Time-delayed influence of urban landscape change on the susceptibility of koalas to chlamydiosis
McAlpine, C, Brearley, G, Rhodes, J, Bradley, A, Baxter, G, Seabrook, L, Lunney, D, Liu, Y, Cottin, M, Smith, AG & Timms, P 2017, Landscape Ecology, vol. 32, no. 3, pp. 1-17.
The effects of changes in landscape or climate on the prevalence of chlamydiosis in koalas may not manifest until several years after the initial change occurred. Disease prevalence and body condition of koalas in southeast Queensland were quantified to determine both the spatial factors that affect these variables and the time delay, if any, of the effect. For landscapes in which the area of suitable habitat increased, koalas had high body condition scores but also high disease prevalence three years later. Alternatively, when the extent of urbanisation in a landscape increased, koala populations exhibited an increased prevalence of chlamydiosis four years later and decreased body condition one year later. An increase in annual rainfall was associated with immediately improved body condition and reduced disease prevalence after two years.
Although at least 90% of free-ranging koalas in southeast Queensland are infected with Chlamydia, the prevalence of clinical disease is much lower at only 9 – 12%. Several factors are likely responsible for the observed variation in the extents of infection and clinical expression, including landscape disturbance. The effects of landscape change on disease prevalence in koalas are both spatial and temporal. Following a physical disturbance, factors such as increased physiological stress, higher contact rates, more frequent sexual or aggressive encounters, and reduced dispersal opportunities may interact in a process that eventually results in increased disease prevalence in a population of koalas. Here, the authors collected georeferenced health records of koalas admitted to or observed by associates of the Moggill Koala Hospital. Time-series data of the explanatory variables of habitat loss and fragmentation, land-use intensity, and climate variability were then used to create a spatiotemporal model of the extent of landscape change and climate variability within southeast Queensland. From this model, time lags between landscape or climatic variation and changes in koala disease prevalence and body condition could be established.
An increased proportion of suitable habitat in a landscape was associated with higher disease prevalence because an increase in resources may lead to a higher population density, causing greater exposure rates to Chlamydia. This change may have also been associated with an increase in body condition because an increase in resources and hence nutrition allows koalas to cope better with infection. The time-lagged increase in disease prevalence and decrease in body condition following growth in urbanisation extent is likely due to the fragmentation of habitats whereby resources become ‘clumped’, which drives increased contact rates between koalas and increases stress induced by the threats of the surrounding urban matrix. Amount of annual rainfall was positively and rapidly associated with reduced disease prevalence and better body condition. This association is likely due to a reduction in nutritional, hydration and physiological stress resulting from improved water availability.
This study highlights not only the implications of landscape change and climate variability for disease prevalence in koalas but also illustrates the value of longitudinal monitoring studies for understanding threats to koala populations that may not be immediately evident following a disturbance.
Summarised by Joanna Horsfall
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