It’s called Darwinism. Survival of the fittest.
If the numbers have fallen off already without a fence, then the species is already at the end of the line.
It would be going against “nature” to extend the bloodline artificially.
The concern isn’t over the animals. It is over those coming here for migrant employment opportunities.
One of the more hilarious arguments I have heard are the Audubon people who are against the fence because it “endangers rare bird species” and haven’t figured out yet that 1) birds aren’t bothered all that much by fences and 2) the bird habitat is being destroyed on a large scale by illegals crossing. Birds will build nests in the fence before it is all over.