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.