Plight of the platypus

By James Bradley

In July last year, a team led by Dr Gilad Bino visited the catchments of the Manning and Hastings rivers, on the Mid North Coast of New South Wales. Only six months previously, bushfires had swept through the area, consuming ecosystems already damaged by prolonged drought, and Bino – a freshwater ecologist at UNSW – was there to assess the effects of these multiplying catastrophes on platypus populations.

Over two weeks, Bino’s team used nets to trap and count platypuses along Dingo Creek, a tributary of the Manning River, and the Thone River, which flows into the Hastings, anaesthetising and examining captured animals before returning them to the water. They also carried out surveys of the populations of macroinvertebrates – crustaceans, insect larvae, worms – that platypuses depend upon for food.

Interestingly, Bino’s team’s findings suggested that even in areas badly damaged by fire, macroinvertebrate numbers were relatively healthy. But when it came to platypuses, it was a very different story. On sections of creek unaffected by bushfires, the team caught six platypuses over three nights. Likewise, in areas that weren’t directly affected by the fires but had suffered indirect impact in the form of run-off and poor water quality, the team trapped three platypuses over two nights. But on sections of creek directly affected by fire the team caught only one platypus over five nights in one location, and another one in a single night’s trapping at a second location. Even more concerning, they found no juvenile platypuses at any location, suggesting none of the populations had bred successfully in the preceding year.

Although disturbing, Bino didn’t find these results surprising: “Intuitively it makes sense that fire and drought pose significant threats to freshwater species, especially in combination.” But his team’s findings were significant. “Previous studies that looked at the impact of bushfires on platypuses didn’t find a strong association between fire and platypus numbers. But in those studies the condition of the rivers wasn’t as dire as it was at the peak of the drought in 2019 when the fires hit.”

As with many locations on the east coast, the effects of the drought on the Manning and Hastings catchments were drastic and unprecedented. “Local people told us that both Dingo Creek and the Thone ceased to flow for the first time in their recollection,” says Bino. “So it was really extreme.”

There seems little question these conditions had already placed pressure on platypus populations. “What we know is that as creeks and rivers dry out, platypuses have to move along the dry stream bed in search of the deep pools they use as refugia. This makes them susceptible to predation, particularly by invasive species such as foxes and cats. We think this was happening in these areas because some of the local people who live along the river mentioned seeing fox dens with platypus carcasses scattered about.”

More importantly, though, Bino’s study suggests the drought alone was not enough to account for the drop in platypus populations. Instead, the number of platypuses in the rivers seemed to be linked to whether the rivers and their catchments had also been affected by fire.

Fire has a variety of direct impacts upon platypuses. The upper sections of riverside plants offer shade, cooling the water and protecting the animals from direct sun, as well as providing habitat for the insects whose larvae form a significant part of the platypus’ diet. Simultaneously, the roots of bushes and trees growing along the riverbank support and stabilise the burrows (some exceeding 10 metres in length) in which the species breeds and rests.

Even six months after the fires, this vegetation was struggling to recover. “In the burnt areas there was a lot of epicormic growth on the trees, but the understory had been overtaken by weeds.” Nor were platypuses the only vertebrates affected: as Bino notes, the fire-affected areas were notable for how quiet they were. “Our surveys are done at night, so you expect very obvious frog calls. But where it had burned there were almost none.”

Fires also affect platypuses in less direct ways. Runoff of ash and increased erosion resulting from loss of vegetation in river catchments reduces water quality, leading to the build-up of toxins and organic compounds, and depleting oxygen, especially when fires are followed by heavy rain. In many instances this combination leads to algal blooms and fish kills such as those observed on the Manning River and elsewhere in early 2020.

But as Bino points out, undertaking the study also highlighted the paucity of our understanding of the plight of the platypus. “After the fires, our first impulse was to go to rivers for which we had data on platypus populations so we could compare before and after. But there wasn’t any.”

It might come as a shock to many that we know so little about the health of the platypus. After all, as well as their significance in many Indigenous cultures, the species has long been a subject of scientific and public curiosity. In the 18th and 19th centuries, much of this revolved around the way the platypus blurred the boundaries between different classes of organism, exhibiting mammalian characteristics alongside ones associated with birds. Eventually the platypus was recognised as one of only five living species of monotremes (the other four are all echidnas), an ancient group that appears to have diverged from the other mammals more than 200 million years ago.

