Abundance is the number of individuals of a species in a given area. Ecologists measure abundance to compare habitats, monitor changes over time and assess the effect of human activity.It is rarely possible to count every individual, so abundance is estimated from samples, which must be taken at random to avoid bias. Common measures include:
- Population density: number of individuals per unit area, e.g. per m2. Best for organisms that are easy to count separately.
- Frequency: the percentage of sampling squares (quadrats) in which the species occurs. Quick, but does not tell you how many there are.
- Percentage cover: the percentage of the ground covered by a species. Used for plants such as grasses and mosses, where individuals are hard to tell apart.
- For mobile animals, mark-release-recapture is used instead.Worked example: ten 0.25 m2 quadrats contain a total of 30 daisies. The total area sampled is 10 × 0.25 = 2.5 m2.density = 30 ÷ 2.5 = 12 daisies per m2 If daisies appeared in 6 of the 10 quadrats, their frequency is 6 ÷ 10 × 100 = 60%.Taking a large number of samples improves reliability, and the mean of many samples gives a better estimate of the true abundance.
- Population density: number of individuals per unit area, e.g. per m2. Best for organisms that are easy to count separately.
- Frequency: the percentage of sampling squares (quadrats) in which the species occurs. Quick, but does not tell you how many there are.
- Percentage cover: the percentage of the ground covered by a species. Used for plants such as grasses and mosses, where individuals are hard to tell apart.
- For mobile animals, mark-release-recapture is used instead.Worked example: ten 0.25 m2 quadrats contain a total of 30 daisies. The total area sampled is 10 × 0.25 = 2.5 m2.