Introduction

The common brushtail possum Trichosurus vulpecula has long been recognised as a harmful browsing species in New Zealand's indigenous forest. In addition, in the last decade it has been identified as both a vector and reservoir of two diseases of farm stock, namely bovine tuberculosis and leptospirosis, and to be an important competitor of farm stock for both pasture and fodder crop species.

The control of possums foraging out onto farmlands relies heavily on the extensive use of compound 1080 applied to vegetable baits and sown throughout adjacent forests. The dramatic rise in control costs (see Coleman, this symposium) increases the need to maximise the percent kill; in particular the need to control that proportion of the possum population which is most "at risk" to Tb. infection. A more efficient approach to aerial control with 1080 clearly requires better information on the ecology of possums in page 52 forest/pasture margin situations. With this control objective in mind we started studying the foraging patterns of possums with the intention of defining the zone of contact on forest/pasture margins between possums and stock, and thus delimit more accurately the width of the adjacent forested slopes requiring control.

The study discussed here is continuing on the slopes of Mt Bryan O'Lynn near Lake Haupiri in central Westland, and involves possums on a 800 m wide strip of land extending from improved stocked pasture at 230 m to alpine grasslands at 1350 m altitude. In this paper we discuss the broad patterns of population movements between forest and pasture, and the implications of these movements from a control perspective. Further seasonal details of movements, other aspects of possum biology, and the interaction between possums and rata/kamahi (Metrosideros/Weinmannia) forest will be published at the completion of the study.

Methods

Seven trap lines have been established; the lowest traverses the forest/pasture margin, the highest traps the alpine scrub zone (Fig. 1). The upper lines run between two adjacent ridge systems and all lines are aligned so as to intersect the numerous forest-to-pasture possum runs. Two hundred and fifty wire-mesh, live-capture traps are permanently positioned on sites with favourable possum sign. These traps have been set for three fine nights per month since August 1975, and produce monthly catches of between 250 and 350 animals.

Newly captured animals are anaesthetised with ether, identified with an ear tag and tattoo number on the inner ear, then weight, length, reproductive status and condition of teeth are recorded. On subsequent captures we weigh animals and determine their reproductive status without using anaesthetic. Non-kill snares have also been set in the lower regions of the hill at different times.

Fig. 1. Percentages of forest-dwelling possums trapped at least once on pasture on the bush edge line. Arrow widths are proportional to total catch and also to Fig. 2 arrows.

Results

The number of possums captured is presented in Table 1.

Great variation in trapability exists between animals. At one extreme are a few animals with over 50 recapture records; at the other are some animals with only two recaptures spread over two years. For the following analysis we included only those animals that had three or more recapture records (to April 1977) which gave a sample size of 412 animals with a total of 4521 capture and recapture records.

For the evaluation of bush/pasture movement patterns it is necessary to determine each possum's "home area" and look at the movements made from that area. There will always be difficulties with trapping data in the determination of "home" or "primary" areas as opposed to "secondary" areas. Both would fall within the animal's "home-range" area as it is normally defined, but the distinction has been made here, perhaps arbitrarily, to distinguish between normal den site areas and what are assumed to be more distant feeding areas.

Data presented in Figures 1, 2 and 3 are based on the following assumptions:

When the 412 animals had been assigned to "home lines" the distribution of animals per line was as follows:

Downhill movements of possums are presented in Fig. 1, uphill movements in Fig. 2. The pool of animals assigned to each "home line" was considered in turn and the number of animals from that line that had been caught, at least once, on each of the other six lines was calculated. These numbers are expressed as percentages in Figs. 1 and 2, or can be estimated since the width of the arrows in the figures are proportional to each other. It is important to stress that Figs. 1 and 2 do not give frequencies of movements but only a minimum estimate of the proportion of animals from any given line that moved, at least once, to other lines.

Thus Fig. 1 shows that of the animals for whom line 9 is their "home line", 76% have also been caught at least once on the bush-edge or pasture. The remaining 24% have not been caught any closer to the pasture than on line 9. Similarly, 29% and 31% of the animals that have J loop and O loop as their respective "home lines" have also been caught on the bush edge or pasture. Since our primary interest in this paper is in bush-to-pasture movements, Fig. 1 shows only the movements from each "home line" down to the bush-edge/pasture traps and omits movements to intermediate lines; for example, movements from 0 loop to line 12 are not shown.

