It can be difficult to establish the conservation significance of endemic infectious diseases – those that are well established in a population – in contrast to infectious diseases that are still invading. The infectious diseases of koalas provide an ideal case study to examine issues involved in identifying the role of endemic disease in conservation biology. The koala Phascolarctos cinereus is an iconic species endemic to Eastern Australia. Koala populations are in decline, particularly in the northern parts of their range, amidst claims for many years that infectious diseases, particularly those with chlamydial aetiology, play a key role in these declines.
We summarised causes of arrivals and outcomes for over 25,000 koalas arriving at care facilities in south-east Queensland between 1997 and 2014 and analysed records using time-series methods to detect both cyclical behaviour and long-term trends.
Results/Conclusions
There are multiple postulated causes of koala decline including drought, habitat destruction (largely associated with urbanisation) and disease. Disease was the leading association with admissions (49% of arrivals). Other major associations included car strike (27.2%), dog attack (9.2%), other injuries (27.8%) and wasting (23%). Individual koalas were frequently associated with multiple causes of hospitalisation.
Using the limited available longitudinal data, we found weak associations between prevalence of infection, clinical signs of disease and population decline, meaning that it remains unclear whether infectious disease is a primary driver of koala population decline. In the few long-term time series of koala population dynamics and disease prevalence, there are signals that clinical chlamydial disease may influence recruitment, but evidence that this drives population decline is equivocal. It may well be that compensatory mortality means that disease related effects on fecundity are not the primary determinants of population dynamics.
Our results show that chlamydial disease is an important source of morbidity in some koala populations. However, they do not demonstrate that infectious disease is a primary driver of population decline. Only carefully designed experimental interventions can do this, and undertaking these should be a high priority for conservation managers.