 of these were aged by sectioning teeth. The animals were trapped and poisoned from pastureland at the edge of the bush to a distance of 3.6 km into the forest (Cook 1975).|
|5.||Tokoroa, September 1974. Clout (1977) poisoned 111 possums in a 3-year-old pine plantation and aged the animals by sectioning teeth.|
|6.||Tokoroa, December 1974. Clout poisoned 133 possums in a mature Pinus radiata forest, 10 km away from the young plantation.|
|7.||Orongorongo Valley, southern Rimutaka Range, 16 km east of Wellington, 1970-1973. 103 possums found dead on the ground or dying in cage traps were collected from broadleaf-podocarp forest near the mouths of Greens Stream and Woottons Stream (see Crawley (1970)). These animals were found during a capture-recapture study running since 1966 (Bell, this symposium) and are presumed to have died of natural causes.|
|8.||Tennyson Inlet, Marlborough Sounds, 1971-75. 68 possums were trapped over five years by Drs R. Bray and G. Struik in bush-covered hills.page 67|
|9.||Wainuiomata Valley, 26 February-5 March 1976. Officers of the Ministry of Agriculture and Fisheries poisoned 280 possums with cyanide along the bush edge bordering the southern Wainuiomata Valley, along the "5-mile track", and up the Peak and Dick Streams, Orongorongo Valley. These populations had been occasionally poisoned and trapped for many years.|
|10.||Ashley Forest, North Canterbury, May 1975-May 1976. Warburton (1977) gin-trapped 472 possums in Pinus radiata forest at fortnightly intervals.|
|11.||Copland Valley, South Westland, January-February 1978. Fraser (1979) sectioned teeth of 185 animals poisoned with cyanide.|
|12.||Between 1966 and 1977, 670 adult or subadult possums were trapped, marked and released in 14 ha of bush adjacent to the DSIR field station in the Orongorongo Valley and most of these were recaptured at monthly intervals for several years (see Bell, this symposium). Detailed dossiers on many animals were built up. A sample of 100 males and 100 females, having been repeatedly recaptured over a three-year period, some time between 1966 and 1973, and therefore considered to be permanent residents in the study area, disappeared from the records or were found dead before 1973. Six years elapsed without further trace of these 200 animals so they are presumed to have died. The date on which they were found dead or the date of their last capture is noted in Table 6.|
A molar tooth was taken from the lower jaw by various authors, ground down, stained, and examined by X40 microscopy to reveal annual layers of cementum as described by Pekelharing (1970).
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 68
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.
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 74
|Age 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|
|)||X2 = 4.166|
|)||p < 0.05|
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 75
|Condition of possums||Years old|
|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 77
Table 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.
The samples considered in this paper were collected in many ways and for diverse purposes. They present a heterogeneous base on which to build any sort of speculative analysis. The main inadequacy is that we know little or nothing of the stability or instability of the populations.
Collections of possums made over several months or years, although useful for investigating, for example, age-specific fecundity or the life expectancy of older animals, are of little use in reconstructing the recent history of the population. So many events and processes, acting separately or in concert, overtake the population during the period of collection as to make interpretation difficult or impossible.
'Instantaneous' samples make it possible to date the effect of recent events on the age structure with some accuracy.
The patterns of age structure were more regular in the female than in the male component in most samples. Males showed greater variations from the expected patterns, presumably because they were less sedentary than the females and because of their greater vagility - young males probably moved quickly to fill depopulated areas and distorted samples because they were more vulnerable to traps, poisons and guns.
Seasonal change in age structure
Two main patterns of age structure have emerged - that prevailing from October to early February, with a predominance of 0–1 year-olds, and a late February to September pattern, with a modal age of 1–2 years.
The 'February' transition comes unexpectedly early as the main birth peak for most New Zealand possums occurs in May (Brockie et al. 1979) and animals should not graduate into the 1–2 year class until May of the following year. The Wainuiomata sample (Fig. 2a) shows that the bulk of the animals had developed their first layer of dental cementum by late February or early March when they were probably 10–11 months old; more known-age yearlings must be page 79 examined to resolve this point and more field samples collected between February and May are required to date the transition in other localities.
