Population density

Population density is the number of individuals in a population per unit area or volume. It describes how crowded a population is within a specified space.

Density estimation: The set of methods used to estimate how many individuals occupy a defined area or volume. Sampling and extrapolation turn counts into estimates of individuals per unit space.
Mark–recapture: A method estimating population size by marking individuals, releasing them, and measuring their later recapture rate. Recapture proportions can estimate abundance, which becomes density when space is known.
Population: A group of individuals of the same species living in a defined area at a given time. Density is calculated for a population whose membership and spatial boundary are specified.
Absolute density: The number of individuals per unit area or volume, including estimates of individuals not directly observed. It contrasts with counts restricted to individuals detected during a survey.
Quadrat sampling: A sampling method that counts organisms in small, systematically placed plots. Plot counts provide a direct estimate of density for stationary or slow-moving organisms.
Line transect: A survey method that records detected organisms along a measured line or route. Detection distances help estimate the density of wildlife across a larger habitat.
Abundance: The total number of individuals in a population or other defined group. Abundance is the numerator; density also requires a measure of space.
Relative abundance: A measure comparing the abundance of a species across places, times, or samples without estimating absolute numbers. It can track change without providing individuals per unit space.
Distance sampling: A family of methods estimating animal abundance from distances between observers and detected individuals. It estimates density while accounting for animals missed farther from a survey route.
Capture–recapture: A family of abundance-estimation methods based on repeated captures and individual identification. It is broader than mark–recapture and includes designs suited to changing populations.

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