Plant and animal species could become extinct in both parts of Ireland because of Brexit, environmentalists have warned.
A fifth are already threatened, and a hard environmental border could undermine common standards and water down protection of those most at risk, according to campaigners.
Lobby groups from both sides of the Border are visiting Brussels on Monday to voice concern.
Chairman of Northern Ireland Environment Link Patrick Casement, said: "Our small island forms a single and unique unit in terms of our natural environment, and our plant and animal species do not recognise the existence of a border.
“Many of these species are currently at risk of extinction on the island of Ireland and any dilution of protection will place them in further danger.
“Any future divergence or lowering of standards on either side of the Border would be bad for the environment, bad for citizens and also bad for business.”
EU environmental standards
More than 650 pieces of EU legislation aid environmental protection in the Republic of Ireland and Northern Ireland.
The North-South green coalition said the island has benefited hugely from this common set of environmental standards.
They created a more co-ordinated and consistent approach to addressing cross-Border environmental issues such as the conservation of species and habitats on an all-island basis, the lobbyists added.
Co-ordinator of Ireland's Environmental Pillar, Michael Ewing, said: "It is only by avoiding a hard environmental border that we can ensure our joint efforts to protect and enhance the environment for the benefit of all is not undermined."
Negotiations and discussions have been focused solely on the economy, with little mention of the potential negative impact on natural heritage, he said.
“Our time in Brussels gives us the perfect chance to highlight the challenges Brexit brings for environmental protection and to articulate how these issues can be averted through continued cross-Border co-operation and the maintenance of high environmental standards both North and South.”
– PA