Abundances

Abundances describe the quantity of a component present in a system relative to a reference. They are commonly expressed as fractions, percentages, concentrations, or counts, and may refer to elements, isotopes, species, or other components. Abundance is distinct from distribution or diversity, focusing on how much of a given component exists rather than where it occurs or how many different components are present.

In chemistry and physics, elemental abundances quantify how much of each element is contained in a material. Common expressions include weight percent, atomic percent, or mole fraction. Isotopic abundances specify the relative frequencies of different isotopes of an element. In astronomy, chemical abundances are inferred from spectra and expressed as metallicity, or in solar logarithmic scales such as [X/H], which compares the abundance of element X to that in the Sun. Cosmic abundances indicate the primordial mix of hydrogen and helium from the Big Bang, with heavier elements called metals produced in stars and dispersed by supernovae. Solar system abundances, meteorites, and interstellar gas provide reference values.

In biology and ecology, abundance refers to the number of individuals of a species in a defined area or volume, or to biomass. It is often reported as density (e.g., individuals per square meter), true abundance, or relative abundance. Estimation relies on sampling methods and can be influenced by detectability. Abundance is a key component of biodiversity measures, interacting with richness and evenness to describe a community.