Three notable exceptions to these age structure patterns require explanations. The Waitotara (1974) sample has a bimodal pattern peaking in the 0–1 year, and the 2–3 year age classes; and males in the Tokoroa sample of September 1974 have a peak in the 3–4 year age class. Clout (1977) explains the large number of 3–4 year old males in the September Tokoroa sample as being due to an influx of young males to fill gaps left by the clearing and burning operations 4 years earlier. The Waitotara animals were subjected to a drastic control program 3½ years previously and the large number of 2–3 year olds in the later sample is also probably due to the rapid influx of one-year olds to fill the gap. The Copland Valley males reveal the usual October-February age structure but the females are quite anomalous (Fig. 1d) with a predominance of 4–5 year olds - cf. 5 males and 20 females in this age class. Fraser (1979) does not attempt to explain this anomaly but it appears as though the population was disturbed in 1973 or 1974 and that it affected males more than females. Other possibilities are that few female young were produced in 1974-77, or that young females were subject to greater mortality than males during those years.
The large number (58.7% of males and 47.8% of females) of 1–2 year-olds from the Tennyson Inlet sample is not approached by any other sample. The trapped area consisted of a narrow strip of seaside forest at the back of which lay an extensive bushed hinterland. The high proportion of 1–2 year-olds probably resulted from the continued but light trapping over a small area, providing empty living space which was continually refilled with youngsters dispersing in from the hinterland. The ranger on Kapiti Island (Peter Daniel, pers. comm. 1978) reported a somewhat similar pattern. Over 18 months he shot some 130 possums near his house at Rangatira Flat. After the first 100 animals, almost every new animal shot was a young male.
Sex-ratio and age classes
The excess of males in the 0–2 year age class of most samples is probably caused by two factors:
Excess of males in the pouch.
Caughley & Kean (1964) confirmed that males slightly outnumbered females in 908 possum pouches, in the ratio of 100 females: 114 males.page 80
|2.||Sampling bias. Young animals, especially young males, experience a dispersal phase in the first year or two of life (Dunnet 1964). Males also exploit a larger home-range than females (Ward 1978) so, moving more widely, are more likely than females to encounter traps, poison or spotlight shooters, a point also made by Fraser (1979).|
Table 3 shows that females dominated all 12 age classes over the age of two years. Without further pooling the differences in the sex-ratio are too small to sway tests of statistical significance but their consistency is impressive.
Over the age of 9 years females dominate the pooled samples by 38:14 and by 19:3 over the age of 10 years. This excess of females in very old age classes is not exceptional in mammals. Dall sheep Ovis dalli, Orkney voles Microtus orcadensis and humans show the same tendency (Caughley 1966).
The high mortality rate of 2 year-old males may be attributed to the extra hazards they meet while dispersing and their continuing decline after the age of 3 years may be caused by their having to maintain a larger home-range than females with its attendant extra demands. Another possibility is that male possums share the kind of reproductive stress shown by the brown antechinus, Antechinus stuartii, which goes into a general decline after mating. The metabolic rate of the male Antechinus shifts up during the breeding season and the animals move into a negative nitrogen balance in April, unlike the females which remain in a positive nitrogen balance during the winter (Woollard 1971). Our records from the Orongorongo Valley and Kapiti Island show that most males lose considerable weight in the winter whereas females generally maintain or increase their body weight (see Bell, this symposium).
The greater longevity of females does not show up in the sample of animals dying natural deaths in the Orongorongo Valley, despite the four thirteen-year-old females. The small size of the over-eight-year-old samples precludes statistical tests of significance.
