(a) If the number of foxes increases, it will lead to foxes consuming more rabbits as food, which will lead to a decrease in the number of rabbits. Hence, grass plants will grow denser as there will be fewer rabbits to consume them.
(b) If the number of foxes decreases, then there will be fewer foxes to consume rabbits as food, so the number of rabbits will increase, which will in turn lead to rabbits consuming more grass plants as food. Hence, grass plants will become sparser.