AP® Environmental Science · Unit 3 · Topic 3.4
Carrying Capacity
Carrying capacity (K) is the largest population size a habitat's resources can sustain long-term. Growth slows and levels off near K because limited food, water, space, and disease spread catch up with a crowding population.
Picture deer on an island. Early on, resources are plentiful relative to the small herd, so births outpace deaths and the population grows fast, often close to its biotic potential. As the herd gets bigger, each deer has less grass, less water, and more competition for space. Growth slows, then stalls around K, the population the island's resources can support over time without collapsing. The curve that describes this is logistic: a J-shaped rise that bends into an S as it approaches K.
The brakes on growth near K are mostly density-dependent: as numbers rise, competition for food and water intensifies, predators have more prey to target, and disease spreads faster through a crowded herd, since animals contact each other more often. These factors hit harder the closer the population gets to K. Density-independent factors work differently: a harsh winter storm or a wildfire kills a fixed fraction of the herd no matter how many deer there are, crowded or not. Both types can act on the same population at once, pushing numbers around K from different directions.
K is not a fixed ceiling stamped onto a place. It tracks whatever resources currently exist there, so it rises in a wet year with lush grass and falls when conditions turn harsh. That variability connects directly to real population swings: a population sitting right at K can be pushed over or under it as resources shift, which is exactly what happens to an island herd when a drought hits.
Key terms
- Carrying capacity (K)
- The population size an environment's resources can sustain over the long term.
- Density-dependent factor
- A limit on population growth, such as competition or disease, whose effect strengthens as population density increases.
- Density-independent factor
- A limit on population growth, such as a storm or wildfire, that affects a population regardless of its density.
- Logistic growth
- An S-shaped growth pattern in which population growth slows as it approaches carrying capacity.
Watch it explained
- How it worksHow do food, water, space, and disease each pull a growth curve flat at K?
- How it worksWalk me through density-dependent and density-independent limits acting on the same herd.
- Common mistakeWhy isn't carrying capacity a fixed number for a given place?
- Going furtherWhat happens to K when a drought cuts the grass on that island in half?
Check yourself
End of topic.