Species density refers to the number of individuals of a species in an area. It is measured in individuals per unit area. It is a useful value that can be used to determine the health of an ecosystem. There are two different ways to measure and calculate species density and the correct method depends upon the type of species being studied. The density of a sedentary species is normally calculated using the quadrant method, while the density of mobile populations is normally calculated using the mark and recapture method.

## Sedentary populations: Quadrat method

Divide the search area into equal sized squares. For example, if your area is 100 square metres you could divide it into 100 sections of 1 square metre each.

Place a square (called a quadrat) around one section of your search area. In this example, the quadrat would be 1 square metre.

Count all of the individuals found within the quadrat.

Move the quadrat and count the number of species again. Repeat this several times. You are not expected to cover the entire sample area, but a greater number of quadrats will give you a more accurate final answer.

Calculate the average number per quadrat by dividing the total number of organisms by the number of quadrats counted.

Calculate the population density using the number of organisms per quadrat and the quadrat size. For example, 10 organisms per 1 square metre quadrat would give a population density of 10 organisms/square meter.

Calculate the total population by multiplying population density by the total sample area size. For example, a population density of 10 organisms/square metre in an sample area of 100 square meters gives a total population size of 1000 organisms.

Capture a number of the species being studied.

Mark the captured animals in a way that will allow you to identify them later, and then release them back into the environment.

Capture a second sample of the population. The second sample should consist of a larger number of animals than the first capture. Determine how many of individuals from the second capture were already marked during the first capture.

Calculate the population size using the following formula: number of individuals (N) = number marked in first capture (m) x number in second capture (n) / number in second capture already marked (r).

Determine the species density by dividing the total population by the sample area.