Some of the richest countries in the world are very reliant on immigrants to fulfil the labour needs of their economies. Dubai is a classic example - the native population is small and wealthy but they could not have possibly built and could not possibly operate, that great big resort city in the desert, without massive amounts of migrant labour.
The Uk is also an economy that relies on immigration. To give an example, the computer software industry, a very important sector, struggles to get good quality staff. This is because we do not produce enough graduates with the skills to meet the needs of this industry. If you go to any software company in the country (north or south) you are likely to find a multi-national group of programmers. My experience is that about 40-50% of programmers are not born in UK. This percentage seems to have climbed over the years as UK universities fail to meet the needs of the economy. Without the immigrants this vital and valuable industry sector would stagnate and decline. Oh and if you try to recruit a specialist software tester and you get 50 applicants in, none of whom were Uk born, because we just don't produce them ourselves. This is my first hand experience, so I know of what I speak.