From another thread:
Using the save file is a good workaround for this sort of thing. It would throw my dynamic reproduction system out the window though... I could do a rapid calculation process to determine animal population when the map is loaded by saving a date-time stamp to the map being saved, but I still won't be able to have marauding bands when population reaches a peak.
You know, given the large map you want to do, and the fact that you will almost HAVE to have a lot of it not be loaded at any one time, I think the thing to do here is to handle most or all of your animal population code separately from the mobs themselves.
You might try having the population code run no matter what, and if that area of the map is loaded, it creates the mobs; if not, it just tracks what their population should be and keeps it up to date until next time the map is loaded.
Not the approach you were planning, I know, but I suspect much more realistic and flexible.