Movement patterns of an arboreal marsupial at the edge of its range: a case study of the koala

Abstract

Background: Conservation strategies derived from research carried out in one part of the range of a widely distributed species and then uniformly applied over multiple regions risk being ineffective due to regional variations in species-habitat relationships. This is particularly true at the edge of the range where information on animal movements and resource selection is often limited. Here, we investigate home range size, movement patterns and resource selection of koalas Phascolarctos cinereus in the semi-arid and arid landscapes of southwest Queensland, Australia. We placed collars with GPS units on 21 koalas in three biogeographic regions. Home range sizes, resource selection and movement patterns were examined across the three regions.

Results: Habitat selectivity was highest at the more arid, western edge of the koala's range with their occupancy restricted to riparian/drainage line habitats, while the more easterly koalas displayed more variability in habitat use. There was no significant difference between home range sizes of koalas at the western edge of the range compared to the more easterly koalas. Instead, variability in home range size was attributed to spatial variations in habitat quality or the availability of a key resource, with a strong influence of rainfall and the presence of freestanding water on the home range size of koalas. Within a 580 m spatial range, movement patterns of male and female paths showed a tortuous trend, consistent with foraging behavior. Beyond this spatial range, male paths showed a trend to more linear patterns, representing a transition of movement behavior from foraging to breeding and dispersal.

Conclusions: The difference in home range movement patterns and resource use among the different koala populations shows that behavior changes with proximity to the arid edge of the koala's range. Changes in home range size and resource use near the range edge highlight the importance of further range-edge studies for informing effective koala conservation and management actions, especially when developing species-specific adaptation responses to climate change.

Keywords: Australia; Home range; Koala; Resource selection; Semi-arid; Spatial distribution.

Figure 1

Structural equation model diagram. The structural equation model diagram with the identified (by the solid and dashed arrows) direct dependences between the variables (shown in the boxes). The numbers next to the arrows show the corresponding regression coefficients between the respective variables in the boxes. The asterisks indicate the levels of significance for the respective regression coefficients: (*) p < 0.05; (**) p < 0.01; (***) p < 0.001; the absence of the asterisks means statistical insignificance. The direction and thickness of the arrows indicate the direction (causality) of the mutual impact of the variables and the approximate values of the regression coefficients, respectively. The dashed arrows indicate the negative regression coefficients. The dotted arrows show the indirect effects of annual rainfall on home range.

Figure 2

The dependence of the home range size on annual rainfall. The grey band shows the 95% prediction interval.

Figure 3

Home range size (males) dependences on two-month rainfall for average, minimum and maximum annual rainfall. The dependences of home range size for male koalas on two-month rainfall for the average annual rainfall of 515 mm (solid curve), for the minimum annual rainfall of 450 mm (dotted curve), and for the maximum annual rainfall of 575 mm (dashed curve) in the presence of freestanding water. The statistical contrasts (differences) between these three curves are significant (with p < 0.01). The grey band shows the 95% prediction interval for the average annual rainfall. No statistically significant dependence of home range size on two-month rainfall was found for female koalas.

Figure 4

Diurnal and nocturnal movement patterns. Mean (± standard error) diurnal and nocturnal movement patterns of males and females. A, B and C identify that significant differences exist from the post hoc test: all pairs of combinations were significantly different except for the male day and night comparison.

Figure 5

Time spent along drainage/riverine habitat. Mean (± standard error) time spent (%) along drainage/riverine habitat (i.e. time spent on the creek/drainage-line rather than in off, non-riverine habitats) for koalas within the Mitchell Grass Downs (MGD), Mulga Lands (ML) and Brigalow Belt South (BBS) bioregions. A and B identify significant differences from the post hoc test.

Figure 6

Plots of fractal D and correlation in tortuosity of successive path segments of male and female koalas. (A) Mean fractal D, and (B) correlation in tortuosity of successive path segments with corresponding 95% confidence intervals for male koalas (n = 8) at varying spatial scales. (C) Mean fractal D, and (D) correlation in tortuosity of successive path segments with corresponding 95% confidence intervals for female koalas (n = 9) at varying spatial scales. Arrows represent point of inflection on plots of mean D ((A) and (C)). Dotted lines represent correlation = 0, and arrows represent drops in correlation below zero on plots of correlation in tortuosity of successive path segments ((B) and (D)). Movement patterns of male koalas showed three changes in movement patterns, at approximately 240, 610 and 950 m ((A) and (B)). Movement patterns of female koalas showed two changes in movement patterns, at approximately 230 – 260 m (similar to males) and the second at path lengths of 360 - 400 m ((C) and (D)).

Figure 7

Study area. Southwest Queensland study area within the Mulga Lands, Mitchell Grass Downs and Brigalow Belt South bioregions. White dots show the site locations within each bioregion where koalas were collared. Map also shows where the study sites are located in relation to the geographic distribution of koalas. (Source: modified from [23, 47]).