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Density-Dependent Factors: Examples, Definition & Quiz

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Taught by

Sarah Friedl

Sarah has a Master's degree in Zoology and a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Populations cannot grow indefinitely because they are limited by resources. When there are too many individuals in a given area, the population may become too dense. However, nature has ways of helping the population return to a more appropriate size.

Definition

When a population of organisms becomes too large, the individuals will suffer because there will not be enough resources for everyone. These resources, such as food, water, and shelter, are essential to life. Each population has a size that is 'just right' for it, and there are natural methods to control population growth.

One very important mechanism for regulating population size is density dependence. The density of a population is simply how many organisms are living in a given area. Density-dependent factors are factors where the effects on the size or growth of a population vary with the density of the population itself.

There are several types of density-dependent factors, but they all have two things in common: they influence the rates of births and deaths, and the effect increases as population size increases.

When the density of a population is low (few individuals in a given area), resources are not limiting. There are plenty of resources for everyone. More individuals can give birth, and fewer individuals will die. Overall, the population will grow in size and become denser.

When the density of a population is high (many individuals in a given area), resources are more limited for each individual. Because of this, more individuals will die, fewer individuals will be born, and the population size will decrease and become less dense.

Examples

Density-dependent factors are most often biotic variables. Biotic variables are all of the living organisms within an ecosystem. Abiotic variables (all of the non-living things in an ecosystem, such as weather, natural disasters, and sunlight) usually affect a population in the same way, regardless of the density.

Disease is one of the most notable examples of density dependence. Disease spreads quickly through dense populations because individuals live in closer proximity to each other.

Parasitism is similar to disease in that parasites also spread faster through populations that have individuals living very close to each other. Parasites are organisms that live off of other organisms, known as hosts. However, since parasites harm their host, they will not do well in an environment that decreases the host population too much.

Predation is another way that population sizes are controlled. A predator will do well in an environment that has a lot of prey available. As the predator eats more prey, the prey population size decreases. This in turn decreases predation, as well as the population size of the predator. As predation decreases, the prey population size increases, and once again provides more prey for the predator.

Density dependence - predation

Competition is another density-dependent factor. There are two types of competition: interspecific competition - competition between two or more different species, and intraspecific competition - competition between members of the same species. In both cases, species are competing for the same food, shelter, water, and other important resources.

Summary

Populations cannot increase their size indefinitely because there simply aren't enough resources for everyone. As the number of individuals increases, resources become less available and after a while the population will begin to get smaller. Though this seems harmful at first, it will eventually allow the population to grow again, and the regulating cycle will continue.

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