Despite doubt about what U.S. President-elect Donald Trump’s pledge for a “great, great” U.S.-Mexican border wall will mean on the ground, green advocates are warning that further fortification of the international boundary will seriously harm the region’s ecosystems.

While Trump initially promised a solid concrete wall up to 45 feet (13.5 meters) high, more recently he has suggested he might simply add fencing. Meanwhile, Republicans in Congress have said they might support expansion of the existing 650-mile (1,000-km) mix of pole barriers, concrete slabs and fencing.

Conservationists argue that more barriers of any sort, apart from being expensive and of questionable utility in deterring migrants, represent major threats to the integrity of animal habitat and border-region biodiversity.