# Density-Dependent Factors: Examples & Definition

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Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, 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.

## Density-Dependent Factors Defined

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 of Density-Dependent Factors

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. Diseases spread quickly through dense populations because individuals live in closer proximity to each other. Parasitism is similar to disease in that it also spreads faster through dense populations. 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.

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