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Lessons from the Koala Coast. Dr Harriet Preece. Part 1

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Published on May 25, 2009

Road kill is familiar sight along highways in many parts of the world. While some studies have documented the impact of linear infrastructure on local wildlife populations, rarely have the impacts on population dynamics been quantified for a regional population. The koala provides an ideal case study for examining species that have large home ranges and are particularly vulnerable to road and rail effects because their frequent movements across roads and rail tracks increases the probability of a collision. Road and rail mortality are particularly insidious because they remove otherwise healthy individuals from the population.

Radio collaring of every koala on a 52ha urban bushland site known as the Greater Glider Conservation Area, revealed the presence of a viable population of 45 koalas in 1996. However, while a few animals lived within the confines of the reserve, the majority of the resident koala population were found to have home-ranges that overlapped an adjacent main road, or extending into the neighbouring urban areas - thus exposing the majority of resident population to an elevated risk of mortality. As a consequence of the high number of road kills and additional anthropogenic mortality, the population density Gradually declined with a number of animals killed on the main road and other animals Succumbing to disease as a result of stress. In 2005 the road was upgraded from two to four lanes and in the following year the koala population halved with only one female seen on the site and nine koalas in total. Consequently the site is now close to effective extinction. This pattern of population decline was evident in surveys conducted in 2005-2006 for bushland remnants on the urban footprint in the Koala Coast and is likely to be indicative of the pattern of decline throughout South East Queensland.

Lessons from the Koala Coast have enabled the delineation of 258 road and four rail blackspots. Radio-tracking demonstrates that wildlife with large home ranges are particularly sensitive to landscape permeability which needs to be enhanced to retain population viability. Crossing structures ideally need to be located every 200m along linear infrastructure to allow for the day-to-day movement of animals such as koalas. While fencing can be beneficial in funnelling wildlife to crossing structures, it can also be detrimental by preventing movement, dispersal and the exchange of recruits. The importance of the urban koala population in bolstering the bushland population has previously been underestimated. This has now been confirmed with the detection of a significant decline in koalas at bushland sites that have not experienced any loss or visible changes to habitat or habitat quality or an increase in anthropogenic influences. This previously large population of koalas, recently found to be genetically distinct from all other koalas in South East Queensland, is likely to follow other urban koala populations in other States towards effective extinction unless there is active management to reduce habitat loss and vehicle related mortality while concurrently increasing landscape permeability.

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