3.4 Carrying Capacity

Carrying capacity (K) is the maximum number of individuals of a species that an environment can support indefinitely. It's set by limiting resources like food, water, nesting sites, space and sunlight. If a resource runs short, birth rates fall and death rates rise until the population roughly matches what the environment can sustain.

Carrying capacity isn't fixed. A wet year with lots of plant growth can raise K for herbivores; a drought or habitat destruction can lower it. Human activities often lower K for wildlife by removing habitat.

Populations don't always stop neatly at K. If a population grows quickly and there's a delay before resource shortages take effect, it can overshoot, meaning it rises above K. While it's above K, it uses resources faster than they can be replaced, damaging its own food supply. Then comes dieback (also called a crash): a sharp drop in numbers from starvation, disease or failed reproduction.

Because the overshoot may have damaged the environment, K can be lower after the crash than it was before. The population may then settle at a lower level or keep oscillating around K.

In 1944, the U.S. Coast Guard released 29 reindeer on St. Matthew Island in the Bering Sea. The island had plenty of lichen to eat and no wolves or other predators. The herd grew to about 6,000 by 1963, far more than the island could feed. The reindeer ate the slow-growing lichen faster than it could regrow. After a harsh winter, the population crashed to just 42 animals by 1966.

This case shows overshoot, resource depletion and dieback very clearly. It also shows why predators and harsh conditions often help keep populations near K.

On a graph of population size over time, carrying capacity is usually drawn as a horizontal dashed line. A population following logistic growth rises in an S-shape and levels off at that line. An overshoot shows the curve rising above the line, followed by a steep drop.

If a graph shows a population oscillating up and down around a level, estimate K as the average level the population fluctuates around.

Scientists debate Earth's carrying capacity for humans. Technology, like fertilizers, irrigation and trade, has raised the number of people Earth can feed. But using up groundwater, eroding soil and changing the climate could lower it. Because humans use very different amounts of resources, estimates depend heavily on how people live.