A coalition of 25 conservation organisations in England has warned the government about the dangers of cutting public funding in the countryside.
The Wildlife and Countryside Link said reducing funding for rural projects would have a "profound" effect on wildlife, landscapes and people.
Short-term savings would create "huge, long-term" economic costs, it said.
The Department for Rural Affairs said the 2011-15 budget would be announced in October following a spending review.
The coalition, which includes the RSPB, RSPCA, Campaign to Protect Rural England, Friends of the Earth England and the Wildlife Trusts, warned that an "austerity countryside" would push species towards extinction, damage rivers and hurt the rural economy.
It said that without funding for protected areas, such as Sites of Special Scientific Interest, reedbeds would dry up, heathlands would vanish and rivers and canals would get clogged up.
Reducing the budget for wildlife-friendly farming and land management measures - known as higher level stewardship - would threaten species such as smooth snakes, bees, butterflies and bats, as well as wildflowers and woodlands, it added.
At sea, a lack of conservation measures could damage reefs and seagrass beds, while an increase in illegal fishing could put pressure on fish stocks.
Squeezing the budget could also derail plans to record footpaths and bridleways and lead to the loss of ancient rights of way, it said.
Paul de Zylva, chairman of Wildlife and Countryside Link, said: "Such a picture is not an exaggeration, nor is it an inevitability.
"There may be a temptation to see cuts in conservation and recreation as an easy win, but in reality ministers need to think very hard before making cuts that could have profound and perhaps irreversible consequences for England's wildlife, landscapes and people.
"Defra and its agencies like Natural England spend just 0.5% of the government's budget, yet their investment in the countryside brings huge benefits in wildlife, clean air and water, flood alleviation, carbon sequestration and pollination," he said.
A spokesman for Defra said: "All Defra's main arm's-length bodies have had their funding reduced by 5% as part of our contribution to the £6.2 billion efficiency savings this financial year.
"It will be up to each individual organisation to revise their plans to reflect this reduced funding, but we expect them to protect front-line services as much as they can by first looking for efficiency savings or reducing back office costs."