How has the increasing number of foreign workers in Europe been a result of declining fertility rates in Western European countries?

All developed countries have lower birth rates because it's expensive to feed, house and clothe children. That's different from agricultural countries, where children start to work on the farm at an early age and contribute to the family. So all developed countries need to import workers. Even Japan has found that out, and is bringing in workers from the Philippines and other places. Without immigrants, the economies of Western European (and the US) would crumble. These imported workers come from cultures and religions that reject birth control, so they continue to have higher birth rates, raising the overall birth rate in the Western European country. 

no

it's a result of there being too much money in western europe, and east-europeans being willing to work crazy hours for a very modest salary