Tree species preferences of koalas (Phascolarctos cinereus) in the Campbelltown area south-west of Sydney, New South Wales
Phillips, S & Callaghan, J 2000, Wildlife Research, vol. 27, pp. 509-516.
In the Campbelltown Local Government Area, the quality of habitat being utilised by a small koala population was assessed, with particular attention paid to food tree species preferences. Forty-five different field sites were examined, revealing that grey gum (Eucalyptus punctata) and blue-leaved stringybark (E. agglomerata) were the most preferred species. However, tree preferences were shown to depend on soil substrate, derived either from shale or sandstone.
Activity levels and tree species preferences of koalas were identified by the presence/absence of faecal pellets using a plot-based methodology. Sample areas were chosen based on mapped geological units, with all trees inspected within a randomly-selected plot site. Overall, 2499 trees were sampled, of which 46% were eucalypts. Tree use was observed in 44% of the field plots and activity levels (trees with faecal pellets/total trees in plot) ranged from 1.2% to 18.4%. Out of a total of seven Eucalyptus species, E. punctata and E. agglomerata were the most frequently utilised trees. Twenty-five of the plots assessed contained sandstone substrates, of which 36% held evidence of koala activity, whereas 55% of shale substrate plots provided such evidence. Mean activity levels of sandstone and shale sites were 3.44% and 9% respectively. Furthermore, findings showed that koalas demonstrated a preference for grey gums of larger size and that tree utilisation rates were significantly greater on shale than sandstone substrates.
In the Campbelltown Local Government Area, koalas appeared to concentrate on two Eucalyptus species, with utilisation rates highest in shale-based substrates. Such findings indicate that, from a koala’s perspective, the relative importance of food trees may change depending on soil quality. This phenomenon is consistent with the ‘resource-availability’ hypothesis, which suggests that plants sustained by low-quality soil are more likely to incorporate chemical defence mechanisms in an attempt to deter folivores. Accordingly, in Campbelltown, where there appears to be areas of differing substrate quality, habitat preferences depend on more than food tree distribution; koalas will preferentially select those trees with greater nutrient levels growing on higher quality substrates. This hypothesis may also relate to the koala’s selection of larger food trees, as large trees are more likely to have access to or successfully compete for soil nutrients and so are not required to produce chemical deterrents.
Ultimately, low koala abundance in the Campbelltown area is a likely reflection of poor habitat quality rather than anthropogenic disturbances such as logging and urbanisation. As such, careful management of this population is required, with koala abundance and distribution limited significantly by the occurrence of E. punctata and E. agglomerata on higher quality, shale-based substrates.
Summarised by Julian Radford-Smith
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