I'm not sure that illegal immigrants drive down the costs of labour when there is little unemployment but I agree that it could be a problem. I doubt if illegals are competing for the same jobs as legals.

I also take issue with your statement about the degradation of the environment. The environment does not have national boundaries, unless you are talking about a socio/cultural environment. If you're talking about sustainablility in a physical environment, it doesn't matter where the people actually live, its a worldwide problem. The damage to the Earth is the same whether it happens in Mexico or if it happens in the U.S. Environmental issues are international.