MAY/JUNE 2017 | | 39fires might have on the long-term viability of frog populations,
is currently unknown.
In February 2009, the Kinglake region of Victoria was devas-
tated 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 esti-
mate 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 popu-
lation 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 popula-
tions 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 undoubt-
edly have long-term consequences, particularly if another fire
swept through the region before populations had fully recov-
ered from the initial fires.
The ability of species to recover from such devastating distur-
bance events is implicitly linked to their ability to move between
areas, enabling animals to recolonise or bolster dwindling popu-
lations 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 persis-
tence of frog populations over time. For all three of the frog“... 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”.