The majority of shallow-water reef species living around Australia's coastlines have declined in number over the last 10 years, and scientists predict extinctions will follow.
In what appears to be a clear signature of climate change, population declines are disproportionately impacting cool-water species. On the flip-side, some warmer-water species have expanded their range.
According to the results of the "most comprehensive assessment of marine species population trends to date", published today in Nature, as many as 138 more shallow-water reef species can now qualify for endangered and critically endangered listing after suffering significant population declines in the last decade.
The populations of more than a quarter of all species in the study declined by 30 per cent or more — a rate that could qualify them for threatened listing on the International Union for Conservation of Nature (IUCN) Red List.
The study looked at over 1,000 species Australia-wide from 1,636 sites. The observations were supplied by more than 100 citizen scientists — trained volunteer divers — under the Reef Life Survey project, which study lead author Graham Edgar from the University of Tasmania helped set up.
"We'd never have had the resources as a scientific research team to cover the whole continent," Professor Edgar said.
"But by headhunting and tapping into the enthusiasm and the skill of recreational divers, and training them to a scientific level of data collection, we were able to complete this audit of Australia's marine life."
In general, warmer-water species tend to be expanding into the cooler margins of their ranges, putting pressure on more temperate species.
Contrary to expectation, coral populations showed a slight increase during the study period off the north-east coast of Australia as they expanded into those cooler margins.
While one species of coral significantly decreased, four species increased.
Where animals and plants can migrate south, they are tending to do so. But cooler-climate species are running into what are called "climate traps" — geophysical barriers stopping them from migrating any further.
In the case of Tasmania, the waters have warmed by around 1.5 degrees Celsius over the past 60 to 70 years.
"There's been this gradual warming effect and it's starting to push species to their limits," Professor Edgar said.
"They're squeezed on the edge of a cliff basically. With higher numbers of the warmer-water species pushing down, there's nowhere for [the cooler species] to go."
Endemic species will be 'gone from Earth'
More than 30 per cent of invertebrates living in the cool, shallow waters of southern Australia are deemed to be at high risk of extinction.
Asta Audzijonyte from the Institute for Marine and Antarctic Studies at the University of Tasmania said unlike in tropical waters, the majority of Australia's southern marine flora and fauna is endemic.
"These cold water species around Tasmania, southern Australia, 70 per cent of these species occur only in Australia, compared to 3 per cent of tropical species," said Dr Audzijonyte, who wasn't involved with the study.
"So if a species goes extinct [in the south], that's it. It's gone from Earth."
The fingerprints of extreme marine heatwaves also showed up in the data, Professor Edgar said.
"The biggest pulse event in the survey was the 2011 heatwave off south-western Australia when the Leeuwin current heated up about 4C.
"That caused a massive influx of those tropical species down to the south-western corner ... our data indicates that the colder water species haven't recovered from that."
And following the 2016 eastern Australia marine heatwave, just a quarter of the 24 warm-temperate species previously observed at subtropical sites on the southern Great Barrier Reef and Coral Sea were observed again at those sites.
Weedy sea dragons, sea urchins, invertebrates show big declines
Charismatic species such as the weedy sea dragon declined by 59 per cent over the 10 years. And overall, populations of large-bodied fish tended to show a greater decline than smaller fish — thought to be a combined factor of fishing pressure and climate change.
But less photogenic and arguably more ecologically significant species featured heavily in the declines in the south.
Dr Audzijonyte said the design of the study — using hundreds of citizen scientists — provided data at a resolution that would usually be "totally missed".
"We don't study how many sea urchins or little invertebrates there are, and their overall abundances on such a large scale.
"Once we put it together, this is fairly worrying because that's actually what is driving the declines in these cool waters."
The study only included species observed on at least 60 occasions, in order to have enough data to establish credible population trends.
The upshot of this is that very rare and potentially disappearing species were excluded.
Given the lack of funding for monitoring, and the challenges of surveying in marine environments, it's likely that marine species have already gone extinct in the south that we're not aware of, Professor Edgar said.
How do we know when marine species go extinct?
We're all dependent on biodiversity in different ways, but no country anywhere in the world is systematically tracking the changes that are going on, Professor Edgar said.
"If a marine species has become extinct in the last five years, nobody will ever know because there's no consistent assessment of what is happening in the marine environment in terms of the biology of the system."
The smooth handfish is one species that Professor Edgar thinks has likely been extinct for some time. It was briefly listed as extinct on the IUCN Red List, but was removed because targeted studies had not been done to confirm it has gone — proving absence is difficult.
He said more needs to be done to stop further southern extinctions.
"What we have [in southern Australia] is this really unique biota that no one else is going to look after. And it's in a heavily populated area of the coast where most of the marine threats are concentrated.
"We really do need to protect these areas because some of the species within it are heading towards extinction."
Barramundi, Irukandji and some coral are increasing
As well as losers, there are some winners as waters warm, at least in the short term.
Anecdotally in southern Queensland, some anglers are reporting increased catches of barramundi, including in the canals of the Gold Coast.
The southernmost captures of barred grunter — a prized angling species in Queensland — were reported in the Nambucca river in New South Wales in 2021 and 2022.
And there's some evidence that Irukandji jellyfish are expanding their range south as well.
Dr Audzijonyte said the biggest surprise to come out of the research, for her, was the expanding range of some corals and other tropical species.
While extinctions can happen in the blink of an eye, new species take millennia to evolve.
"For sure, diversity is disappearing, and new species won't evolve in our lifetimes," Dr Audzijonyte said.
"The main thing is we're not protecting these [southern reefs]. A lot of attention goes to the coral reefs and that's great. Of course they should be protected and should be protected more.
"[But] it's only about 1 per cent that we are actually strictly protecting in these cool, temperate coastal waters."
