Carrying capacity and density-dependent factorsSpec C4.1.5, C4.1.6
In short
Carrying capacity is the maximum population size that an environment can support sustainably. It is set by limited resources such as food, water, space, light or nesting sites. Density-dependent factors such as competition, predation and the spread of pathogens push a population back towards the carrying capacity by negative feedback, while density-independent factors cause fluctuations.
- Carrying capacity
- The maximum population size of a species that an environment can support.
- Density-dependent factor
- A factor whose effect on the population increases as population density increases, such as competition, predation or disease.
- Density-independent factor
- A factor that affects a population regardless of its density, such as a flood, fire, drought or very cold winter.
Every environment has a carrying capacity for each species because resources are limited. When a population reaches it, individuals compete for the resources in shortest supply. Examples of limiting resources: food for animals; light, water and mineral ions such as nitrate for plants; space for sessile animals on a rocky shore; nesting sites for birds; oxygen in water for fish.
Negative feedback control
Density-dependent factors act as negative feedback: they reduce the birth rate or raise the death rate more strongly when the population is dense, and relax when it is sparse. This tends to push the population back towards the carrying capacity.
| Factor | Effect in a dense population |
|---|---|
| Competition for limited resources | Less food, water or space per individual: lower survival and fewer offspring |
| Predation | Prey are easier to find and catch, and predator numbers rise, so more prey are killed |
| Pathogens and pests | Close contact transfers infections and parasites between individuals more easily, so disease spreads faster |
- Population rises above the carrying capacity.
- Competition, predation and disease increase.
- Death rate rises and/or birth rate falls.
- Population falls back towards the carrying capacity; the factors then weaken.
Density-independent factors can make numbers fluctuate, for example a storm killing many seabirds at once, but they do not regulate the population around a set level.
Linking question: what factors can limit capacity in biological systems? Compare carrying capacity with limiting factors in photosynthesis or enzyme saturation.
Written and checked against the IB Biology HL specification · Updated October 2026