The EU’s common agricultural policy (CAP) should be overhauled urgently to stop the intensification of farming practices that is leading to a steep decline in wildlife, scientists from across the bloc have urged.

Five organisations representing more than 2,500 experts have written to Ursula von der Leyen, the incoming president of the European commission, and the European parliament, to demand major changes to the way the CAP operates.

Under current rules, farmers are rewarded for the land they farm, and can apply for supplementary subsidies based on implementing measures to protect the environment and preserve wildlife and flora. This system is regarded as an improvement on its predecessor, which encouraged over-supply in some areas by awarding production-based subsidies, leading to the infamous butter mountains and wine lakes of previous decades.

However, the current CAP – which costs EU taxpayers €60bn a year – is “turning rural areas into green deserts of uninhabitable maximum-yield monocultures”, according to the European Ornithologists Union, the European Mammal Foundation, the Societas Europaea Herpetologica, the Societas Europaea Lepidopterologica and Butterfly Conservation Europe.

There is now an “unequivocal scientific consensus” that the plummeting numbers of European farmland birds, populations of which more than halved between 1980 and 2015, and a decline of more than three-quarters among insects in some areas, have happened because of intensified farming, the signatories wrote.

“The greening measures in the CAP are largely ineffective at retaining or restoring biodiversity and are too often poorly controlled,” they wrote. “The current agri-environmental schemes are both underfunded and insufficiently targeted to meet the scale of the damage to farmland biodiversity.”

Proposed changes that would place more emphasis on such measures are still inadequate, the experts found. Instead, they want to see fundamental reforms that support smaller farms that use sustainable methods and maintain high biodiversity.

Harriet Bradley, the EU agriculture policy officer at BirdLife Europe, said: “The scientific evidence speaks for itself – intensive farming is killing nature. If the CAP isn’t fundamentally reformed – if we don’t make space for nature – then Europe’s ‘Green Deal’ promises on biodiversity and climate will just be empty words on another piece of paper.”

In the UK, farmers will no longer receive CAP subsidies of about £3bn a year after Brexit, but the government has pledged instead to offer payments to farmers who meet certain environmental targets, which have not yet been set out.

A spokesperson for the Department for Environment, Food and Rural Affairs said: “Leaving the EU presents an opportunity to design a bespoke agricultural policy and reward our farmers for enhancing the environment, including protecting and improving biodiversity.

“Our new environmental land management scheme is intended to be a powerful vehicle for driving this change and achieving the goals of the 25-year environment plan. We are now working closely with a range of environmental and agricultural stakeholders to collaboratively design this new scheme.”