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By Dr Viki Cramer 28 February 2020 5 min read

A freshwater crocodile. Image: Ruchira Somaweera/CSIRO

There’s a large, long-lived reptile with deadpan eyes and an exquisite set of sharp teeth that’s found pretty much anywhere there’s water in Australia’s tropical north.

But it’s likely not the one you’re thinking of.

Freshwater crocodiles (Crocodylus johnstoni) are found in rivers, creeks and wetlands from the south-west Kimberley in Western Australia, across the Top End and into the northern parts of Queensland’s Cape York Peninsula. They make their homes away from the lower tidal areas dominated by the larger and more aggressive saltwater crocodile (Crocodylus porosus).

Despite their ubiquity across the north, their smaller size, delicate snouts and shy demeanour have meant that freshies do not loom as large in the public’s consciousness as their potentially life-ending saltwater cousin. They are also not as commercially important as salties, so attract less research funding.

While we do know a good deal about what freshies eat and when and where they nest, the long-term survival of the species is not as well understood as we think, says CSIRO’s Dr Ruchira Somaweera.

“We don’t know many of the threats they face and how a changing environment is directly and indirectly affecting their populations.”

Such a widespread species cannot be living the same life in all of the locations it is found, he explains. A freshwater crocodile found in a seasonal, shallow river will live a completely different life to one found in a deep, permanent lake.

“They are a species that is widely regarded as being OK from a conservation perspective,” adds Dr Bruce Webber, who leads CSIRO’s Ecosystem Change Ecology team. “But no one has delved into actually putting numbers against that.”

In a recent paper, Dr Somaweera, Dr Webber and their colleagues identified a complex web of interactions between species and with human activities that are likely to affect freshwater crocodiles.

“The interactions between species are the glue that holds ecosystems together,” Dr Webber says.

“If you can prioritise your management decisions on interactions, then you’re managing the things that either provide or take away ecosystem resilience. The challenge with managing interactions is they’re notoriously hard to understand.”

Unexpected interactions

We are in the midst of an era of global environmental change where many of the established interactions between species are breaking down, and the rapid spread of invasive species is forcing new and modified interactions at a rate that is unprecedented, Dr Webber says.

The research, funded by the Australian Government’s National Environmental Science Program and supported by the WA Department of Biodiversity, Conservation and Attractions (DBCA), showed that the ways in which freshwater crocodiles interact with other species can be complex and unexpected. A species that poses a direct threat to freshwater crocodiles may, through its interactions with a third species, provide freshies with an indirect benefit.

A freshwater crocodile skeleton next to a cane toad. Image: Ruchira Somaweera/CSIRO

The spread of poisonous cane toads poses one of the most important direct threats to freshwater crocodiles. A freshie eats a large cane toad, and that freshie dies. Goanna populations across the north have also been decimated by the arrival of cane toads. But goannas are the major predator of freshwater crocodile eggs, and so fewer goannas means fewer raids on crocodile nests: an indirect benefit to freshies. But over time, some goanna species have learnt not to eat cane toads, and researchers are actively ‘training’ populations of goannas to avoid them. While this is great conservation outcome for goannas, it could lead to a perverse outcome for freshies: they continue to die after eating cane toads while the numbers of the main predator of their eggs increase.

Silent threats

“Cattle and weeds are two other silent threats,” says Dr Somaweera.

Of particularly concern is the invasion of stinking passionflower (Passiflora foetida) in the Kimberley, along the sandy river banks where female freshies build their nests. Dense infestations of stinking passionflower persist into the dry season long after native vegetation has either died off or become dormant.

“Crocodiles are leaving the nest sites that they used to use and nesting somewhere else – because crocodiles don’t clean up and nest, they just want pristine open banks,” Dr Somaweera explains.

“Passiflora – absolutely – is a difficult weed,” says DBCA’s Tracy Sonneman, also an author on the paper. “If we saw stinking passionflower in areas where we knew crocodiles were nesting we’d try to pull it back, but it grows faster than we can tackle with our resources.”

The research on freshwater crocodiles stemmed from a Northern Australia Environmental Research hub project to develop targeted control options for stinking passionflower at Geikie Gorge, where freshwater crocodiles are abundant but passionflower infestations are heavy.

Yet where there’s both passionflower vines and feral cattle, the implications for freshies becomes more complicated. As with cane toads, feral cattle pose both a direct threat and yet provide an indirect benefit to freshwater crocodiles (although the researchers are quick to point out that neither cane toads nor cattle should be considered ‘good’ for the environment in northern Australia). Feral cattle trample crocodile nests as they graze along river banks or come down to the water to drink. Yet grazing cattle also create passionflower-free patches, providing an open space where freshies can build their nests.

Crocodiles, cattle and stinking passionflower are symbolic of the way in which we need to be tackling a lot of environmental threats, Ms Sonneman says.

“You can’t just attack the problem from one angle. The more we can understand, the more we’re able to paint a better picture and know how to manage the range of threatening processes.”

Complex and local

Climate change adds yet another level of complexity to the interactions between freshwater crocodiles, feral cattle and stinking passionflower.

The temperature the eggs incubate at determines both their viability and the sex of the hatchlings. The researchers found that, in some locations, the shade provided by stinking passionflower kept nests cool enough for the eggs to remain viable at times when temperatures were literally high enough to cook the eggs in exposed nests. Yet in other places, that shading keeps nest temperatures too low, and the eggs either don’t develop or they incubate for too long and the river floods before they hatch.

“It’s a very complex, location-based scenario,” Dr Somaweera says.

A failure to consider these location-specific details means that we are likely to misunderstand – or simply miss – important interactions that may weaken or strengthen the future prospects of freshwater crocodiles.

“We hope this research is a wake-up call – as well as a call-to-arms – for what we need to do for freshwater crocodiles, to avoid ongoing complacency around what may be a greater conservation need than we think,” Dr Webber says.

The research on freshwater crocodiles was funded by the Australian Government’s National Environmental Science Program as part of the Northern Australia Environmental Research Hub.

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