Continually banging your head on the ceiling is not very pleasant. This is also the case when wild animal stocks hit the ceiling of the ecosystem's capacity for that particular species. In a certain area there is only room for a certain number of animals of a certain species. Limitations appear in the form of available food, lack of space, and competition with other species.

When a species approaches this limit, starvation and disease cause an increasing mortality rate and decreasing fertility. Finally, the same number of animals will die as is born. If there is widespread consensus that stocks should be kept at such a level, there will be nothing to harvest.

If the stock is reduced, the mortality rate will sink, fertility increase and each year, more animals will be born than die. It is possible to stabilize the stock at such a reduced level and harvest the annual surplus instead of allowing the stock to grow.

Some species provide the biggest possible surplus when the stock is at 50% of the ecosystem's capacity for that particular species. Other species provide the biggest surplus when at a somewhat higher level in order to the ecosystem's capacity.