Mortality rate and life expectancy
The calculations on mortality rate and life expectancy of possums in the Orongorongo Valley (Table 5) give a spurious appearance of accuracy and may be wide of the mark because (1) the sample size is so small, (2) both sexes are run together in the calculations but there is every possibility that the sexes suffer differential mortality rates, and (3) the table assumes a constant page 81 population density before and during the period of collection. We have evidence, however, that possum numbers rose from about 7 to about 15 animals per hectare in 1972 and fell away again after that date (unpublished data). The calculations therefore give a clue to mortality rates and life expectancy but the details are open to question.
However, Boersma (1974), who also supposed his mixed samples were from a stationary population, obtained rather similar results. He calculated that females suffered high mortality in their first year, a reduced rate between their first and fourth years and an increased rate after the age of four. These similarities between very different populations perhaps give some plausibility to the calculations.
We must concur with Caughley (1974) that the age structure of possum populations is of little value in estimating whether a population is on the increase or decrease. Age-specific birth and death rates, the incidence of double breeding and spring births, the migration or dispersal of certain age classes, overwhelming degradation or improvements in the habitat, control operations, food crop successes or failures, and perhaps predation and disease, shape the pattern of age structure. Add to these factors the bias of most sampling methods and the usual demographic changes which overtake an animal population from month to month, and the hazards of interpretation become apparent.
Age estimation is nevertheless a valuable exercise as it throws light on the age of maturity, age-specific reproductive performance and mortality, the potential and actual rates of increase, the age-specific effects of diseases and control operations.
Our thanks go to Dr J.M. Bamford, Dr M.N. Clout, B.R. Cook and B. Warburton for allowing us to use unpublished data. Dr Bamford, T. Ball, Dr Clout, M.G. Efford, Dr J.A. Gibb and S. Pledger kindly commented on an earlier draft of this paper.
Bamford, J.M. 1972. The dynamics of the possum (Trichosurus vulpecula Kerr) populations controlled by aerial poisoning. Unpublished Ph.D. thesis. Zoology Department, University of Canterbury.
Bell, B.D. 1981. Breeding and condition of possums Trichosurus vulpecula in the Orongorongo Valley, near Wellington, New Zealand, 1966–1975 . In Bell, B.D. (Ed.) Proceedings of the first symposium on marsupials in New Zealand. Zoological Publications from Victoria University of Wellington 74: 87–139.
Boersma, A. 1974. Opossums in the Hokitika River catchment. New Zealand Journal of Forestry Science 4: 64–75.
Brockie, R.E. , Cowan, P.E. , Efford, M.G. , White, A.J. , & Bell, B.D. 1979. The demography of Trichosurus vulpecula in Australia and New Zealand. Ecology Division, DSIR, Report.
Caughley, G. 1966. Mortality patterns in mammals. Ecology 47: 906–918.
Caughley, G. 1974. Interpretation of age ratios. Journal of Wildlife Management 38: 557–562.
Caughley, G. & Kean, R.I. 1964. Sex ratios of marsupial pouch young. Nature 204: 491.
Clout, M.N. 1977. Aspects of the ecology of possums in pine plantations. Proceedings of the New Zealand Ecological Society 24: 128–129.
Cook, B.R. 1975. Unpublished report. Condition index of possums. Section 8 of the Epidemiology of Tuberculosis on the West Coast of the South Island. Hohonu Mountain MAF/NZFS Project 117.
Crawley, M.C. 1970. Longevity of Australian brush-tailed opossums (Trichosurus Vulpecula) in indigenous forest in New Zealand. New Zealand Journal of Science 13: 348–51.
Dunnet, G.M. 1964. A field study of local populations of the brush-tailed possum Trichosurus vulpecula in eastern Australia. Proceedings of the Zoological Society of London 142: 665–695.
Fraser, K.W. 1979. Dynamics and condition of opossums (Trichosurus vulpecula Kerr) populations in the Copland Valley, Westland, New Zealand. Mauri Ora 7: 117–137.
