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Population Size

Limiting factors determine the carrying capacity for a population in an environment.

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Population Size

How many penguins are the right number for this beach?

As many as can survive and have healthy offspring! A population will tend to grow as big as it can for the resources it needs. Once it is too large, some of its members will die off. This keeps the population size at the right number.


Biotic and abiotic factors determine the population size of a species in an ecosystem. What are some important biotic factors? Biotic factors include the amount of food that is available to that species and the number of organisms that also use that food source. What are some important abiotic factors? Space, water, and climate all help determine a species population.

When does a population grow? A population grows when the number of births is greater than the number of deaths. When does a population shrink? When deaths exceed births.

What causes a population to grow? For a population to grow there must be ample resources and no major problems. What causes a population to shrink? A population can shrink either because of biotic or abiotic limits. An increase in predators, the emergence of a new disease, or the loss of habitat are just three possible problems that will decrease a population. A population may also shrink if it grows too large for the resources required to support it.

Carrying Capacity

When the number of births equals the number of deaths, the population is at its carrying capacity for that habitat. In a population at its carrying capacity, there are as many organisms of that species as the habitat can support. The carrying capacity depends on biotic and abiotic factors. If these factors improve, the carrying capacity increases. If the factors become less plentiful, the carrying capacity drops. If resources are being used faster than they are being replenished, then the species has exceeded its carrying capacity. If this occurs, the population will then decrease in size.

Limiting Factors

Every stable population has one or more factors that limit its growth. A limiting factor determines the carrying capacity for a species. A limiting factor can be any biotic or abiotic factor: nutrient, space, and water availability are examples (Figure below). The size of a population is tied to its limiting factor.

Water is the limiting factor on plant population in the desert

In a desert such as this, what is the limiting factor on plant populations? What would make the population increase? What would make the population decrease?

What happens if a limiting factor increases a lot? Is it still a limiting factor? If a limiting factor increases a lot, another factor will most likely become the new limiting factor.

This may be a bit confusing, so let’s look at an example of limiting factors. Say you want to make as many chocolate chip cookies as you can with the ingredients you have on hand. It turns out that you have plenty of flour and other ingredients, but only two eggs. You can make only one batch of cookies, because eggs are the limiting factor. But then your neighbor comes over with a dozen eggs. Now you have enough eggs for seven batches of cookies, but only two pounds of butter. You can make four batches of cookies, with butter as the limiting factor. If you get more butter, some other ingredient will be limiting.

Species ordinarily produce more offspring than their habitat can support (Figure below). If conditions improve, more young survive and the population grows. If conditions worsen, or if too many young are born, there is competition between individuals. As in any competition, there are some winners and some losers. Those individuals that survive to fill the available spots in the niche are those that are the most fit for their habitat.

A frog in frog spawn

A frog in frog spawn. An animal produces many more offspring than will survive.


  • Biotic factors that a population needs include food availability. Abiotic factors may include space, water, and climate.
  • The carrying capacity of an environment is reached when the number of births equal the number of deaths.
  • A limiting factor determines the carrying capacity for a species.


  1. Why don't populations continue to grow and grow?
  2. What happens if a population exceeds its carrying capacity?
  3. What happens if a factor that has limited a population's size becomes more available?

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Use this resource to answer the questions that follow. (Note: that when he says "people," he's really talking about any population of organisms.)

  1. Under what circumstances can population growth be exponential?
  2. What is carrying capacity?
  3. What does reaching the carrying capacity do to population growth?
  4. What does carrying capacity depend on?
  5. What happens if a population exceeds its carrying capacity?
  6. Is the carrying capacity constant? What changes it?
  7. What are the two ways to eliminate a pest from your home?
  8. Give the definition of density dependent factors that are limiting to population growth.
  9. Give four examples and explain them for density dependent factors.
  10. How do natural disasters affect the population size in a region?

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carrying capacity The number of individuals of a given species a particular environment can support.
limiting factor The one factor that limits the population of a region. The limiting factor can be a nutrient, water, space, or any other biotic or abiotic factor that the species need.

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