I suppose that if wages at the bottom are less, those folks have less disposable income, thus they spend less, so people in jobs that service them (grocery clerks are an excellent example as everyone buys groceries) may well make less.
susie
That's one way of looking at it, but I don't think it's right. An employer is interested in paying the least amount possible that will still attract qualified people to take the job.
Illegal aliens aren't generally equipped to compete for grocery clerk jobs, at least not until they've been here long enough to speak passable english, etc. So their presence isn't really relevant to what employers must pay to attract labor for that position.
I guess it's true that if a large portion of the population doesn't make enough money to buy stuff, that puts a squeeze on employers, but that doesn't seem apparent yet. Witness the lines at any store this week.