Diversity isn't a big problem. Conflict is a problem. People will always find reasons to conflict. That's why people in remote villages from the beginning of time fought people in the next village over. Not because the diversity was too great to manage. But because conflict and looking for reasons to fight seems to be inherent in people. 

Infighting now isn't just about diversity, any more than it was in 1603. Look at the DP discussion boards. I'd bet the majority of commenters here are white Christian Americans from the Western United States, if not Colorado, and yet because of political partisanship, or sports team affiliation, or school pride, or class affinity or some other man-made demarcation, they go at each other every day, day after day as if they were from different species.

"Diversity" is all around us. The search for homogeneity and purity is a never ending because it's a false, non-existent state of being; there will always be differences for cranky humans to fight over.