Abundance is an ecological concept referring to the relative representation of a species in a particular ecosystem. It is usually measured as the large number of individuals found per sample. How species abundances are distributed within an ecosystem is referred to as relative species abundances.
Abundance is contrasted with, but typically correlates to, incidence, which is the frequency with which the species occurs at all in a sample. When high abundance is accompanied by low incidence, it is considered locally or sporadically abundant.
A variety of sampling methods are used to measure abundance. For larger animals, these may include spotlight counts, track counts and roadkill counts, as well as presence at monitoring stations. In many plant communities the abundances of plant species are measured by plant cover, i.e. the relative area covered by different plant species in a small plot.
Relative species abundance is calculated by dividing the number of species from one group by the total number of species from all groups.
ACFOR is an acronym for a simple, somewhat subjective scale used to describe species abundance within a given area. It is normally used within a sampling quadrat to indicate how many organisms there are in a particular habitat when it would not be practical to count them all. Instead, a smaller representative sample of the population is counted instead.
The ACFOR scale is as follows:
- A - The species observed is "Abundant" within the given area.
- C - The species observed is "Common" within the given area.
- F - The species observed is "Frequent" within the given area.
- O - The species observed is "Occasional" within the given area
- R - The species observed is "Rare" within the given area.
This method of sampling is simple and easy to implement, but can be subjective. Species frequency is the number of times a plant species is present in a given number of quadrats of a particular size or at a given number of sample points. Frequency is usually expressed as a percentage and sometimes called a Frequency Index. The concept of frequency refers to the uniformity of a species in its distribution over an area. No counting is involved just a record of species present. Each individual of the species present is recorded, is a more accurate and reliable method of sampling.
See also 
- Living Planet Index
- Occupancy-abundance relationship
- Range (biology)
- Relative abundance distribution
- Species richness
- Plant cover
- Bartelt, Gerald A.; Rolley, Robert E.; Vine, Lawrence E. (2001). Evaluation of abundance indices for striped skunks, common raccoons and Virginia opossums in southern Wisconsin (Research report (Wisconsin. Dept. of Natural Resources), Report 185). Madison, Wisconsin: Wisconsin Dept. of Natural Resources, Bureau of Integrated Science Services. Retrieved 2006-12-15.
- Wright, David Hamilton (July 1991). "Correlations Between Incidence and Abundance are Expected by Chance". Journal of Biogeography (Journal of Biogeography, Vol. 18, No. 4) 18 (4): 463–466. doi:10.2307/2845487. JSTOR 2845487.
- Damgaard, Christian (2009). "On the distribution of plant abundance data". Ecological Informatics 4 (2): 76–82. doi:10.1016/j.ecoinf.2009.02.002.
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- "Abundance in ecology" (article, with works cited)