When the creeks run dry and the temperature rises, where do all the frogs go?
Amphibians are among the first animals to respond when water arrives in a wetland. Their croaking chorus creates a din that can be heard kilometres away. But how do inland frogs survive Australia’s hot dry summers when running water can all but disappear?
Scientists have had their theories, but until now, solid evidence has been hard to come by. It’s this lack of formal information that prompted environmental scientist Carmen Amos to dig a little deeper.
Carmen works for the Office of Environment and Heritage evaluating the effects of watering on wetlands, creeks and rivers in the Lachlan and Murrumbidgee valleys. At the same time, she is finalising her PhD through Charles Sturt University’s School of Environmental Science on the impact of environmental factors on frog communities in the Lachlan valley.
As part of her PhD, Carmen has assessed 49 sites across the valley to determine which factors encourage frogs to occupy a particular location. She has listened to hours of frog recordings in an effort to identify the factors that may encourage frogs to call when their habitat is already wet. And she has spent many long, dark nights on the trail of these elusive amphibians in a bid to understand their preferred hideouts when water is scarce.
Carmen’s methods are ‘colourful’ and provide a valuable insight into the survival strategies of frogs in semi-arid Australia.
“I’ve always loved frogs but there are so many unanswered questions, particularly when it comes to frogs in arid and semi-arid environments,” Carmen said.
“How long will frogs stay in a drying wetland before they move? How far can they move between permanent habitats? Can young frogs move long distances?
“In the course of my work, I’ve seen frogs crawling out of cracks in the dirt, but there isn’t a lot of literature to support the observation. So, as part of my PhD, I decided to tackle of the idea of where frogs go during the day to shelter from drying conditions.
“My focus was on terrestrial microhabitat – places on land where the frogs might seek shelter during the heat of the day. The habitat types included deep soil cracks and holes, vegetation and coarse woody debris.
“I chose six sites, each with a variety of potential terrestrial habitats within close proximity of one another. Water had been delivered to these sites within the past year so the soil was wet but in the process of drying down.
“I focused on the spotted marsh frog because it is relatively common and present at each of the sites.
“On four separate occasions, I visited each site and captured up to 10 spotted marsh frogs by hand. Each frog was weighed, measured and dusted with a pink fluorescent powder. The frog was then returned to the spot where it had been captured for release.
“Early the following evening my research assistant and I returned to the release sites and used black light torches to follow the fluorescent paths left by each frog to their hot-weather hideouts.
My research showed that spotted marsh frogs use mainly deep soil cracks and holes as their daytime micro-habitat. In terms of temperature, these microhabitats sit at around 25 degrees Celsius with humidity between 50 and 60 per cent – so, not too dry and not too wet.
“Two thirds of the frogs surveyed chose cracks and holes to retreat to during the day. This is interesting because when you look at the ground surface, those cracks and holes make up only one per cent of the visible surface microhabitat available.
“Potentially, there is a large and complex labyrinth of deep soil cracks below the surface, but that’s a theory for a future study.
“Most of the frogs that formed part of the research were found using sub-soil habitat within 10 metres of the wetland but one was found 30 metres away. For a frog that measures just 4.5 centimetres in length, that’s a long way.
“The frogs may have chosen this habitat because the evaporation of soil moisture decreases the deeper you go.
“This provides a cooler, moist environment that offers some protection to the frog during drying weather conditions,” Carmen said.
This research has the potential to influence future deliveries of water to rivers, creeks and wetlands that dry down during warmer weather.
“By understanding how these frogs use the terrestrial habitat surrounding wetlands, water managers can make decisions which provide the best opportunities for frog survival,” Carmen said.
“This may influence the frequency or duration of watering and the rate at which these wetlands are allowed to dry down. This research is best applied when conditions within the environment might be very dry and there is not a lot of environmental water available.
If we have a greater understanding of the conditions frogs need in order to shelter from drying weather conditions then available water can be applied strategically to promote these sort of environments.
“This may not lead to breeding, but will help support frogs during drier times, she said.
For environmental water managers, Carmen’s research reinforces the need for both wet and dry phases within the wetland environment. Her findings may also influence decisions which help maximise the cost effectiveness of environmental flows.
Senior environmental water manager in the Lachlan valley, Paul Packard, said increased knowledge could help to decide the timing, duration and frequency of watering events.
“By identifying the needs and drivers of particular species and landscapes, we can target water more effectively and in concert with other sources of water in the system to achieve the best possible outcomes,” Mr Packard said.
Carmen’s research reinforces the importance of soil cracks as frog habitat and the understanding that we don’t need to provide non-stop inundation to ensure the survival of these frogs.
“By using water efficiently we can provide sufficient habitat to allow frogs to rebound after a dry spell when more water becomes available,” he said.