Back to the original subject of this thread, is stress involved when predator and prey species can see each other? Based on personal experience, I think it depends on a lot of a lot of different factors. The mere sight or smell of a lion might not be stressing in a negative way for a zebra, but combined with other factors it might be disasterous. Same goes for the predators. They get used to seing prey species very fast and they learn (just like predators in the wild) which individuals they have a chance of catching. Obviously not the ones on the other side of the fence or moat... The key word here is habituation. Both predator and prey learn that they have nothing to gain/fear and I believe that if they live in an otherwise good enclosure, it can be enriching for them. Obviously there is no rule here, some very nervous individuals or species may never adapt to a setting like this.