The importance of forest area and configuration relative to local habitat factors for conserving forest mammals: A case study of koalas in Queensland, Australia
McAlpine, CA, Rhodes, JR, Callaghan, JG, Bowen, ME, Lunney, D, Mitchell, DL, Pullar, DV & Possingham, HP 2006, Biological Conservation, vol. 132, pp. 153-165.
Koalas were used as a model species in this study to represent forest-dependent mammals particularly affected by forest clearing and fragmentation. The study was conducted in south-east Queensland and compared the importance of forest area and configuration with nine fine-scale habitat features for koala occurrence. Results found that koala occurrence was positively associated with the area of forest habitat, habitat patch size and the proportion of primary eucalypt species. In contrast, forest patch density, mean nearest neighbour distance (between forest patches) and road density had a negative relationship with koala occurrence.
Logistical modelling and hierarchical partitioning analysis were used to determine the importance of forest area and configuration relative to fine-scale habitat variables for koala occurrence. Surveys of faecal pellets were used to determine the occurrence of koalas throughout the sites. Habitats were classed from highly suitable to unsuitable based on the proportional abundance of identified food tree species. The explanatory variables used to determine koala occurrence included: proportion of primary tree species (those favoured as food sources), habitat patch size, proportion of forest habitat (1km extent), forest patch density (2km extent), forest mean nearest neighbour distance (4km), road density, interaction of proportion of forest habitat and forest mean nearest neighbour distance, the spatial autocorrelation term, and the year of the survey. The proportion of forest habitat had the strongest independent effect (7.5%) on occurrence, followed by the proportion of primary tree species (4.5%). Mean nearest neighbour distance and forest patch density combined accounted for 7% of the variation, and road density accounted for 3.6% of variation.
Given that koalas are specialised folivores that rely on specific eucalypt trees for food, it is no surprise that the proportion of primary tree species (those favoured by koalas as a food source) influenced koala occurrence. The findings also show the importance of simply having forest habitat within the landscape even if it is not a highly suitable food source, as this allows koalas to disperse and travel safely between areas of high suitability. In contrast, patches that are more isolated (less dense and further away from other patches) and areas with a high density of roads create a landscape more difficult for koalas to traverse. Moving through such landscapes increases the risk of vehicle strikes and dog attacks, leading to increased mortality rates.
The findings from this study reveal that both fine-scale habitat features and landscape features like forest area are important factors in determining occurrence of koalas. To better conserve koalas, habitats of all suitability levels should be preserved, as they provide important connectivity. The spatial configuration of a landscape and its intensity of use by humans were important in determining presence of koalas and could be better incorporated into conservation actions for koalas and other forest-dependant mammals.
Summarised by Miranda Rew-Duffy
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