Ethos, by one definition, is the distinguishing character, sentiment, moral nature, or guiding beliefs of a person, group, or institution. With this in mind, think how immigration patterns can change the ethos of a country like the US, Europe, and Australia. Australia, in particular, has both a constant outflow of citizens, and a inflow of new migrants. This doubles the rate of change of the ethos in the country.
Without proper control of the mix of outflowing "ethos" (for want of a better word) and inflowing ethos, Australia will gradually experience a change which may or may not be welcome by the older inhabitants. This can create social unrest, which will in turn erode the economic advantages that the immigration policy is trying to create; i.e. skilled workers, job creation, increased tax revenue. Social unrest tends to become more pronounced in times of economic crisis.
Negative ethos that might build up over time includes extremist religious beliefs, which can be very incendiary; or simply cultural behaviors that are more easy to live with in this age of globalization. I propose that immigration policies should also take care of the changing ethos in the country and not just be focused on economics.