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Ward, G.D. 1978. Habitat use and home range of radio-tagged opossums Trichosurus vulpecula (Kerr) in New Zealand lowland forest. In Montgomery, G.G. (Ed.) The Ecology of Arboreal Folivores. Smithsonian Institution Press, Washington, D.C., pp. 267–87.
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FORDHAM. Nearly four years after you had cleared the Waitotara population you still had animals of 4 years and more in the population. This suggests an ability to disperse is not restricted to one-year-olds. What do you know about the inequalities of these animals apart from that related to sex? Do you have data comparable to that collected from small mammal studies?
BROCKIE. We know rather little about these aspects of the animals we studied.
ANONYMOUS. Winter has got some information in his thesis about changes in the home-ranges of adult animals.
B.D. BELL. Much home-range data has been collected in the Orongorongo live-trapping study and I feel sure this could throw further light on differences and relationships between animals and on changes throughout the life-history of individual possums. Regarding the Waitotara kill in 1970, while it may have been quite a good one I do not think the population was entirely cleared, so there would presumably be residual animals of all ages left.
SPURR. I have also been looking at age structure between different possum populations. When you say that you have compared age structures statistically and do not obtain significant results, what do you mean?
BROCKIE. We tested statistically using X2 tests.
SPURR. Do you have age structure data for the live Orongorongo possums?
BROCKIE. We have a fairly good measure of the age structure of the resident animals but we have less information on the transient animals which pass through our study areas.
FITZGERALD. You suggested that by controlling one and two-year-olds you may make drastic inroads into your population if the animals are not in very good condition. I think this is a debatable point. A high fat index does not necessarily mean the animal is in good condition for all purposes. If you have animals in what we call 'not such a good condition' which will not breed until their second or third year, then by removing your one and two-year-olds you are not going to eliminate your breeding animals. So really the impact of such removals depends to some extent on the condition of your animals.
BROCKIE. Mine was a wide generalisation and I agree it may need qualifying along the lines you suggest.
FITZGERALD. Any inroads into the population would not be permanent would they?
FITZGERALD. So it would only be a temporary situation rather than a permanent control measure?
BROCKIE. Yes, you would have to come back in later years and do the same thing. Mind you, you might have fewer animals then.
KEBER. But with large scale movement and dispersal that may make up for any loss.page 85
YOUNG. Auckland wouldn't believe that.
BROCKIE. It would depend on a variety of factors, including the scale of the operation. It would only work if you could work on a large area so that relatively few animals would be moving in.
YOUNG. I think we shall need a workshop discussion because we must differentiate between established animals and home-ranges as opposed to your first and second age class groups which are wandering and subject to greater mortality. The mortality younger animals suffer may only be because they cannot become established due to the adult population already there.
CUMMINS. I'm intrigued that a major controlling influence on the population seems to be the New Zealand winter. Has there been any research into the possible effects of for example the effects of detergents on thermo-regulation? I know this has been done in birds but it seems we might very well look at this in terms of the possum losses.
BROCKIE. I can't quite follow the mechanics of this.
CUMMINS. Detergents reduce the insulating qualities of the fur.
KEBER. There is no oil in the fur anyway so you would not be gaining much from it. The animals are very susceptible to wetting with water.
CUMMINS. So we need to increase the rainfall even more!
HATHAWAY. You mentioned a population of good-conditioned animals had a high mortality rate?
BROCKIE. Yes, this was the study of Boersma on the West Coast and referred to mortality in the first year of life.
HATHAWAY. Is it possible that your good-conditioned animals entered the ranks of the poor-conditioned animals after a hard year or winter? They swell the ranks of the poor-conditioned animals so this group appears to have high mortality but in fact there is a change in the composition of the population of good-conditioned to poor-conditioned ones.
BROCKIE. If I understand you right, that could be a possibility. My remarks regarding the West Coast study were an over-simplification. There are many catchments and tributaries to the Hokitika River. Some populations have been poisoned there for 10 years almost regularly while others have been poisoned occasionally, so there is a mosaic of different sub-samples.page 86