This is one of the most impressive developments in the game PERIOD! Ultimately, this is what DH and I had discussed on a number of occasions, a biological spawning system like this. You've done very well with it but I do have a couple of issues I'd like to point out.
At current, my experience of the game suggests that we are causing animals to spawn based on the resources that have been placed on the map (whether we see them yet or not is irrelevant.) Now that seems to work fine except that when your map fails to place certain game-critical resources, such as deer, sheep, horses (as our current game has failed to) it leads to the world flat out not having any of those resources - ever. Mind, this is probably more of a problem to be addressed with the mapscripts but could be overcome by some consideration for it here.
Otherwise... very well done. I agree that an initial spawn is good to work on and have nothing to really add to what you're working on now except to say its spot on to what I thought it needed.