Australasian Science Magazine

The effective population sizes of brown tree frogs crashed following the Black Saturday fires and had not fully recovered at the end of the study. Credit: D. Paul/Museums Victoria

By Jane Melville & Dominque Potvin

The increasing risk of bushfires due to climate change is escalating the risk of extinction for frogs in Australia’s south-east.

Walking through the bush on a hot summer’s day in south-eastern Australia, you can’t help being aware of the lurking threat of bushfires. This awareness is part and parcel of living in this region where the hot, dry summers have frequent days of extreme bushfire risk.

In the tall forests of Victoria, severe bushfires are a reasonably common event, with at least five significant fires in the past 100 years. Alarmingly, climate change models are predicting increases in the frequency and ferocity of bushfires in this region in the future. Like other ecological disturbances, bushfires can have profound impacts on both individual species and the composition of whole biological communities.

Our research sought to find out how severe bushfire events impact the long-term survival of frogs in the tall forests of south-eastern Australia. Frogs are in trouble globally, with up to 40% of species facing imminent extinction. In Victoria the situation is bleak, with 50% of Victoria’s 30 frog species listed as threatened.

The threats frogs are facing include habitat destruction and fragmentation, climate change and disease. However, whether bushfires should also be included in this list, and what impact fires might have on the long-term viability of frog populations, is currently unknown.

In February 2009, the Kinglake region of Victoria was devastated by extensive bushfires that have been labelled the worst in Australia’s recorded history based on area burnt (430,000 hectares) and the loss of life and property. The devastating “Black Saturday” fires burnt through populations of tree frogs that we had been studying in an ongoing research program investigating ecological and genetic diversity. We continued this research for a further 5 years after the fires to assess the initial impacts and then the ongoing recovery of two common and abundant species: the brown tree frog (Litoria ewingii) and Victorian tree frog (Litoria paraewingi). The results of our study were both surprising and unexpected.

Our assumptions about how the frogs would survive and recover after the Black Saturday fires were based on previous research that estimated post-fire frog abundances by counting individuals from their calls. These ecological surveys concluded that frog species can be resilient to fire.

Indeed, when we used these traditional ecological survey techniques in the Kinglake area after Black Saturday we also found minimal impacts on population estimates. Four months after the fires, the “advertising”’ calls of male frogs during the breeding season (May–August) from 11 study sites, nine of which were burnt, showed only a small effect of the fires on the number of breeding males. We felt relieved and happy that the frogs we had been studying over the previous 2 years had survived the fires.

However, our elation was short-lived. When we went back to the DNA lab at Museums Victoria and analysed the genetic profiles of the populations we saw a very different and very disheartening story!

Genetic fingerprinting techniques allow us to assess the “genetic health” of populations, by revealing the level of genetic diversity present in a population. This also allows us to estimate the effective population size, which reflects the ability of a population to breed and sustain itself long-term.

Using these techniques on the frog populations around Kinglake, we found that both of these common and abundant frog species were significantly more inbred after the fires. Even more worrying were the dramatic declines in the effective population sizes of the frogs. The effective population size of the brown tree frog decreased by 98% immediately following the fire in 2009, then showed steady recovery in subsequent years, while Victorian tree frog populations decreased by 74% in 2009 followed by a further decrease of 99% in 2010, with populations not fully recovered at the completion of the study.

These genetic results, which contrast so strongly with the ecological survey data, highlight that some of the traditional survey techniques that are usually used to assess animals after environmental disturbance may not give the full picture of the genetic health and future resilience of species. Such dramatic declines in the genetic diversity of populations would undoubtedly have long-term consequences, particularly if another fire swept through the region before populations had fully recovered from the initial fires.

The ability of species to recover from such devastating disturbance events is implicitly linked to their ability to move between areas, enabling animals to recolonise or bolster dwindling populations from unaffected areas. We found strong evidence that this connectivity between populations decreased in the years post-fire, indicating that frogs were not moving between sites as they had done previously.

This decrease in connectivity could be for a number of reasons, such as burnt landscapes between breeding ponds being inhospitable to the movement of frogs, or increased mortality of frogs when moving between ponds. Whatever the cause, reductions in the ability of frogs to move across the landscape and an associated increase in the isolation of ponds is linked to declines in genetic diversity.

Like other ecological disturbances, bushfires can not only drive declines in the abundance of species but also the genetic diversity of populations. Genetic diversity is fundamental to individual survival, population viability and species adaptability to environmental change. With low genetic diversity and high inbreeding, populations can be at significant risk of localised extinction, and dramatic declines in genetic diversity can severely affect the long-term persistence of species. In a future of predicted escalating fire frequencies with anthropogenic climate change, such dramatic responses to bushfires in common and abundant frogs is of concern.

To further explore the uncertain future of these frogs we used predictive modelling to ask how fires of varying frequency would affect frog populations. We expanded our study to include a related but more vulnerable species, the growling grass frog (Litoria raniformis), which occurs in the area.

We found an important effect of fire frequency on the persistence of frog populations over time. For all three of the frog species, the predicted population size decreased and the probability of extinction increased as fire frequency increased from an average of one fire every 100 years to one every 10 years. This means that as severe fires, like those of Black Saturday, become more frequent there is a greater likelihood of frog populations becoming extinct. However, we did find that these effects were substantially reduced if the rate of migration between populations was sufficiently high.

Under all our modelling scenarios, the conservation-listed growling grass frog had a higher risk of extinction than the other frogs. The results from this predictive modelling indicate that species with an initially lower genetic diversity and/or isolated populations of frogs with low rates of migration are more vulnerable to extinction under the increasing fire frequencies expected in south-eastern Australia due to climate change.

While natural bushfires are considered an ecologically important part of the Australian landscape, it is predicted that current and future climate changes will have unprecedented impacts on the frequency and severity of fires in south-eastern Australia. However, it is difficult to predict how changing fire regimes might impact populations and species – even those thought to have evolved in areas that benefit overall from natural fire over the long-term. In the context of global change, it is important to consider how seemingly distinct human activities, such as changing climate, landscape modification and fragmentation, interact when impacting natural systems. Understanding how predicted changes might contribute to a species’ likelihood of disappearance or persistence is a vital part of making informed decisions about management and conservation.

Previous studies have shown an intricate relationship between climate change, habitat loss and disease in the extinction risk of frogs. Our study on the Victorian frogs found evidence of an additional cause of extinction risk for frogs with anthropogenic climate change. The additional pressures of increasing bushfire frequency could have a profound impact on the persistence of many small vertebrates. While it is true that many small vertebrates living in fire-prone areas may benefit, or simply show higher genetic diversity, from infrequent fire regimes, increasing fire incidence is likely to be detrimental to even well-adapted species. Our results support the disturbing suggestion that, at least in some environments, extinctions may result from climate-mediated causes, such as more frequent fires, long before the physiological inability to adapt to high temperatures becomes an issue.

Although these results portray a gloomy future for Victorian frogs with a multitude of compounding extinction risks, it does provide a warning call that some factors are particularly important in the management of the long-term prospects of our fauna. In particular, maintaining connectivity between populations and minimising fragmentation seem to be reoccurring themes when assessing the ability of frogs to recover from the impacts of ecological disturbance. We hope that our study, and others that provide strong predictive evidence of extinction risks in our already dwindling frog species, will be taken into account in the development of future management and conservation strategies.