While the barbarians have means to add wildlife, the other tribes don't. Fine from a gameplay perspective, but it can make the map look a bit empty.
Suggestion: Once in a while a sheep escapes from the empire sheep farm, and a boar escapes from the empire pig farm. This should be rare enough not to affect balance much. But it would add a nice touch, making the map a bit more lively. And in a big empire with many pig and sheep farms it might even make the hunter a not-totally-useless building even in the later game.