A new species of "killer fungus" has been discovered in Britain that destroys an aggressively invasive plant, raising hopes that decimated native habitats could be restored.
It infects the heath-star moss that is damaging UK environments by taking over and turfing out other species.
Scientists say it could represent a rare fightback by the British environment against invasive species.
The scientist tracking the spread of the moss die-back fungus across the UK took BBC News to see its "fairy rings of death" in south Wales.
Dr George Greiff, 30, made the discovery while walking in the Isle of Wight four years ago.
He spotted dead invasive mosses on a cliffside and took samples, but couldn't work out the killer.
But he kept seeing more cases of decaying moss. Working with scientists in the UK and France, he has now managed to piece together the puzzle.
The culprit was a potent fungus never seen before by scientists, now named moss die-back.
We often think of fungus as bad but consider this one to be a "goodie".
Its victim, the heath-star moss, is all over Britain. Look on hillsides, sand dunes, or a simple garden fence. "It's aggressive. I've even seen it growing in tarmac," says Greiff.
"It is a pretty-looking thing," he concedes, plucking fragments off a clump torn from a carpet of green moss. "But it's caused a lot of problems," he says.
Around 2,000 non-native plants and animals have been brought to Britain from all over the world by people, accidentally or sometimes deliberately.
Some end up dominating the environment. The most famous is probably the North American grey squirrel, which has nearly wiped out our native red squirrel populations.
In the plant world, amongst the biggest offenders are the invasive Japanese knotweed which damages people's homes, or rhododendron that leaves toxins in soils.
But not a lot is known about the heath-star moss. Scientists think it arrived on British soil in the 1940s from somewhere in the southern hemisphere. By 1990 it was everywhere.
"This moss has just exploded. In the 1930s, native mosses would have been growing here instead," Greiff explains.
There are more than 1,000 types of moss in the UK. They are the backbone of our most precious habitats like rare temperature rainforests (damp coastal woodlands) and peatlands that store carbon. But they are threatened by non-native plants.
The heath-star moss sends out spores far and wide and reproduces quickly, making it a successful invader.
"The first time I saw it, I had no idea what it was. I threw it in the bin," says Grieff, who has been working at Amgueddfa Cymru museum in Cardiff.
He's guiding me along a path in the Bannau Brycheiniog national park near Abergavenny, south Wales.
"In heathlands like this one, native mosses have gone locally extinct or reduced significantly in their populations," he says.
His eyes are fixed to the ground. He scans the low banks for "patches of death".
His talent for quiet, careful observation of plants no taller than our ankles is on show.
Just a few metres into our walk, Greiff finds what I've come to see. "It's a big as my hand," he says, spreading his fingers over a brown ring of dead moss.
Honestly, it is less dramatic than I expected. But our cameraman Gwyndaf Hughes has brought a macro lens, and when I peer through, I see white blobs suspended on the moss tips. It is the fungus at work.
Greiff points out baby heather plants nearby now able to grow in gaps left by decayed moss.
Now Greiff knows what the helpful fungus can do, he looks for it anywhere he goes. He jokes that the map of where it has been found is also a map of his holiday destinations.
"It's taken a lot of DNA sequencing to fully identify this fungus," he says, showing me dead moss under a microscope in a lab at Amgueddfa Cymru museum.
The fungus clings onto the moss stem, ballooning like candy floss around a stick. It's even penetrated some of the moss cells.
He's now worked out that it is a close relative of the ash die-back fungus that has killed up to 80 million ash trees in Britain.
A powerful fungus destroying plants sounds like a potential threat to other species and biodiversity.
But Greiff's analysis so far suggests it only affects the heath-star moss, and to a limited extent one other type of moss, although more work is needed to confirm the findings.
He believes that the origins of this fungus may be in a native species that has since adapted to kill the heath-star moss. This could be a "rare example of the British environment fighting back", he says.
Usually, invasive species are deliberately targeted by humans to try to bring them under control, like the initiative to feed contraceptives to grey squirrels.
"Some people might try to collect this moss to try and get rid of it, but it's not very effective. It would be very resource-intensive, very expensive," says Greiff.
"To have a natural biological control agent doing it for us is really valuable," he explains.
In cabinet drawers holding the oldest collection of mosses in the UK, the museum's Head of Plant and Earth Science Dr Nathan Smith shows me dried mosses in paper envelopes, some dating back to the 1880s.
By looking for evidence of the moss die-back fungus on these samples, scientists want to pinpoint exactly how and when it appeared.
"This fungus gives an opportunity to save these unique moss landscapes that are homes for insects, fungi, molluscs, other plants," explains Smith.
"It gives a real chance to preserve and present their uniqueness and beauty," he says.
Mosses are just one part of Britain's complex biodiversity. But as our nature is classed amongst the most depleted in the world, with one in six species at risk of extinction, discovering a control on habitat destruction is a cause for hope.
"It's exciting in so many respects. Though I am the only one looking for the fungus. It would be nice if some other people did too," says Greiff.
