I thought the laws of economics told us that immigration and international trade increase prosperity.
What a gross way to misrepresent that article, the source of that quote, and the established consensus. That quote is not "from Wikipedia" any more than it is from yourself (or from HN for that matter) just because you happened to copy-paste it here. That quote is from one critic whom Wikipedia happens to cite just for the sake of illustrating what counterclaims exist out there.
If you really want to quote Wikipedia, why not quote the couple paragraphs before, which actually address what the consensus is:
Several arguments have been advanced against using comparative advantage as a justification for advocating free trade, and they have gained an audience among economists. [...] However, the overwhelming consensus of the economics profession remains that while these arguments are theoretically valid under certain assumptions, these assumptions do not usually hold and should not be used to guide trade policy. Gregory Mankiw, chairman of the Harvard Economics Department, has said: ″Few propositions command as much consensus among professional economists as that open world trade increases economic growth and raises living standards.″
Now your quote doesn't look so much like an established fact, does it?
The one and only thing I'm saying here is that you need to represent your sources and the sides of an argument honestly.
The effects of migration and trade are based on a slew of factors, including supply/demand for labour in both the host country and the home country, wages, benefits in both countries, etc... Trade effects depend a lot on tariffs, costs in countries on each side of the trade, and those in turn are largely the effect of supply/demand both for goods and labour in countries on both sides of the trade.