Biology · Ecology

Species Richness & Evenness

How ecologists compare the variety and balance of species in a community

Species richness and species evenness describe different aspects of species diversity. Richness asks how many different species are present, while evenness asks how evenly individuals are distributed among those species.

Species Richness & Evenness: The Big Picture
Community ACommunity BSame richnessSame richness4 species4 speciesLow evenness: one species dominatesHigh evenness: abundances are similar

Follow the arrows to see how matter or diversity changes within an ecological system.

In This Article

Species Richness & Evenness

  1. What Are Species Richness and Species Evenness?
  2. Species Richness
  3. Species Evenness
  4. Richness vs. Evenness
  5. How to Recognize Richness and Evenness in Data
  6. How Richness and Evenness Contribute to Species Diversity
  7. Common Misconceptions
  8. If You See This on a Test, Think This
  9. Quick Practice
  10. Key Takeaways
The Big Picture

What Are Species Richness and Species Evenness?

Species richness and species evenness are two related ways ecologists describe species diversity within a community. Richness measures the number of different species present. Evenness describes how evenly individuals are distributed among those species.

These concepts are easy to confuse because both describe a community's species composition. The key distinction is simple: richness is about the number of species; evenness is about their relative abundances.

The Core Idea

Ask “How many?” for richness. Ask “How balanced?” for evenness.

Counting Species

Species Richness

Species richness is the number of different species present in a defined area or community. If a forest contains oak trees, maples, pines, ferns, and mushrooms, then its species richness for that sample is based on how many distinct species were identified.

Richness does not tell you how many individuals belong to each species. A community containing four species has the same species richness whether one species contains 97% of the individuals or all four species are equally abundant.

Richness = 4

Four species present

The community contains four different species.

Species A + B + C + D
What richness ignores

Relative abundance

Richness alone does not tell you how many individuals belong to each species.

4 species ≠ equal abundances
Relative Abundance

Species Evenness

Species evenness describes how similar the abundances of the different species are. A community has relatively high evenness when no single species overwhelmingly dominates and the species have similar numbers of individuals.

Evenness is lower when one or a few species account for most of the individuals. In a very uneven community, several species may be present but only a small fraction of the organisms belong to most of those species.

Same Richness, Different Evenness
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Both communities contain four species, so richness is the same. Community B is more even because the species have more similar abundances.

Put Them Together

Richness vs. Evenness

Species Richness

Counts species. It increases when a new species is added to the community.

Question: “How many species are there?”

Species Evenness

Compares abundance. It increases when species become more similar in their relative abundances.

Question: “How evenly are they represented?”

A community can therefore have high richness and low evenness, low richness and high evenness, or many combinations in between. The two measurements provide different information about community structure.

Reading Data

How to Recognize Richness and Evenness in Data

When you are given a table, graph, or species-abundance diagram, first count the distinct species. That gives you the basic idea of richness. Then compare the number of individuals represented by each species to evaluate evenness.

Species A = 25, B = 25, C = 25, D = 25
Richness = 4; high evenness.
Species A = 90, B = 4, C = 3, D = 3
Richness = 4; low evenness.
Species A = 40, B = 35, C = 25
Richness = 3; relatively high evenness.
Species A = 70, B = 30
Richness = 2; lower evenness than a 50/50 community.
A Broader Measure

How Richness and Evenness Contribute to Species Diversity

Species diversity considers both richness and evenness. In many ecological contexts, a community with more species and a more balanced distribution of individuals is described as having greater species diversity.

More advanced biology courses may use mathematical diversity indices, such as the Shannon or Simpson indices, to combine information about species richness and relative abundance. The important conceptual foundation is still the same: how many species are present and how evenly are individuals distributed?

Watch Out

Common Misconceptions

“Richness measures how many individuals are present.”
Richness measures how many different species are present, not the total number of individuals.
“If two communities have the same richness, they have the same diversity.”
They can have very different evenness and therefore different overall species diversity.
“Evenness means every species must have exactly the same number of individuals.”
Higher evenness means abundances are more similar; exact equality is not required.
“A community with one dominant species has low richness.”
Dominance affects evenness. The community could still have high richness if many species are present.
Test-Day Connections

If You See This on a Test, Think This

“Number of different species in the sample.”
Species richness.
“Relative abundance of each species.”
Species evenness.
“Same number of species, but one community is dominated by one species.”
The dominated community has lower evenness.
“A community has many species with similar abundances.”
High richness and high evenness.
Check Your Understanding

Quick Practice

1. What does species richness measure?
Answer: The number of different species present.
2. What does species evenness measure?
Answer: How evenly individuals are distributed among the species present.
3. Two communities each contain five species. Can one have greater evenness?
Answer: Yes. The community whose species have more similar abundances has greater evenness.
4. A community contains 100 individuals: 90 are species A and 10 are species B. What is its richness?
Answer: Its species richness is 2 because two species are present.
5. Why is richness alone not enough to describe species diversity?
Answer: Richness does not describe how individuals are distributed among the species, so evenness provides additional information.
Remember These

Key Takeaways

  • Species richness is the number of different species present.
  • Species evenness describes how evenly individuals are distributed among those species.
  • Two communities can have the same richness but different evenness.
  • A dominant species can lower evenness without changing richness.
  • Species diversity considers both the variety of species and their relative abundances.
  • When reading data, count species first, then compare their abundances.
Continue Learning
Biodiversity →Community Interactions →Ecosystems →Habitat Fragmentation →Food Webs →Trophic Levels →
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