Determine the biodiversity of an ecosystem using measures of species richness, species evenness and Simpson's diversity index, and explain the limitations of these measures

What this dot point is asking

QCAA wants you to quantify biodiversity using three measures (species richness, species evenness and Simpson's diversity index), apply them to data from a sample, and critique what they do and do not capture. Calculation questions appear most years; the limitations are reliable short response material.

The answer

Biodiversity in a sample is measured by counting species and weighting them by abundance. Three measures appear in the QCAA syllabus.

Species richness (S)

Species richness is the total number of different species in a defined area or sample.

How to calculate. Count the species. That is it.

Example. A quadrat contains 12 grass plants of species A, 4 of species B, and 1 of species C. S = 3.

Why it is limited. Richness ignores abundance. A community of 3 species with abundances (12, 4, 1) has the same richness as one with (5, 6, 6), but they feel ecologically very different to anything living in them.

Species evenness

Species evenness describes how evenly individuals are distributed among the species present.

Conceptually. A community is most even when all species are equally abundant. A community dominated by one species, with the others rare, has low evenness even if richness is high.

Why it is limited. Evenness on its own does not capture richness. A community with 2 species in equal abundance is perfectly even but not very biodiverse.

Simpson's diversity index (D)

Simpson's diversity index combines richness and evenness into a single number between 0 and 1.

Formula (QCAA form).

D = 1 - sum( n(n - 1) / N(N - 1) )

where n is the number of individuals of one species and N is the total number of individuals across all species.

Interpretation. D close to 1 means high diversity (a randomly selected pair of individuals is likely to belong to different species). D close to 0 means low diversity (most pairs belong to the same species, so one species dominates).

Worked calculation

A rockpool quadrat contains: limpets (n = 8), snails (n = 5), crabs (n = 2), barnacles (n = 5). Total N = 20.

Step 1. Compute n(n - 1) for each species.

• Limpets: 8 x 7 = 56
• Snails: 5 x 4 = 20
• Crabs: 2 x 1 = 2
• Barnacles: 5 x 4 = 20

Step 2. Sum: 56 + 20 + 2 + 20 = 98.

Step 3. N(N - 1) = 20 x 19 = 380.

Step 4. D = 1 - (98 / 380) = 1 - 0.258 = 0.742.

A second rockpool with the same N = 20 but abundances (16, 2, 1, 1) gives sum = 16 x 15 + 2 + 0 + 0 = 242, D = 1 - 242 / 380 = 0.363. Same richness (4), much lower evenness, much lower Simpson's index.

Sampling methods that feed these measures

To compute any of these, you first need a sample. QCAA expects you to know:

• Quadrats for sessile or slow-moving organisms (plants, barnacles, corals).
• Transects when abundance varies along a gradient (intertidal zone, altitudinal gradient up a hill).
• Capture, mark, release, recapture (Lincoln index, N = M x C / R) for mobile animals.
• Random sampling (random number generator over a grid) to avoid observer bias.
• Stratified sampling when the habitat has obvious sub-areas (e.g. canopy vs understorey) so each sub-area is represented in proportion.

A well-designed survey will state sample size, sampling technique, replicates and how randomisation was achieved.

Limitations students must mention

Limited to one level of biodiversity. Simpson's index quantifies species-level diversity in a single sample. It says nothing about genetic biodiversity within those species or about ecosystem-level diversity across the region.

Sensitive to sample size and effort. Rare species are easily missed. Larger and more numerous samples generally increase richness; index values from samples of different size are not directly comparable.

All species treated equally. A keystone species, an introduced weed and a common native are counted the same. An ecosystem dominated by an invasive species can score a high Simpson's value while being ecologically degraded.

No information about ecosystem function. Two communities with the same index can differ in productivity, nutrient cycling, pollination services and resilience.

Spatial and temporal snapshot. A single survey reflects one place at one time. Seasonal variation (flowering, migration, larval recruitment) can change the value substantially.

Common traps

Using N when you should use n. Inside the bracket, use the number of individuals of each species (n). Outside, use the total (N).

Forgetting the "1 minus" step. The raw quotient is the probability that two individuals belong to the same species. Subtract from 1 to get the diversity index. Some textbooks use D = sum( n(n - 1) / N(N - 1) ) directly, where higher values mean lower diversity. QCAA uses the "1 minus" form. Read the question carefully.

Comparing samples of different size. A 0.5 m by 0.5 m quadrat in a forest will record fewer species than a 5 m by 5 m quadrat in the same forest, and the indices are not directly comparable. Always standardise sample size.

Quoting "high biodiversity" without a number. Examiners want the index value, then an interpretation, then a comparison.

In one sentence

Species richness counts species, evenness describes how equally they are represented, and Simpson's diversity index (D = 1 - sum(n(n - 1) / N(N - 1))) combines both into a single value between 0 and 1, but the index reflects only species-level biodiversity in one sample and cannot speak to genetic variation, ecosystem function or sampling bias.