4.2.1(c)-(d) - Species richness, evenness and Simpson's Index

Richness And Evenness

Species biodiversity depends on two linked ideas: how many different species are present and how evenly the individuals are shared between those species. These are called species richness and species evenness.

Richness gives one part of species biodiversity: the species list. The second part asks whether the individuals are spread evenly across that list.

To measure species richness, collect a representative sample and count how many different species are recorded. If a quadrat survey records daisy, clover, buttercup, plantain and ryegrass, the species richness for that sample is 5.

To measure species evenness, do not just count species names. Count the number of individuals of each species, or another valid abundance measure if individuals cannot be separated easily, then compare the relative abundances. A habitat with 10 individuals of each of five species has high evenness. A habitat with 46 individuals of one species and only one or two individuals of the other species has low evenness, even if the richness is still 5.

[DIAGRAM: richness_evenness_simpson_comparison: Lesson 081: Species richness, evenness and Simpson's Index - diagram 01; asset_slug: 081_m04_2_1_species_richness_evenness_and_simpsons_index__diagram_01; recommended_method: drawn_biology; description: 16:9 NovaLearn-style deterministic comparison of two habitat samples with the same five-species richness but different abundance evenness, showing simple bar patterns and Simpson's Index values high versus low.]

The diagram shows the trap: the two habitats have the same richness, but they do not have the same species biodiversity. The more even habitat is more diverse because a randomly encountered individual is less likely to belong to one dominant species.

Richness and evenness describe different features of the same sample.

Preparing Count Data

Simpson's Index uses the abundance of each species compared with the total abundance of all species in the sample. The two key symbols are:

The fraction $n / N$ is the relative abundance of one species. It tells you what proportion of the sample belongs to that species. If one species has $n = 18$ and the total sample has $N = 30$, then that species makes up $18 / 30 = 0.600$ of the sample.

Good Simpson's Index work begins with a tidy table:

The $n / N$ values should add to 1, allowing for rounding. This is a useful check because it means all the species counts have been included.

The sampling behind the table matters. Two habitats should be compared only if the sampling effort is comparable, for example the same number and size of quadrats, the same trap duration, or the same sweep-net effort. If one area is sampled more thoroughly, its richness and evenness may look different because of the method rather than the habitat.

This short check keeps the focus on the data preparation step before the full index is calculated.

Calculating Simpson's Index

Simpson's Index of Diversity, $D$, uses the relative abundance of each species to give one value for species biodiversity in a habitat.

The formula will be provided where needed in assessments, so the skill is using and interpreting it. The symbol $\sum$ means "add all the values". For each species, calculate $n / N$, square it, add the squared values, then subtract the total from 1.