The carrying capacity is the size of a population that can live
indefinitely using the resources available where that population
lives. For example, consider an island onto which is dropped a colony
of rabbits. As long as there is an adequate supply of food and water,
the rabbits will not only survive but they will reproduce and the
colony will get larger. The rabbit population can continue to grow as
long as food and water are adequate. However, if at some point,
there are more rabbits that there is food to feed them, then the
rabbit population will start to decline.
This limit is called the carrying capacity. Carrying capacity is
not a fixed number; it depends upon factors such as how much each
rabbit eats, how fast the food grows, and how well the natural systems
of the island can handle the waste produced by the rabbits. Obviously,
in a drought year less food would grow and the island would support
fewer rabbits. In good years, the island would support more rabbits.
The earth is our island. We have an advantage over the rabbits in
that we have developed technology to grow, process, and store food so
that we can survive the bad years. We have also developed
technologies for handling wastes that we create. However, there is
still a carrying capacity that the earth can support. That carrying
capacity is a function of the number of people, the amount of
resources each person consumes and the ability of the earth to process
all the wastes produced. Sustainability is about finding the balance
point among population, consumption, and waste assimilation.