How does competition influence population size

Say you have foxes and rabbits. They have a steady eco system because the rabbits breed and the foxes consume. This keeps the population consistent. If you add wolfs into the equation it completely throws off the balance because the wolfs and foxes will have competition. They will start to eat all the rabbits before they can multiply and the food sources will grow scarce thus upsetting the balance of that ecosystem. Hope this helped!