The goal is not to model the life inside an ecosystem in which the player would act.
The objective is try to make the behaviour of Creatures, their evolution and the surrounding fauna and flora as realistic to the point of view of the players as possible.
The average player is not interested in the way the Creatures are reproducing. The only things he sees, is that when he kills a large number of rabbits, their number returns slowly to its initial level.
If he decides to exterminate all rabbits of the area, they will not come back.
In the same way, if he kills a creature X, he will not find it again in good health at the other side of the city (and a fortiori at the same place).
As a consequence, we will try to simplify at a maximum the modeling of the fauna and the flora by neglecting a number of parameters.