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Population growth and carrying capacity

Review your understanding of population growth and carrying capacity in this free article aligned to NGSS standards.

Key terms

PopulationA group of organisms of the same species living together in a particular area
EcosystemAll of the organisms in a particular area, along with the nonliving parts of the environment with which they interact
Population sizeThe total number of individuals in a population
Population densityThe number of individuals in a population, per unit area
Exponential growthPopulation growth that is unlimited by resource availability, which allows the population growth rate to increase continuously over time
Logistic growthPopulation growth that is limited by resource availability, causing the population growth rate to slow as population size increases
Limiting factorA feature of an ecosystem that restricts a population’s size
Carrying capacityThe maximum number of organisms or populations an ecosystem can support

Exponential growth

Exponential growth describes a hypothetical model for population growth in which space and resources are available in unlimited supply. As a result, the population growth rate increases with each new generation. Under this model, a population rapidly grows quite large, and continues to grow indefinitely:
A line graph titled Exponential Growth showing population size over time. The line curves upward with an increasing slope, showing that population grows exponentially.

Logistic growth and carrying capacity

In reality, there is simply not enough space or resources for natural populations to continue to grow unchecked. Limiting factors within every ecosystem, such as the availability of food or the effects of predation and disease, prevent a population from becoming too large. These limiting factors determine an ecosystem’s carrying capacity, or maximum population size the environment can support given all available resources.
Logistic growth describes a model for population growth that takes into account carrying capacity, and is therefore a more realistic model for population growth. According to the logistic growth model, a population first grows exponentially because there are few individuals and plentiful resources. As the population gets larger and approaches the environment’s carrying capacity, resources become more scarce and the growth rate slows. This leads to the logistic growth model’s characteristic S-shaped curve:
A line graph titled Logistic Growth showing population size over time. The population grows exponentially until it nears the carrying capacity, which is shown by a separate horizontal line. As the population nears the carrying capacity, population growth slows significantly.
The logistic growth model reflects the natural tension between reproduction, which increases a population’s size, and resource availability, which limits a population’s size. The result of this tension is the maintenance of a sustainable population size within an ecosystem, once that population has reached carrying capacity.

What else should I know about population growth and carrying capacity?

  • Real population growth often deviates from the ideal logistic model. The ideal logistic growth curve shows population size leveling off as a flat line just below carrying capacity. However, a real population’s size typically oscillates around its carrying capacity. This means it’s common even for a stable population to briefly exceed or dip below its carrying capacity, even though the average growth rate of the population is zero.
  • Carrying capacities can change. An ecosystem’s carrying capacity may fluctuate based on seasonal changes, or it may change as a result of human activity or a natural disaster. For example, if a fire destroys many trees in a forest ecosystem, the forest's carrying capacity for tree-nesting birds will decrease.

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