@TLD: I am much more optimistic about the bull situation in Europe because the last decade has shown that young bulls can easily be kept with older bulls. That works in a bachelor group, but also in the context of a breeding group. When a young bull can`t be housed with the female herd any longer, he an almost always be housed with his father. Cologne and Emmen are already housing their own bachelor groups (breeding bull(s) with male offspring).That works fine and is usually a situation that is stable for many years. I think this is the new strategy of the EEP; in the future all (or most) breeders are required to also house their own bachelor group. Which is not as difficult and space-intensive as it sounds because you can just put the boys all together. You still need space in the bull barn, of course. But many if not most zoos could house at least one juvenile bull with their breeding bull without too much problems, and most are not using these reserves right now.
It is easier too than in gorillas. Male Elephants of different ages associating in male groups is natural, with gorillas it isn't.