What is ecological density dependency

Regulation of population growth

The growth of a population is regulated by various factors. The population density, which relates the number of individuals to the existing habitat, has a major influence.

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Population density = number of individuals per area

How different the population density can be within a species is shown by humans themselves:

  • in Germany: 230 inhabitants per km2
  • in Australia: 3 people per km2

Age of individuals within a population

If you look at the age of the population of the Federal Republic of Germany, you can create a graphic that shows the number of individuals of a certain age. In Germany we are currently finding an urn shape, i.e. many people are between the ages of 35 and 65, few are older, fewer individuals under 35 are following.

In a young, growing population, one would expect a pyramid shape.

Pyramid shape and urn shape are extremes of the age structure within a population.

Population pyramids

For humans it is clear: the individuals of a population are of different ages. This is not necessarily the case with insects. The oak moth, a butterfly that uses oak leaves as an egg-laying place, only shows individuals of the same age within a population. Here the hatching of the oak moth is linked to the development of the oak leaves. The life cycle of the oak moth is linked to the course of the year. There is no age structure.

Pyramid shape and urn shape are extremes of the age structure within a population.

Regulation of population density

The individuals of a population are exposed to abiotic and biotic environmental factors. Optimal growth can only take place under conditions that are favorable for this species. In other words:

Abiotic factors have to be right:

  • suitable temperature
  • Air or soil moisture
  • adequate salt / mineral content

Biotic factors must not represent a competitive situation:

  • adequate food
  • sufficient nesting places
  • sufficient mating partner


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So there are two general ways to regulate population size:

  • depending on the population density
  • regardless of the population density