In ecology, local abundance is the relative representation of a species in a particular ecosystem, usually measured as the number of individuals found per sample. The ratio of the abundance of one species to that of other species in the same ecosystem is referred to as relative species abundance. Both indicators are relevant for computing biodiversity.

A variety of sampling methods are used to measure abundance. For larger animals these include spotlight counts, track counts, roadkill counts, and observations at monitoring stations. In many plant communities, abundance is measured by plant cover—the relative area occupied by different species in a small plot. Abundance can also be measured by identifying and counting every individual of every species in a given sector, although species distributions are often skewed so that a few species account for most individuals.

Relative species abundance is calculated by dividing the number of individuals of one species by the total number of individuals of all species.

These measures are part of community ecology. Understanding patterns within a community is easier when species richness is low, but most communities are species-rich. Measuring species abundance helps reveal how species are distributed within an ecosystem. For example, salt marshes receive an influx of seawater, so only a few species adapted to both salt and fresh water tend to be abundant. Conversely, in land-locked wetlands species abundance is more evenly distributed among those that live there.

In most ecosystems where abundance has been measured, a small number of species are abundant while many are rare. Abundant species are often generalists, whereas many rare species are specialists. A species that occurs at high density in multiple localities is likely to be relatively abundant across the region; high local abundance therefore tends to be linked to broad regional distribution. Species with high abundance also tend to produce more offspring, which increases the chance of colonizing new areas, creating a positive feedback loop that produces a few widespread "core" species and many restricted, scarce "satellite" species.

Species abundance distribution (SAD) describes how common or rare species are within an ecosystem and allows researchers to assess how species are distributed. SAD is one of the most basic measurements in ecology and is used frequently; many methods for measurement and analysis have developed. One example is semi-quantitative abundance ratings, which involve estimating abundance within a specified area (quadrat). Two commonly used scales are A.C.F.O.R. and D.A.F.O.R.

The A.C.F.O.R. scale:
A — Abundant
C — Common
F — Frequent
O — Occasional
R — Rare

The D.A.F.O.R. scale:
D — Dominant
A — Abundant
F — Frequent
O — Occasional
R — Rare

These methods are useful for obtaining rough estimates of species abundance, but they are not exact or fully objective. If a more quantitative method is available, it should be used to produce more reliable and measurable data. See also: Abundance estimation; Cover-abundance; Living Planet Index; Occupancy–abundance relationship; Plant cover; Range (biology); Relative abundance distribution; Species richness; References; External links; "Abundance in ecology" (article, with works cited); Ecology terminology; Biodiversity.