Dominance Calculators

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Dominance in ecology measures the degree to which one or a few species account for most of the individuals or biomass in a community. High dominance indicates low diversity, with one species greatly outnumbering others. Dominance indices such as Simpson's D and the Berger-Parker index quantify this pattern and are used to compare community structure across sites and over time. Dominance shifts — particularly increases in single-species dominance — often signal environmental stress, habitat degradation, nutrient enrichment (eutrophication), or invasive species takeover.

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Simpson's Dominance Index

Simpson's D = Σ[nᵢ(nᵢ−1) / N(N−1)], where nᵢ is the count of species i and N is the total count. D ranges from 0 (no dominance) to 1 (complete dominance by one species). The complementary diversity index 1−D (Simpson's diversity index) or 1/D (reciprocal Simpson) are more commonly used as positive diversity metrics.

Berger-Parker Dominance Index

The Berger-Parker index is simply d = Nmax / N — the proportion of individuals in the sample belonging to the most abundant species. It is the simplest dominance measure and ranges from 1/S (equal abundance) to 1.0 (complete dominance). A high Berger-Parker index signals that one species monopolizes the community.

Ecological Interpretation

Dominance patterns reflect competition outcomes, resource availability, and disturbance history. In physically stressed environments — polluted water, nutrient-enriched soils — tolerant species often dominate while sensitive species disappear, reducing both evenness and richness. The Intermediate Disturbance Hypothesis predicts that moderate disturbance prevents monopolization by the strongest competitor, maximizing diversity.

Dominance vs. Diversity

Dominance and diversity are inversely related. Communities with high Simpson's D or high Berger-Parker index tend to have low Shannon diversity (H'). Both metrics are needed to fully characterize community structure: richness tells you how many species are present; dominance tells you how evenly they are distributed.

Glossary

Simpson's Dominance Index (D)
D = Σ[nᵢ(nᵢ−1)/N(N−1)]; the probability that two randomly chosen individuals belong to the same species; values near 1 indicate high dominance.
Berger-Parker Index
d = N_max/N; the simplest dominance metric, expressing the proportion of the most abundant species in a community.
Evenness
A component of diversity describing how equally individuals are distributed among species; low evenness means a few species dominate while others are rare.

Frequently Asked Questions

Simpson's dominance index D = Σ[nᵢ(nᵢ−1) / N(N−1)], where nᵢ is the count of the ith species and N is the total count. D equals the probability that two randomly selected individuals belong to the same species. Values close to 1 indicate high dominance (one species is very abundant); values close to 0 indicate high diversity with even distribution among species.

The Berger-Parker index (d) = N_max / N, where N_max is the count of the most abundant species and N is the total count. It is the simplest dominance metric — essentially the proportion of the community made up by the single most abundant species. Values near 1 indicate extreme dominance; values near 1/S (where S is species richness) indicate even distribution.

High dominance typically occurs when one species has a strong competitive advantage over others. Common drivers include environmental stress (pollution, nutrient enrichment, salinity), habitat homogenization, invasive species outcompeting natives, or removal of predators that would normally keep dominant species in check (trophic cascade). After major disturbances, early successional species often dominate before diversity recovers.

Dominance and diversity are inversely related. A community where one species accounts for 90% of individuals will have low Shannon diversity despite potentially having many rare species. Evenness — how equally individuals are distributed among species — bridges dominance and diversity. The Simpson diversity index (1−D) and Shannon's H' both incorporate evenness and are more informative than species richness alone.