How can species richness and evenness describe a community differently?
Quick answer
Species richness is the number of different species in a community; species evenness describes how individuals are distributed among those species. Two communities can have the same richness but different evenness: each might contain five species, while one has similar numbers of each and the other is dominated by a single species. Richness tells you how many species are present; evenness adds information about their relative abundance. [S1]
The question, the details, the next step.
I keep mixing up species richness and species evenness. What does each one tell me?
Think of them as two different questions about a community. Richness asks, “How many species are there?” Count the distinct species, without considering how many individuals belong to each. Evenness asks, “How are the individuals divided among those species?” A community is more even when the species have more similar abundances; it is less even when some species are much more abundant than others. These are separate dimensions of biodiversity, so richness alone does not describe how balanced the counts are. [S1]
For a simple comparison, imagine counting individuals in two sample plots. The numbers below are hypothetical and are just for illustrating the distinction.
Can you show two communities with the same richness but different evenness?
Suppose each plot has a total of 100 counted individuals and contains these five hypothetical species:
- Plot A: Species A: 20; B: 20; C: 20; D: 20; E: 20.
- Plot B: Species A: 84; B: 4; C: 4; D: 4; E: 4.
Both plots have richness of five because each contains five distinct species. Their evenness differs: Plot A has equal counts, while Plot B has many more individuals of Species A than of the other four. So the plots match in richness, but Plot A is more even. The particular counts are made-up examples, not observations about real communities.
How should I compare them without accidentally treating the species count as the whole story?
Write down both results separately. First list each distinct species to establish richness. Then list the number of individuals counted for each species and compare how those counts are distributed. In the example, the richness comparison is simply 5 versus 5. The evenness comparison is 20, 20, 20, 20, 20 versus 84, 4, 4, 4, 4: the first set is balanced, and the second is strongly dominated by one species.
Keep the scope of the comparison clear: these counts describe the samples you chose to compare. The example illustrates how richness and evenness can differ; it does not establish how either plot would look at another time or under a different sampling approach.
What’s a short way to explain the distinction in a class note?
You could write: “Species richness is the number of different species present. Species evenness describes how similarly individuals are distributed among those species. Two communities can have equal richness but different evenness: a community with five species at 20 individuals each is more even than one with counts of 84, 4, 4, 4, and 4.”
That wording gives the definitions and a paired example. It does not claim that one community has more species, because both examples have five.
A practical checklist
Count the distinct species to determine richness.
Record the number of individuals for each species separately.
Compare the abundance distribution, not just the number of species.
When comparing samples, state which counts belong to which community.
Label illustrative counts as hypothetical rather than observed data.
A few more useful answers
Can two communities have the same species richness but different evenness?
Yes. They can contain the same number of distinct species while having different distributions of individuals among those species. For example, five species with equal counts and five species dominated by one have equal richness but different evenness. [S1]
Does richness tell me which species is most common?
No. Richness counts distinct species; it does not describe how many individuals belong to each one. To compare abundance distribution, look at the counts by species as well.
What is the quickest way to remember the difference?
Richness is the number of species; evenness is how evenly individuals are distributed among them. Keep both in view when describing a community.
Sources
References used for this guide.
- [S1] 45.6 Community Ecology — Biology 2e | OpenStax openstax.org · Checked