Carrying capacity

Carrying capacity, written K in population ecology, is the maximum number of individuals of a species that a particular environment can sustain over time. It is set by limiting factors such as food, water, nesting sites, shelter, predators and disease, and it can change when the environment changes, for example after a drought or when waste products build up.

When resources are plentiful, a small population can grow almost exponentially. As numbers rise, individuals compete more intensely for what is left, so birth rates fall or death rates climb and growth slows. Plotted over time, this logistic growth traces an S-shaped curve that levels off near K. Factors whose effect depends on how crowded a population is, such as competition, predation and disease, are called density-dependent, while those that strike regardless of numbers, such as fires and storms, are density-independent.

Real populations rarely sit exactly at carrying capacity. Harbor seals, for example, briefly overshoot it and then fall back, so their numbers fluctuate around K. Yeast grown in a test tube shows the classic S-shaped curve most clearly, leveling off as its nutrients run low.