The affected animals would include such rare species as the jaguar, Mexican gray wolf, mountain lion, and whitetail deer.
The live on both sides of the border with the exception of the Jaguar which is a jungle cat.
The last time I looked the border between Mexico and the United States is a desert. The Jaguar of Mexico is many of hundreds of miles to the South. The author pulled that one out of his butt. Once the wall is built what would occur is two distinct breeding populations instead of one for the animals mentioned with the exception of the Jaguar.
There is still a small population go Jaguars in the Sonoran desert but they are unlikely to be affected by the fence since they live in the most remote mountain ranges.
In reality, the illegals are an ecological disaster and they wantonly kill the wild life they encounter on the trip to El Norte either for food or for entertainment.