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By Mary O’Callaghan 3 December 2015 5 min read

Crocodile sitting in a hole filled with mud surrounded by dry dirt and man taking photo
A croc in a mud hole on Cape York Peninsula, awaits the arrival of the wet season. Image: Justin Perry (CSIRO)

CSIRO scientist Justin Perry is just back from Cape York Peninsula where he saw saltwater crocodiles sitting in tiny mud holes, waiting for the wet season to kick off. It’s already a couple of weeks late this year, thanks to El Niño.

“We see drought portrayed in images of dying cattle and failing crops all the time”, says Justin. “But seeing those crocs made me think, what do extreme weather events mean for the broad array of animals across Australia?“

Every species has its extreme weather limits

The late onset of the wet season in northern Australia may play out alright for the crocs, but for some animals a change in weather patterns, even for a couple of weeks, can be fatal.

“For a flying fox, it might be three days over 40 degrees and they fall out of the trees, dead”, says Jeremy VanDerWal, Director of the Centre for Tropical Biodiversity & Climate Change at James Cook University.

“This is very different to a green ringtail possum, whose limit is five days at 30 degrees. At 30 degrees, the possum can’t regulate its body temperature, so it needs to find somewhere cool to hide in, such as a shady hollow.”

Every species has its limits. In Tasmania, 6–8 months of warmer, wetter weather drove the spotted quoll population down to what Chris Johnson of UTAS calls ‘quasi extinction‘.

“The number of quolls was so low and the quoll’s lifecycle is so long that the species remains near extinction”, says Jeremy.

Short-term weather extremes like these can drive animals outside of their normal range. The boundaries can shift significantly and populations can contract and expand based on whether they find places of refuge where the microclimate suits them.

“A small skink might exist within the 10–20 metres around it”, explains Justin. “It can go under a leaf or into a clump of grass looking for the microclimate that suits it. It’s weather that’s driving the species’ boundaries and we need to treat those boundaries as dynamic or we will get a skewed view of their distribution.”

Justin and Jeremy are embarking on research to reveal what constitutes extreme weather for species across Australia. Their aim is to start looking at weather patterns from 60–100 years of historical data and start to identify extreme events, first for some iconic species such as koalas, kangaroos and flying foxes.

“Only by knowing the true limits to a species’ distribution can we can understand the real threats to that species over time”, says Justin. “We’ve got to be looking at these shorter events that can cause quasi extinction”.

Jeremy is already seeing a pattern: “If you look at climate variability in Australia over 30 years, whether it’s in rainforest or desert, the 95th percentile seems to be a threshold where things start to die off. If we find that holds true across many species, that becomes a threshold we can look at and consider anywhere in Australia.”

The mystery of the disappearing bettong

For three years, as part of her PhD, Brooke Bateman from the University of Wisconsin-Madison scoured the rainforests of far north Queensland in search of the endangered northern bettong (Bettongia tropica), a small jumping marsupial about the size of a rabbit. Yet, in spite of reports of intermittent sightings over the past 25 years, it was only after she had submitted her thesis that she finally managed to trap one.

A small rodent creature standing on hind legs in grass and twigs
Northern Bettong looking for food in the Dinden National Park. Image: Angus McNab ©  ANGUS MCNAB

Why had the northern bettong almost disappeared when the common rufous bettong was doing fine? Curiously, the two species overlapped in their distribution, but were never observed in the same place at the same time.

The frustration of her search led Brooke to try to unravel the puzzle of the disappearing bettong. Poring over weather records going back to 1900, she uncovered clues that very short dry periods lay at the heart of the mystery.

More than 60% of the northern bettong’s diet consists of ‘truffles’—the fruit of underground fungi. The truffle itself, to survive, needs consistent rainfall throughout the year, in large quantities.

What Brooke discovered is that it takes only two weeks of dry weather (in this case, < 50 mm of rainfall a day) for the truffle to disappear—for a truffle, this is what constitutes an extreme weather event. And even with subsequent good rain, it takes up to two months for truffles to reappear. This means that in areas where rainfall is variable, a long run of wet weather is needed to support the return of the truffle.

The ripple effect is that during these dry spells the northern bettong can no longer find its favourite delicacy. It can resort to eating grass, and it does, but finds itself outcompeted by the common rufous bettong.

Close up of rodent creature showing head and forearms
The common Rufous Bettong wins the grass hunt during dry times. Image: Anders Zimny

A sort of tug-of-war occurs between these two species of bettong where, in the consistently wet times, the northern bettong wins the truffle hunt, but in the dry truffle-free times, the common rufous bettong wins the grass hunt—hence the intermittent disappearance and reappearance of the two species.

Will truffles remain on the menu?

The current El Nino will drive the northern bettong into the smallest of suitable refuges, says Justin. But, despite the existence of good connected rainforest through which they can travel, the refuges are not always there.

“What’s good for the northern bettong is that rainfall isn’t expected to change too much, so those refuges will hold out for a while”, he says. “But if we lose the truffles, we lose the species. Yes, they’ll eat grass, but they can’t compete with the rufous bettong for grass. And even if the truffles come back, it takes time for a population to recover.”

Truffles may be on the menu for our world leaders in Paris this week. Whether they stay on the menu in the rainforests of far north Queensland remains to be seen.

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