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.