The comparative ecology and population dynamics of koalas in the koala coast region of south-east Queensland

Thompson, Jim, (James Athol) (2006). The comparative ecology and population dynamics of koalas in the koala coast region of south-east Queensland PhD Thesis, School of Integrative Biology, University of Queensland.

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Author Thompson, Jim, (James Athol)
Thesis Title The comparative ecology and population dynamics of koalas in the koala coast region of south-east Queensland
School, Centre or Institute School of Integrative Biology
Institution University of Queensland
Publication date 2006
Thesis type PhD Thesis
Supervisor Dr Francis Carrick
Total pages 300
Language eng
Subjects 0501 Ecological Applications
Formatted abstract This study addressed aspects of the population dynamics and ecology of selected koala sub-populations in the Koala Coast Region of south-east Queensland, approximately 30 km from the centre of Brisbane. This work formed part of a larger study program addressing the management of the regional population of approximately 6000 animals (Thompson et al. 2002, Dique et al. 2004) considered to be a single genetic management unit (Sherwin et al. 2000).

Between 1996 and 2000, detailed ecological and population studies based on intensive koala and vegetation surveys were conducted at five sites within the region. Four sites (two habitat types, two replicates) were selected for comparative studies of koala populations inhabiting either extensive bushland areas (Sites: Burbank and Sheldon) or remnant bushland areas surrounded by urban areas (Sites: Gravel Reserve and McDonald Rd). The fifth site (Ney Rd) was used to assess the impacts of development on a koala population of approximately 80 animals inhabiting a 69ha remnant bushland block. An intensive koala capture, radio-collaring and monitoring program was established at three sites involving 610 captures of 210 individual koalas. Collared koalas were used to compare movement patterns and habitat use between sites and to estimate population parameters including dispersal, mortality and survival rates. Information on 83 known mother-offspring relationships facilitated an assessment of the breeding dynamics of koalas.

The study reports on six major themes: movement patterns and population spatial structure (Chapter 4); resource selection and use (Chapter 5); breeding dynamics (Chapter 6); population structure and function (Chapter 7); population rates of change (Chapter 8); and the impacts of development (Chapter 9).

The results support the model of koalas being predominantly solitary in nature and living within a relatively long-term stable population structure (Mitchell 1989). In this study koalas lived within flexible and overlapping home ranges of between 5-17ha. These ranges reflected the habitat used by koalas over a 2-3 year period. Evidence exists for some competition between males for space. Site fidelity of koalas remains strong amongst adult animals, with sub-adults being the predominant group to disperse. Koala home ranges are not static with animals moving between years and continually adjusting their use of space in the environment. Koalas regularly incorporate human modified landscapes into their normal movement patterns, even though this comes with some risk. Perhaps most critical to management is that the movement of animals is influenced by local conditions, with differences detected between populations in terms of home range size and use, home range overlap and dispersion. 

The local identification of ‘preferred’ tree species remains the most consistently requested and used information for management. Yet conflicts still exist between the concepts of ‘preference’ and ‘importance’ and the methods used for determining tree use (Hasegawa 1995). In this study trees were assessed for importance as a daytime roost tree, with importance being determined by a combination of preference (as determined by radio-telemetry location data) and availability. Within the Koala Coast region the trees most important for koalas are Eucalyptus fibrosa, E. tindaliae, Corymbia citriodora, E. racemosa, E. microcorys and E. resinifera. This differs slightly from previous studies, which have identified E. tereticornis, E. propinqua and E. microcorys as the most preferred trees (Melzer and Houston 2001). Across the sites assessed other species including Glochidian sumatranum, Lophostemon confertus, E. siderophloia and E. carnea were locally preferred but not widely available. There was a significant difference between tree species selected by individual koalas, indicating that the identification of preferred trees from population data should be used with some caution. The use of some tree species such as G. sumatranum is most likely related to its benefit as a shelter tree; however the importance of these trees, which are rare in the environment, should not be overestimated. These results suggest that at some sites koalas utilise a wide range of species and are not restricted to a few species as previously proposed (Hindell and Lee 1990). Koalas preferentially selected taller and larger trees.

The advantages and disadvantages of monitoring techniques for determining tree use have been well discussed (Dique 2004, Phillips 2000, Ellis et al. 2002b) and are summarised in this study. However, relying on the presence of a small number of preferred tree species as a determinant of good koala habitat is considered too simplistic and is not consistent with the results from this study. Block size, fragmentation, connectivity and the impact of disturbance on nearby areas are issues that require further assessment. The results from this study indicate that the appeal and continued development of simple assessment methodologies, threaten to override a better understanding of the potentially complex make-up of good koala habitat.

The genetic relationships between koalas, movement patterns and known mother-offspring relationships were assessed to determine both the genetic diversity of a fragmented regional koala population and the role of social dominance in the reproductive success of male koalas. Genotypes from hypervariable microsatellite loci were used to identify individuals and analyse parentage through paternity exclusion. Even at a local scale there was measurable genetic differentiation associated with population isolation.

