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More than 40% of UK species are in decline while 15% are threatened with extinction, according to the latest State of Nature report.
Many species, particularly top predators such as lynxes and wolves, have been extinct here for centuries and this has knock-on effects for the wider ecosystem.
Known as keystone species, their hunting stops other animals from overbreeding and dominating their ecosystems.
The Wildlife Trusts are not currently proposing to reintroduce lynxes or wolves but keystone herbivores instead, such as beavers, highland cows and Exmoor ponies.
By damming rivers and streams, beavers create wetlands and provide healthy habitat in which other species thrive.
The rewilding programme, called Transforming Nature’s Recovery, is to help the UK fulfil its international obligation of protecting 30% of land for nature by 2030 and net-zero carbon emissions by 2050.
It will take £56 billion to realise this aim, The Wildlife Trusts said, as it announced £6 million coming from the Ecological Restoration Fund, which supports organisations worldwide that revive nature and protect biodiverse areas.
Dr Rob Stoneman, director of landscape recovery at The Wildlife Trusts, said: “Extinctions, pollution and wildfires make headlines with increasing frequency and so it’s vital to go much further with nature recovery across the UK.
“We must create a place where wildlife, farming and people thrive together, where beavers work their magic and benefit communities, where seas are abundant with marine wildlife and where there’s easy access to nature to improve people’s lives.
“This game-changing donation from the Ecological Restoration Fund will bolster our work when nature needs us most.”
Beyond funding the release of beavers and grazing animals, the money will also go towards regenerating marine ecosystems off the Welsh coast, a nature recovery corridor along the length of Hadrian’s Wall and improving chalk streams, peatland and grasslands across the country.
The Wildlife Trusts also want to restore Atlantic rainforests in the damp, western regions of the British Isles.
It is thought these forests once carpeted much of Cornwall, Wales, the Lake District and Western Scotland though only 1% of this habitat now remains.
Daniel Hotz, chair of the Ecological Restoration Fund, said: “We’re proud to support The Wildlife Trusts in their ambitious vision for UK nature recovery.
“Recognising that our collective futures are intrinsically bound to the health of our environment, the urgency to address the UK’s troubling species decline and habitat degradation is paramount.
“By knitting together and rejuvenating wild spaces, we’re not only creating richer habitats for wildlife but also bringing tangible benefits to local communities.”