A bit of brainstorming:
Creature Dispersal Code Considerations
Starting areas: 1, 2, 3, or more? There should be an increasingly low chance of (more) starting areas, with the vast majority of creatures only having. However, some species should probably start out in multiple places to represent conglomerate species, i.e. those similar to rats, the peregrine falcon, etc, which can be found virtually everywhere on earth, or to emulate the separation and evolutionary branching of a species due to phenomena such as continental drift or natural disasters.
Habitat availability and 'leaps': Should creatures be able to spread to distant habitats? Not sure about this one. It seems like the likelihood should be very low, except for migratory or otherwise very mobile species. There should probably still be a small chance of this happening, however, since it has certainly occurred in real-life evolutionary history. All but the hardiest and most adaptable of species should be restricted to a single or small set of habitats/niches to represent specialization.
ETA: The bio major girlfriend says this is actually fairly likely over a long period of time if two suitable habitats are in a semi-reasonable proximity to one another even if creatures would normally never migrate between the two, because being forced out of habitat A by natural disaster, predation, our overpopulation can result in a creature ending up in habitat B. Cool.
Competition: Creatures that draw from the same food source(s) should be considered competition and should either slow the spread of species or out-compete them if there is not enough food availability.
Possibility of extincion: Animals should probably be able to go extinct, although I'd like this to be an extremely small chance, since presumably the sentient races of Cult are (mostly) not industrialized enough yet to represent a serious threat to nature. But being out-competed by other, similar, more successful species and/or being hunted into extinction a la the dodo bird are both possibilities to implement.
Some additional thoughts: The result of this code being well-thought out should be a world in which an adventurer can go anywhere and encounter new creatures with their own niches and behavioral considerations. Eventually, hunting guilds, medicinal or magical recipes, and the like should also give players a reason to seek out such rare creatures in inhospitable/difficult to reach environments.