Despite the difficulties inherent in the analysis of trap data some interesting comparative points emerge in Figs. 1 and 2. The "magnet effect" of the pasture is most obvious; relatively few possums in the low or mid forest are captured above their "home lines" compared with the larger proportions that travel down to the pasture. We have even caught two of the 60 "slip-line animals" in pasture traps (not shown in Fig. 1). The pasture feeders from the low-forest area (Level 9, Level 12) are moving 400–700 m from their den site areas. Pasture feeders from the mid-forest J and 0 loop areas have to cover, on average, 1200 m ground distance and a vertical drop page 56

Fig. 2. Percentages of possums trapped at least once on lines deeper into the forest than their "home line". Arrow widths are proportional in Figs. 1 and 2.

page 57 of 350 m during their travels to the bush-edge. As these animals generally return to den sites the same night then the total distance for one pasture feeding excursion may exceed 2.5 km through mature rata/kamahi forest and some dense cutover vegetation adjacent to the bush-edge.

Two features of the uphill movement patterns are worth comment. First, the low occurrence of uphill movements when compared with downhill movements has already been mentioned. Secondly, there is a virtual absence of uphill movements in the mid-forest zone, particularly by Level 9 and Level 12 possums. One explanation for this discontinuity may be that dietary requirements of low-forest animals are fully met by species in their immediate or pasture environment.

The recapture data can also be used to give some indication of the frequency of movements between lines whereas Figs. 1 and 2 only show the proportion of animals that move between lines. Hence Fig. 1 suggests 76% of level 9 animals forage out onto pasture but does not indicate if these animals travel to pasture every night or once a month. An estimate of movement frequencies was obtained by summing the capture/recapture records for resident animals of each "home line" and calculating the percentage of captures that were made on other lines. The results for bush to pasture movements are shown in Fig. 3 by the solid arrows; dotted lines give Fig. 1 data for comparison. The pattern for uphill movement changes in a similar manner and is not presented here.

Frequency data suggest the amount of travel between lines represents 10%-25% of the activity of possums that make such moves. As might be expected O and J loop animals are less likely to travel to pasture when compared with Level 9 and 12 animals. Only if possums are equally trapable in both "home line" areas and pasture areas will these data accurately reflect the frequency of travels to pasture. Further analysis of trap data does not test this assumption and only through methods such as radio-telemetry tracking can we begin to resolve the problem.

Discussion

Preliminary investigations of the foraging movements of possums on Mt Bryan O'Lynn suggest quite different patterns exist for peripheral and deep forest-dwelling animals. While the majority of possums denning within 700 m of adjacent pasture make frequent return forages out to the pasture page 58

Fig. 3. Captures made on pasture or the bush edge line as a percentage of all capture records for forest-dwelling possums. Arrow widths are proportional; Fig. 1 data are given by dashed arrows for comparison.

page 59 and back, other possums denning between 700 and 1200 m from the pasture come out less frequently. Possums denning in deeper forest rarely make such extended journeys and in contrast to possums denning near to pasture, have local home ranges. Overall, there is a great deal of individual variability in the foraging patterns of possums; a few animals nest within 300 m of the pasture and yet ignore it, while a few that den between 700 and 1200 m seem to feed on the pasture almost every evening.

Direct comparisons with other published movement studies should be treated cautiously. Winter (1963) worked in a forest remnant habitat close to urban areas and estimated range sizes of 1.1 ha for males and 2.1 ha for females, but recognised that these were underestimates because of movements into surrounding scrub areas. In an area of indigenous forest in the Orongorongo Valley Crawley (1973) estimated that range sizes were slightly smaller; around 0.9 ha for males and 0.6 ha for females. Crawley also gives figures for the average distances moved from the initial point of capture. For males 95% of the movements were less than 115 m; the corresponding value for females was less than 90 m. Dunnet (1956, 1964) reported larger range areas, 3 ha for males and 1 ha for females, in open eucalypt forest near Canberra. These values are similar to Jolly's (1976) estimates for a Banks Peninsula population (males, up to 3.6 ha, females, up to 1.2 ha) in a mixed pasture, bush and scrub habitat.

The Bryan O'Lynn population is a markedly more mobile population than any previously studied; especially when the animals within 700 m of the forest/pasture margin are considered. Range movements are five to ten times longer than those for Orongorongo Valley animals (Crawley 1973) and home range areas are several times larger. Jolly (1976) reports range movements for males that approach the corresponding values for Bryan O'Lynn animals, but in a habitat of widely scattered food sources. In fact, previous workers had not studied populations occupying indigenous forest adjacent to pasture, despite the ubiquitous nature of this habitat throughout New Zealand. The disparities in movement between the populations in these various studies exemplify, once again, the remarkable adaptive abilities of the possum to a diversity of habitats.

The striking difference in mobility between the Orongorongo and Bryan O'Lynn animals in indigenous forest might be explicable entirely in terms of the attraction of the pasture forage at Bryan O'Lynn. However, just as page 60 colour and size differences distinguish these two populations, so might more subtle genetic differences be having an effect on the behavioural responses of the respective populations. There are no data to test this hypothesis at present.

