By creating thoroughfares for the illegals to transit and not closing those gaps, between mere trampling the indigenous flora and whatever fauna could not escape, and the sheer amount of refuse and rubbish discarded by the illegals and what ever damage they, individually and directly did, I can easily see that allowing that traffic to transit would cause significant damage.
Just a few hundred thousand feet tramping through would change the surface conditions allowing wind or rain to erode land more effectively, killing off vegetation and promoting erosion, increased sediment load in any water courses, and disrupting the food chain and behavioural habits of the local fauna.
Certainly, that would have far more environmental impact than the completed construction of the barrier, which would have already been studied in the project EIS.
