Relative Abundance Calculators

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Relative abundance is the proportion of one species (or taxon) relative to the total number of individuals in a community sample: pᵢ = nᵢ / N, where nᵢ = count of species i and N = total individuals. Relative abundance is the fundamental input for all diversity indices (Shannon H', Simpson D, Bray-Curtis dissimilarity). Species abundance distributions (SADs) describe how relative abundances are distributed among species in a community — most communities follow a log-normal or log-series distribution, with a few dominant species and many rare ones. Relative abundance is used in community ecology, metagenomics (16S rRNA sequencing), and conservation to characterize and compare communities.

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Relative Abundance Formula

pᵢ = nᵢ / N

nᵢ = count of species i; N = Σnᵢ (total individuals); Σpᵢ = 1. Example: bird survey: robins 45; sparrows 30; finches 15; crows 10; N = 100. p_robin = 0.45; p_sparrow = 0.30; p_finch = 0.15; p_crow = 0.10.

Relative Abundance in Diversity Indices

Shannon H' = −Σpᵢ ln(pᵢ) = −(0.45×ln0.45 + 0.30×ln0.30 + 0.15×ln0.15 + 0.10×ln0.10) = 1.27. Simpson D = Σpᵢ² = 0.45² + 0.30² + 0.15² + 0.10² = 0.203 + 0.090 + 0.023 + 0.010 = 0.326. 1/D = 3.07 (effective species).

Species Abundance Distributions

Most natural communities: few dominant species + many rare species → right-skewed (log-normal or log-series). Rank-abundance plots (Whittaker plots): species ranked by abundance on x-axis; ln(abundance) on y-axis. Steep slope = high dominance (low evenness). Flat slope = even distribution (high evenness).

Metagenomics Application

16S rRNA amplicon sequencing: relative abundance (%) of each OTU/ASV. Compositional data: relative, not absolute; analyses require special methods (ALR, CLR transformation; phyloseq, vegan R packages).

Glossary

Relative Abundance (pᵢ)
pᵢ = nᵢ/N; proportion of species i in a sample; Σpᵢ = 1; input for Shannon H', Simpson D, and Bray-Curtis; compositional in metagenomics (relative, not absolute counts).
Species Abundance Distribution (SAD)
The pattern of relative abundances across species in a community; most communities are log-normal or log-series — few dominant, many rare; visualized as rank-abundance (Whittaker) plot.
Rank-Abundance Plot
Species ranked by decreasing abundance (x-axis) vs. ln(abundance) (y-axis); steep slope = high dominance; flat slope = high evenness; used to compare community structure without calculating indices.

Frequently Asked Questions

Relative abundance (pᵢ) = proportion of species i among all individuals: pᵢ = nᵢ/N. nᵢ = count of individuals of species i; N = total count of all individuals. Σpᵢ = 1 (proportions sum to 1). Example: 50 oak, 30 maple, 15 birch, 5 pine (N = 100): p_oak = 0.50; p_maple = 0.30; p_birch = 0.15; p_pine = 0.05. In metagenomics (16S rRNA sequencing): relative abundance = reads assigned to OTU/ASV i / total reads × 100%. Important: relative abundance in metagenomics is compositional data — if one taxon increases, others mathematically decrease even if unchanged.

All major diversity indices use relative abundances as input: Shannon H' = −Σpᵢ × ln(pᵢ). Each species contributes −pᵢ × ln(pᵢ) — maximized for intermediate abundances. If one species has pᵢ = 0.99 and others are very rare: H' will be very low. Simpson D = Σpᵢ². Dominated by the most abundant species (pᵢ² amplifies large values). 1−D: probability two randomly chosen individuals are different species. Bray-Curtis dissimilarity = Σ|aᵢ − bᵢ| / (Σaᵢ + Σbᵢ): uses relative abundances between two communities. Without relative abundances, diversity indices cannot be calculated — they require the proportional representation of each species.

A rank-abundance plot displays species' relative abundances in decreasing order. X-axis: species ranked from most to least abundant. Y-axis: ln(relative abundance) or log₁₀(relative abundance). Shape reveals community structure: Steep slope: high dominance — one or few species dominate; low evenness. Gentle slope: even distribution — many species with similar abundances; high evenness. Shape types: geometric series (very steep; early succession, harsh environments). Log-normal (humped then declining; typical of many mature communities). Broken-stick (flat; highly even; rare in nature). Use: visually compare diversity and evenness between communities without computing indices; identify dominant and rare species; visualize the SAD (species abundance distribution).

Absolute abundance: actual count or density of organisms (individuals/m², CFU/mL, reads/sample). Relative abundance: proportion of a taxon among all organisms (pᵢ = nᵢ/N). Why the difference matters: In metagenomics: DNA sequencing gives reads per sample; if one taxon increases, others' relative abundance decreases even if their absolute numbers are unchanged (the 'compositional' problem). Example: if bacteria A increases 10-fold but you only measure relative abundance: bacteria A appears to increase, but all other bacteria appear to decrease — even if their absolute numbers haven't changed. Solution: pair relative abundance data with total microbial load measurements (qPCR of 16S); use compositional-aware statistical methods (ALR, CLR transformations, ANCOM-BC for differential abundance).