Radio-telemetry studies are now underway on selected Bryan O'Lynn possums which will contribute significantly to our further understanding of movement patterns. Nonetheless we can suggest, with some confidence, the width of the zone that will require control in forest/pasture margin situations. Elimination of a majority of the pasture foraging animals should follow from a control operation on a 1200 m wide forest zone starting at the pasture margin. Successful control of this zone would create a buffer zone between stock and the deep forest possum population. The long-term effectiveness of this buffer zone will depend on the rate of reinvasion of the area from peripheral possum populations and on the amount of dispersal and successful establishment by sub-adult animals from the deeper forest.

References

Coleman, J.D. 1981. Tuberculosis and the control of possums Trichosurus vulpecula - an expensive business. In Bell, B.D. (Ed.) Proceedings of the first symposium on marsupials in New Zealand. Zoological Publications from Victoria University of Wellington 74: 211–219.

Crawley, M.C. 1973. A live-trapping study of Australian brush-tailed possums, Trichosurus vulpecula (Kerr), in the Orongorongo Valley, Wellington, New Zealand. Australian Journal of Zoology 21: 75–90.

Dunnet, G.M. 1956. A live-trapping study of the brush-tailed possum Trichosurus vulpecula Kerr (Marsupialia). Csiro Wildlife Research 1: 1–18.

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.

Jolly, J.N. 1976. Habitat use and movements of the opossum (Trichosurus vulpecula) in a pastoral habitat on Banks Peninsula. Proceedings of the N.Z. Ecological Society 23: 70–78.

Winter, J. 1963. Observations on a population of the brush-tailed opossum. Unpublished M.Sc. thesis, University of Otago, New Zealand.

General Discussion

PRACY. Am I right in assuming that you accept that the habitat was a good one with good condition animals?

GREEN. Yes, certainly the animals were in good condition for their mean weight was 3 kg or more, and many males weighing 4 kg were caught. The condition of animals varied with their position on the hill. Those in the middle region tended to be in poorer condition than those nearer the pasture edge. I would say the habitat is good at this stage compared with many other West Coast areas nearby.

PRACY. Would this be in relation to food availability?

GREEN. Yes.

PRACY. The reason I asked the question is that it is not uncommon for established possum populations to move distances of 1 km or more from forest to pasture margin, but I have never seen it with a colonising population.

GREEN. Clearly the food supply has changed a great deal from the peak population time. I am not suggesting it is as good as it used to be, but to the eye there is still a great amount of greenery which we normally class as palatable.

MORIARTY. Would you be prepared to guess how frequently a possum might journey down to the pasture and back again?

GREEN. We certainly have had some animals in the mid-hill zone caught on the pasture edge and then up in their forest area two nights later - some 1200 m away. Whether they do it nightly or whether they 'stopover' I cannot say until we have done radio-telemetry work. I would not be surprised if some do it nightly.

YOUNG. You have not opened up tracks on the slope which would facilitate possum movement?

GREEN. No, only in the alpine scrub zone did we actually cut a track. We have been well aware of the danger of influencing movements through cutting tracks. We have not really opened up the understorey to any extent.

WODZICKI. Many of your possums are coming to the grass, which is a renewable crop, and you actually find that they do not do much damage. What would be their range inside native forests? Do they do the damage they might have been expected to do had they not moved but had remained in their one area of native forest?

GREEN. It is a difficult argument because one would have to show that the numbers would be as high if there was no pasture. If there was no pasture then presumably it would be a more difficult habitat for them and numbers might be therefore lower. So I cannot follow that argument through to say that if they did not have the grass they would be wiping out all the forest.

B.D. BELL. In the Orongorongo study we have no evidence of such long movements in the adult animals. For 3½ years we trapped possums in two lowland forest areas on river terraces some 3 km apart and there was no evidence of lateral movement between areas. There is some evidence of dispersal by younger animals, of both sexes, up to distances of page 62 10.5 km. Within the forest study areas on reaching maturity the possums appear to settle into discrete home-ranges of a few hectares, perhaps making periodic excursions of only a hundred or so metres. We have little evidence of vertical movements from higher altitudes down to the river terraces, although only a little trapping has been carried out above our main study areas.

GREEN. This more sedentary pattern would suggest what? The drawing power of our pasture, or the food supply in the Orongorongo forest?

B.D. BELL. I think it likely that your pasture draws the possums down hill. In our area the Orongorongo river bed is relatively sparsely vegetated, and is possibly less of a draw being mostly visited by possums living in the immediate vicinity. However more extensive studies of movements would be worthwhile in the Orongorongos to clarify the situation.