Zoo population maintenance?

If you had to choose (in many cases release into the wild and transfer to another zoo is not an option) how would you manage zoo populations?

Circle of Life - more natural, animals live more closely to how they would in the wild including some being killed.

Birth control - more humane, animals live out thier lives but can't have babies.

This is apparently sort of a European vs American view. So few free where you are posting from :) If you want more background Google "zoo birth control vs. culling" and there will be plenty of articles!