The snakes keep the mice from overpopulating, which could deplete their resources.
Explanation:
The mice and the moles are not competing against each other as they feed on different food. The problem is that if the mice population is not regulated, their very quick reproduction will cause overpopulation in very short space of time. The more mice there will be , the more food will be needed, so very soon the resources will be depleted, resulting in collapse of the mice population. This is where the snakes come in action, as they eat mice, so they are the ones that control and keep their population stable, thus not allowing the mice to overpopulate the area and destroy themselves.
