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Proceedings of the First Symposium on Marsupials in New Zealand

General Discussion

page 84

General Discussion

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?

BROCKIE. No.

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.

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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.

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