Species richness and biodiversity are closely related concepts, but they do not mean the same thing. Species richness is simply the number of different species in a particular area or community. Biodiversity is a broader measure of biological variety that can include species richness, the relative abundance of species, genetic differences within species, and the variety of ecosystems.
The distinction matters because two ecosystems can contain the same number of species while differing greatly in how those species are distributed or how much biological variety they contain.
What is species richness?
Species richness is the number of species present in a defined area, habitat, or ecological community.
For example, imagine two ponds. Pond A contains 20 species of organisms, while Pond B contains 35. If the areas being compared are defined in the same way, Pond B has greater species richness.
Species richness does not depend on how many individual organisms belong to each species. It counts species, not individuals.
Suppose a forest contains 10 species of birds. If one species accounts for most of the birds and the other nine are rare, the forest still has a species richness of 10. If the numbers of individuals change but all 10 species remain present, its species richness has not changed.
This makes species richness relatively straightforward to understand and measure, but it also means that richness captures only one part of biological diversity.
What is biodiversity?
Biodiversity is the variety of life within a particular place, ecosystem, region, or across Earth as a whole.
The term can be considered at several levels. At the most familiar level, biodiversity refers to the variety of species. But biological diversity also exists within species and among ecosystems.
Three commonly recognized levels are:
- Genetic diversity: variation in genes among individuals and populations of the same species.
- Species diversity: variation among different species, including both the number of species and their relative abundances.
- Ecosystem diversity: variation among different ecosystems, habitats, and ecological communities.
Because biodiversity encompasses more than simply counting species, species richness is best understood as one component of biodiversity rather than a synonym for it.
Species richness and species diversity are not identical
There is also an important distinction between species richness and species diversity.
Species richness asks a simple question: How many species are there?
Species diversity considers both the number of species and how evenly individuals are distributed among those species. Measures of species diversity therefore contain information that species richness alone does not.
Consider two communities, each containing four species and 100 individual organisms.
In the first community, the 100 individuals are distributed fairly evenly among the four species. In the second, one species accounts for almost all of the individuals while the other three are rare.
Both communities have a species richness of four. But their species diversity differs because their distributions of individuals among species are different.
This property is often described as species evenness. Higher evenness means that species have more similar abundances; lower evenness means that a few species dominate while others are uncommon.
Why the distinction matters
Counting species can give a useful first picture of an ecosystem, but it can hide important ecological differences.
Imagine two grasslands that each contain 50 plant species. Their species richness is the same. Yet one grassland might have a relatively even mixture of those species, while the other might be dominated by just a few species.
The two grasslands therefore have the same richness but different patterns of species abundance.
The same principle applies to biodiversity more broadly. An ecosystem can have many species but relatively little genetic variation within some of those species, or it can contain substantial genetic and ecosystem-level variation that a simple species count cannot capture.
For this reason, ecologists choose biodiversity measures according to the question they are trying to answer.
How species richness is measured
Species richness is usually determined by identifying the different species found within a defined sampling area or community.
The boundaries of the comparison matter. A small patch of forest, an entire watershed, and a continent can all have different species counts because they encompass different amounts and types of habitat.
Sampling effort also matters. More intensive surveys are generally more likely to detect rare or difficult-to-observe species. Two locations can therefore appear to have different richness partly because they were sampled differently.
Ecologists may use standardized sampling methods or statistical approaches to make comparisons more meaningful.
A place can have high richness without having high evenness
One of the most important limitations of species richness is that it gives every species equal weight.
If a habitat contains 30 species, each species contributes one to the richness count whether it contains thousands of individuals or only a single detected individual.
That can be useful when the central question is simply how many species occur in an area. But it provides little information about which species dominate the community.
Measures that incorporate both richness and abundance can reveal patterns that a species count misses.
Biodiversity extends beyond species counts
Two regions can contain the same number of species yet differ in biodiversity for reasons that species richness does not capture.
For example, populations of the same species can contain different genetic variants. Genetic diversity can affect a population’s ability to respond to environmental change, disease, or other pressures.
At an even broader scale, one region may contain forests, wetlands, grasslands, rivers, and other distinct ecosystems, while another region with the same number of species may contain far less ecosystem variety.
Thus, asking whether one place is “more biodiverse” than another requires specifying what aspect of biodiversity is being compared.
Species richness is useful, but it is only part of the picture
Species richness is valuable because it provides a clear and intuitive measure: the number of species in a defined community or area. It is particularly useful for comparing how many species occur in different habitats or how the number of species changes over space or time.
But richness alone cannot describe the full structure of a biological community. It does not tell us how common each species is, how genetically varied populations are, or how many different types of ecosystems occur in a region.
The simplest way to keep the concepts straight is:
Species richness = how many species are present.
Biodiversity = the broader variety of life, including diversity within species, among species, and across ecosystems.
Species richness is therefore an important measure of biodiversity, but a species count by itself is not a complete measure of biodiversity.
