I think its because of too many Guests in the Zoo. They really should add something like a Button to hide the Guests or a Button which immediately gets rid of the Guests. I think the Guests are the Reason why my first Zoo is unplayable (I have more than 3000 Guests there) I also have this Problem with one of my other Zoos (more than 2000 Guests). I tried closing the Park to get rid of the Humans but the Game crashes too fast. Even if I pause the Game and go to my new Enclosure where no Guests are, it crashes after a few Seconds
Edit: I was able to save my Zoo in Africa from Crashing with the lowest possible Graphic-Settings while the People walked out of the Park.
But sadly I wasn't able to save my first Zoo with this Method. Maybe 3000 People are even to much for the lowest Graphic-Settings