Nature regulates itself
Excellent! Your answer is correct.
When there are more foxes, rabbits have more predators and get eaten more often, so the number of rabbits declines.
Not all of the rabbits get eaten by foxes because...
a)...when there are fewer rabbits left, they reproduce faster. The foxes simply cannot eat enough of them.b)...when there are fewer rabbits, there are fewer foxes as well because the foxes do not find enough prey.