Abundance Distribution Calculators

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A species abundance distribution (SAD) describes the pattern of relative abundance among species in a community — how many species are very rare, how many are moderately common, and how many are dominant. SADs are universal features of ecological communities: most communities have many rare species and few very abundant ones. Common SAD models include the log-normal distribution, geometric series, broken stick, and log series. Rank-abundance curves (Whittaker plots) display each species' relative abundance in rank order from most to least abundant and are widely used to visualize community structure and diversity.

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Rank-Abundance Curves (Whittaker Plots)

Plot species ranked by abundance (x-axis, rank 1 = most abundant) vs. log(relative abundance) or log(abundance) on y-axis. Shape reveals community structure:

  • Steep gradient, few species: Low diversity, high dominance — geometric or log series distribution
  • Gentle slope, many species: High diversity, high evenness — log-normal distribution
  • Nearly flat (S-shaped): Broken stick distribution — high evenness

Common SAD Models

Log-normal: Most common in nature; many rare species and few common; species abundances log-normally distributed; arises from multiplicative independent factors affecting each species. Preston's octave plot (log₂ scale of abundance classes) reveals the log-normal bell shape.

Geometric series: Each successive species takes a fixed proportion k of remaining resources; produces a straight rank-abundance line; characteristic of early successional or harsh environments with few species.

Broken stick: Resources randomly divided among species; high evenness; straight or slightly convex curve.

Measuring Abundance

Absolute abundance: count of individuals (e.g., CFU/mL, stems/m², individuals/ha). Relative abundance (RA): proportion of total individuals belonging to species i (RA_i = n_i/N). Relative abundance is used for diversity calculations (Shannon H', Simpson D).

Glossary

Species Abundance Distribution (SAD)
The frequency distribution of species with different abundances in a community; almost universally shows many rare and few common species; modeled by log-normal, log series, or geometric distributions.
Rank-Abundance Curve
A Whittaker plot of log(relative abundance) vs. species rank; shape reflects diversity and evenness; steep gradient = high dominance; gentle gradient = high evenness.
Relative Abundance
The proportion of total individuals belonging to a species: n_i/N; used in diversity calculations (Shannon H', Simpson D); dimensionless; indicates dominance relative to other species.

Frequently Asked Questions

A species abundance distribution (SAD) describes how individuals are distributed among species in a community — the frequency distribution of species with different numbers of individuals. SADs almost always show a characteristic pattern: many rare species (with very few individuals) and a few very common (dominant) species. This pattern is consistent across ecosystems — tropical forests, marine plankton communities, bird assemblages — suggesting universal ecological processes generate it. SADs are summarized by rank-abundance curves (Whittaker plots) and fitted to theoretical distributions (log-normal, log series, geometric).

A rank-abundance curve (Whittaker plot) plots log(relative abundance) vs. species rank (1 = most abundant). Shape reflects community structure: steep, short curve = few species, highly uneven (one or few dominants) — typical of disturbed, early-successional, or harsh environments. Gradual long curve = many species, more even distribution — characteristic of mature, diverse, stable communities (tropical forests, grasslands with many coexisting grasses). Nearly flat curve = very even community — unusual in nature, expected from broken stick model. Changes in rank-abundance curve shape over time reveal succession, disturbance, or pollution effects.

The log-normal distribution is the most commonly observed SAD in nature. When species abundances are log-transformed (log₂ or log₁₀), they follow a bell-shaped normal distribution — a few very rare species, many species at intermediate abundances, and a few very common ones. Preston's canonical log-normal hypothesis formalized this and predicted the number of rare and common species. The log-normal arises theoretically when many independent multiplicative factors (resources, space, climate tolerance) influence each species' abundance — consistent with the complexity of ecological communities. In Preston's octave plots, the bell shape is visualized by grouping species into doubling (log₂) abundance classes.

Absolute abundance = the actual count of individuals per unit area or volume (e.g., 450 trees/ha; 2.3 × 10⁶ bacteria/mL). It tells you how many there are. Relative abundance = the proportion of total individuals belonging to a species (n_i/N × 100%). It tells you how dominant a species is relative to others. A species with absolute abundance of 1000 can have low relative abundance (2%) if the total community is 50,000 individuals. Relative abundance is used in diversity index calculations (Shannon H' = −Σpᵢ ln(pᵢ); Simpson D = Σpᵢ²). Both metrics are needed for complete community description.