Analysis of parentage, as an indicator of social and breeding behaviour, confirms that koalas are polygamous with a number of males siring several young. However koala populations do not conform to a strictly polygynous social structure where individual males exhibit a relatively stable breeding relationship with multiple females. Non-resident animals sire between 20% and 60% of all young born at some sites refuting assertions that dominant males monopolise mating opportunities. Some local males do contribute significantly to breeding success suggesting that in some situations there are breeding advantages associated with residency. The variation in breeding patterns between geographically close sites suggests that local issues, particularly barriers to movement, should be considered in management programs intended to maintain genetically viable populations. 

Koala densities at the sites studied ranged from 0.35-0.81 koalas per ha, which is considered high for the region and in comparison to other Queensland populations (Melzer and Lamb 1994, Dique et al. 2004). Differences in population structure were recorded between sites with higher proportions of older animals in the ‘secure’ bushland sites. Across all study sites adult sex ratios favoured females over males which is consistent with previous studies (Martin and Handasyde 1990) although the sex ratio of young was close to parity. Significant differences in fecundity between sites and years indicated the importance of local influences, particularly the prevalence of chlamydial disease. Apart from one site, fecundity rates were relatively high for most years (0.6-0.9) and despite the presence of chlamydial disease were consistent with previously reported disease-free populations (Martin and Handasyde 1990).

Mortality rates were highest in the winter/spring period as previously suggested from Moggill Koala Hospital records (Nattrass and Fiedler 1996). It is likely that this reflects the increased movement of animals during the breeding season, especially dispersing animals. Disease was the single greatest cause of death, although a number of other causes, particularly dog attacks and car accidents, were prominent. Mortality rates were higher in the remnant bushland and urban areas compared to the ‘secure’ bushland areas, as the number of deaths from cars and dogs was greater in these areas. Previously published mortality figures overemphasized the impact of cars and dogs across the region (Kraschnefski 1999). The relationship between chlamydial disease and reproduction and mortality in koalas is well documented (e.g. Augustine 1998) and is discussed in relation to trends recorded in fecundity and mortality rates at each site. Generally, however, an increase in the prevalence of disease corresponded with a decline in reproductive output and an increase in mortality.

From a management perspective, dispersal and immigration rates, which are poorly reported in the literature, present an interesting dilemma. Rates of both dispersal and immigration were found to be relatively high in this study suggesting a large pool of ‘nomadic’ animals in the fragmented landscape. Dique et al. (2003b) proposed a regional model of koalas moving from the high-density source areas (bushland habitat) to the low-density areas (remnant bushland/urban) where mortality rates are high. Based on population rates of increase recorded in this study using time-series data, this hypothesis appears reasonable. However, this method of population monitoring lacks statistical power and is sensitive to demographic and environmental stochasticity.

Population modelling suggests a more pessimistic scenario than presented by time-series predictions. Modelling indicates that the populations in the so-called source habitats of secure bushland are functioning at close to a stable population growth rather than as a provider of excess animals for sink areas. Parameters most sensitive to change are adult survival rates, emigration and immigration. Net migration onto the study sites is identified as critical to long-term population maintenance. These results indicate that management strategies need to be tailored to discrete areas, based on factors most affecting local population dynamics. However the reliance on high immigration rates to maintain population size across the region means that previously unacceptable management options including translocation of some individuals and the identification of ‘sacrifice’ areas should be considered. 

Results from one component of this study confirm the perception of wide ranging impacts of development on resident koala populations (Martin and Handasyde 1999) and highlight potential impacts for surrounding populations. This part of the study monitored the urban development of a 69ha remnant bushland block where components of the block were retained as a conservation reserve or developed in a koala sensitive manner. Initially population decline mirrored the proportion of lost habitat (i.e. approximately a 40% decline over three years), however, eight years after the commencement of the development the population had dropped to 25% of the original population size. While there was a tendency during the study for a reduction in fertility rates and an increase in mortality rates the greatest impact on the decline in population size was due to dispersal. Dispersal from the site was approximately three times greater than at similar sites in the region. The negative impact of people and alternative land uses on dispersing animals threatens to isolate some remnant bushland populations. The management implications and options are discussed.

The contribution of habitat disturbance and other stressors on increased disease rates has long been postulated but rarely recorded. Clinical signs of chlamydial infection increased from 0% at the commencement of the study to 25% during the final stages of development. The most obvious impact of increased levels of disease was a decline in the reproductive rates of female koalas. This component of the study highlights the importance of monitoring populations which are the subject of an innovative and ‘koala sensitive’ development approach. ‘Koala sensitive’ developments may not result in the benefits to the local population that are promoted and require more work.

This study provided a unique opportunity to address a range of issues related to the management and understanding of a regional koala population. Specifically this study has addressed the following three principal objectives: It has clarified the role of dominant males in the reproductive output of koala populations in the Koala Coast Region and linked this to habitat and resource use (Chapters 4-6); It has confirmed that many local koala populations in the Koala Coast Region are not sustainable without management intervention (Chapters 7 and 8); and It has quantified the impacts of development on a local koala population and provides directions for the conduct of future ‘koala sensitive’ developments.

Linked with the work of Dique (2004) and Preece (in prep) a comprehensive information base has now been developed for koalas in the Koala Coast Region of south-east Queensland. This information needs to be used to improve and assess management options.
Keyword Koala -- Ecology -- Queensland, Southeastern.
Koala -- Habitat -- Queensland, Southeastern.
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