Plant and animal species could become extinct in both parts of Ireland because of Brexit, environmentalists warned.
A fifth are already threatened and a hard environmental border could undermine common standards and water down protection of those most at risk, campaigners added.
Lobby groups from Northern Ireland and the Republic visited 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."
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 coordinated and consistent approach to addressing cross-border environmental issues like the conservation of species and habitats on an all-island basis, the lobbyists added.
Coordinator 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 warned.
"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 cooperation and the maintenance of high environmental standards both north and south."
