What are the main causes and effects of growing ethnic diversity in contemporary Europe?

As the twenty-first century opened and ongoing globalization transformed European society and politics, Europeans also saw the ethnic makeup of their communities change. On the one hand, Europe experienced a remarkable decline in birthrates that seemed to predict a shrinking and aging population in the future. On the other hand, the peaceful, wealthy European Union attracted rapidly growing numbers of refugees and immigrants from the former Soviet Union, the Middle East, Africa, and Asia. The unexpected arrival of so many newcomers raised perplexing questions about ethnic diversity and the costs and benefits of multiculturalism.