Species Richness and Diversity Indices

The metrics used to count species and to summarize how diversity combines richness with the relative abundance of species.

Definition

Species richness is the number of species in a defined assemblage; diversity indices are summary statistics that combine richness with evenness — the equitability of species abundances — to express how diverse a community is in a single value.

Scope

Covers the quantification of diversity at the species level: raw richness counts, the problem of incomplete sampling and richness estimators, rarefaction, and the family of diversity indices that integrate richness and evenness. Includes the interpretation and limitations of common indices and the idea of Hill numbers as a unified framework. Excludes spatial diversity gradients and hotspot analysis, treated in sibling topics.

Core questions

• How is species richness estimated when sampling is incomplete?
• What do the Shannon and Simpson indices each emphasize?
• How does evenness change interpretation of diversity?
• How can diversity be compared fairly across samples of different size?

Key concepts

• Species richness
• Species evenness
• Shannon diversity index
• Simpson diversity index
• Rarefaction and richness estimators
• Hill numbers

Key theories

Richness-evenness decomposition

Diversity reflects both how many species are present and how evenly individuals are distributed among them; indices weight these components differently, so no single index captures all aspects of diversity.

Hill numbers (effective number of species)

Diversity indices can be unified as a family parameterized by sensitivity to rare species, expressed as the equivalent number of equally abundant species, allowing consistent comparison across indices.

Clinical relevance

Diversity indices are the standard currency for comparing sites before and after disturbance, ranking areas for protection, and detecting biodiversity decline. Because index choice and sampling effort strongly affect results, understanding their properties is essential for sound conservation inference and monitoring.

History

The Shannon index was adapted from information theory in the 1940s-1950s, and Simpson proposed his dominance-based measure in 1949. Concerns about sampling bias drove the development of rarefaction in the 1960s-1970s and, more recently, the unification of indices under the Hill-number framework.

Debates

Which diversity index should be reported?

Shannon weights rare species more heavily while Simpson emphasizes dominant ones; because they can rank communities differently, there is ongoing debate over standardization and the merits of reporting effective numbers of species instead.

Key figures

• Robert H. Whittaker
• Edward H. Simpson

Seminal works

• primack2014
• groom2006
• pimm2014

Frequently asked questions

Why not just count species?

Raw counts ignore abundance. Two forests with ten tree species each can differ greatly if one is dominated by a single species while the other has them in equal numbers. Diversity indices capture this difference; counts alone do not.

Why is rarefaction used?

Larger samples almost always contain more species, so comparing raw counts from samples of different size is misleading. Rarefaction standardizes comparisons to a common sampling effort.