Endangered water voles are among the creatures thriving on a stretch of wetlands dubbed “Wales’ Amazon rainforest”, which is under threat of destruction.

Conservation groups are up in arms to protest the planned construction of a motorway that will cut across vast swathes of the Gwent Levels, north of Newport.

Home to otters, rare bees and wildflowers, they say this crucial site for Welsh wildlife will be ruined “for the sake of saving 10 minutes’ journey time”.

Six years ago Gwent Wildlife Trust reintroduced water voles to a site on the edge of the Levels, and a new study has revealed that since then the rodents bred successfully and have spread six miles from the launch site.

The success was particularly welcome as these rodents are some of the UK’s most vulnerable mammals and have been lost from 94 per cent of their former rage.

Critics of the planned construction hope this good news will bolster their case to preserve the precious wetlands.

Local conservationists fought the new 14-mile-long, six-lane M4 corridor during a public inquiry that concluded in March, and a decision on the final fate of the Levels is expected soon.

British mammals’ fight for survival Show all 11 1 /11 British mammals’ fight for survival British mammals’ fight for survival The Mammal Society’s assessment of Britain’s mammal populations is the first such review in over 20 years. The results reveal winners and losers, and highlight some of the key threat’s facing the country’s wildlife such as habitat loss, invasive species and the spread of diseases. British mammals’ fight for survival Hedgehogs Populations of the much-loved insectivore may have declined by as much as 73% Richard Bowler British mammals’ fight for survival Greater mouse-eared bats Only one of these flying mammals has been recorded in the whole country, in a railway tunnel in West Sussex Andrew Harrington British mammals’ fight for survival Rabbits Despite being a common sight across the countryside, rabbits have decreased in number by nearly 10% Danni Thompson British mammals’ fight for survival Black Rats Though not generally considered threatened animals, black rats number have collapsed in recent years due largely to pest control measures. However their far more common cousins, the brown rats, are thriving. Tim Melling British mammals’ fight for survival Red Squirrels Though it is difficult to measure changes in squirrel numbers, their range has shrunk considerably due largely to the spread of squirrel pox virus by invasive grey squirrel Alistair Marsh British mammals’ fight for survival Wildcats There has been a marked decline in wildcat populations, with only around 200 left in Scotland Rachel Profit British mammals’ fight for survival Badgers Due to legal protections badgers have recovered from past persecution and the density of their setts (holes) has increased by over 100% in recent decades Phil Mumby British mammals’ fight for survival Otters The banning of persistent organic pesticides has had a positive population effect on otters Allan Chard British mammals’ fight for survival Fallow Deer The 2018 review estimates a population size of fallow deer almost three times the size of that estimated in the 1995 review – a positive trend that is seen in other deer species. James Shooter British mammals’ fight for survival Eurasian Beavers These aquatic mammals were nearly extinct in Europe at the start of the 20th century, but their reintroduction to the UK have been largely successful Paul Scott

The Welsh government has argued that the M4 is a route of “strategic importance” for the region, and is not capable of dealing with the current volume of traffic that uses it.

“Opponents of the scheme have generally been reluctant to acknowledge the environmental benefits which it would bring,” the government concluded in its closing statement at the inquiry.

“Notably in terms of improved air quality and noise conditions for the thousands of people who live close to the current, overloaded M4.”

The new project has been estimated to cost between £1bn and £2bn, and the environmentalists have backed future generations commissioner for Wales Sophie Howe in a call for this money to be spent on an improved public transport system instead.

Fearing the worst from the government’s final decision, Ian Rappel, chief executive of Gwent Wildlife Trust, said the success of the water vole reintroduction programme was “bittersweet”.

“If the new road gets the go-ahead billions will be spent destroying a very special place for the sake of saving 10 minutes of commuting time,” he said.

Besides water voles, common cranes have recently begun breeding on the proposed M4 route after dying out in the UK 400 years ago.

In total, 125 hectares of specially protected wildlife zones will be lost or damaged by the project. Threats from the motorway will include collisions with traffic, toxic fumes from vehicles and disruption of water flow.

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The Wildlife Trusts have launched a campaign to save the Levels in order to preserve the habitat for future generations and to protect rare Welsh wildlife.

“When future generations realise the extent of the damage this generation has wreaked on our finite, beautiful, natural world they will ask ‘why?’ Why, for the sake of a few minutes knocked off journey times, did we destroy the unique and irreplaceable Gwent Levels,” said Wildlife Trusts chief executive Stephanie Hilborne.

“We do not have the right to destroy this treasure or deprive future generations of its joy.”

In response to the group's water vole findings, the Welsh government spokesman said: “Replacement water vole habitat is integrated into the M4 Project, as well as new habitat to ensure their numbers are maintained and increased for the long-term."