a) Rabbits eat grass and if they increase in number, soon the grass population would decline because rabbits will feed on grass faster than they can regrow.

b) Fox feeds on rabbits. If the number of rabbits decreased, soon the fox population will decrease because it will not get to feed on rabbits and foxes would begin to die because of starvation. And if the rabbits will decrease, the grass plants will increase in number because there will be less population of rabbits to eat grass plant.