The fossil record tells us little about what longextinct monotremes looked like, although in the 1980s miners in Lightning Ridge came upon the opalised jawbone of a platypus-like creature dating back 110 million years. Yet there is little question the modern platypus is a singular creature, not just laying eggs but possessing 10 sex chromosomes instead of the two of most mammals, lacking a stomach (their food travels directly to their intestines) and, in males at least, boasting venom glands attached to spurs on their hind legs. There is even evidence their duck-like bills are designed to detect changes in electrical currents, and the distinctive manner in which they sweep them back and forth underwater is a means of sensing prey.

A lack of baseline data on this exceptional creature’s health and habitat has created a situation in which platypus populations could suffer rapid declines or local extinctions without scientists or policymakers noticing. This concern prompted Dr Tahneal Hawke, an ecologist and one of Bino’s colleagues at UNSW, to try to develop a better understanding of platypus distribution and abundance over time. To do this, Hawke scoured newspaper reports, natural history books, explorers’ journals and museum records for references to platypuses. The locations of the nearly 26,000 reports found were then mapped onto the relevant water catchments so they could be tracked over time.

The results were stark. Over the past 20 years platypuses appear to have disappeared from 21.3 per cent, or 159,358 square kilometres, of their known historical range. This loss was most marked in the Gulf of Carpentaria and Murray–darling Basin, where recorded sightings dropped by 40.5 per cent and 27.9 per cent respectively. It was also highly geographically dependent: waterways in New South Wales showed a decline of 28.5 per cent and Queensland 29.4 per cent, while Victoria dropped by just 5.3 per cent, although this statewide figure was undercut by declines of more than 50 per cent in many urban catchments around Melbourne. Only Tasmanian catchments showed no signs of decline.

Hawke’s research also offers a disquieting glimpse of a time when platypuses were far more abundant. Records from the 19th century frequently describe sightings of as many as 20 platypuses in an hour or two, often in daylight. Likewise, thousands of platypus skins passed through markets in Sydney and elsewhere, with one furrier in Nowra claiming to have sold a staggering 29,000 furs in the years before World War One. “The records suggest a shifting baseline phenomenon is at work,” says Hawke. “When people go out and see two or three platypuses, they think that’s indicative of a really healthy population, when in reality it’s likely there would once have been many more.”

These declines are likely to accelerate in years to come. Bino’s modelling predicts that, without significant changes to land and water management practices, platypus populations could drop by as much as half over the next 50 years, leading to widespread local extinctions. “Climate-change projections show humanity is driving a future that’s hotter and drier, and that means more fires and more frequent and more severe droughts,” says Bino. But as Bino’s research demonstrates, these impacts are not experienced in isolation. Instead they interact, creating damaging synergies that further increase the pressure on animals and ecosystems. “It’s not just drought and fire, but rivers that are being exploited at unsustainable rates and heavily regulated by dams and other obstructions that prevent platypuses moving when the rivers dry out or recolonising after conditions improve, as well as

Platypus populations could drop by as much as half over the next 50 years, leading to widespread local extinctions.

changes to sedimentary process that are making rivers shallower. So it’s just death by a thousand cuts.”

With this in mind, Bino, Hawke and their colleagues at UNSW’S Centre for Ecosystem Science recently made a submission to have the platypus listed as a threatened species by the federal government, a move they hope will lead to better monitoring of platypus populations, and more careful consideration of threats from development and water management regimes. For Hawke, this is a critical move. “Often we only really become aware a species is in trouble when it’s already at the point of no return. But although with the platypus there’s evidence of long-term decline, there are still strongholds across eastern Australia. So, if we make the appropriate interventions now, by making sure we’re looking after the health of the rivers, then there’s still the potential to improve the situation for the platypus before we reach the tipping point.”

Later this month, Bino and his team will be returning to the catchments of the Manning and Hastings rivers to conduct follow-up surveys. What they find will contain important lessons about the future of this remarkable animal in a climate-changed world. And while Bino is cautiously optimistic, he is also realistic about the challenges ahead. “My gut feeling is things will be better, although numbers will probably still be low. And I’m really hopeful we’ll pick up at least a few juveniles. Because if numbers are still low and we don’t get any juveniles, then that will be a really big concern.”