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.

Examples in context

Example 1. Daintree quadrat survey for IA1 data test. A typical IA1 data test on biodiversity gives students raw counts from a 10 m squared quadrat survey in the Daintree: 30 Argyrodendron, 20 Castanospermum, 10 Toona, 5 Macaranga, 5 Endiandra (total 70 individuals across 5 species). Students calculate species richness ($S = 5$), proportional abundances $p_i$, and Simpson's index $D = 1 - \sum p_i^2 = 1 - (0.184 + 0.082 + 0.020 + 0.005 + 0.005) = 0.704$. A nearby cleared site shows $S = 3$ and $D = 0.42$. The IA1 then asks for limitations: small quadrat size, single sample, time of day, and exclusion of cryptic species, judged under the ISMG knowledge and analysis criterion.

Example 2. Moreton Bay seagrass evenness monitoring. Queensland Parks staff monitor Moreton Bay seagrass meadows using line transects measuring per-cent cover of Zostera, Halophila, Halodule, Cymodocea and Syringodium. In 2010 the cover percentages were 40, 25, 15, 10 and 10, giving high evenness and Simpson's $D \approx 0.74$. After the 2011 floods, Zostera declined to 5 percent while opportunistic Halophila surged to 70 percent: richness unchanged at 5 species, but evenness collapsed and $D$ fell to 0.46. The case shows that two measures of biodiversity can move independently and that Simpson's index detects evenness changes that simple richness misses.

Try this

Q1. Define species richness and species evenness and explain how Simpson's diversity index incorporates both. [3 marks]

• Cue. Richness counts species; evenness compares abundances; Simpson's $D = 1 - \sum p_i^2$ rises when both rise.

Q2. Two reef sites both contain 6 species. Site A has counts 50, 50, 50, 50, 50, 50; Site B has counts 250, 10, 10, 10, 10, 10. Calculate Simpson's index for each and interpret. [3 marks]

• Cue. A: $D = 1 - 6 \times (1/6)^2 = 0.833$. B: $D = 1 - (0.694 + 5 \times 0.0011) = 0.300$. A more diverse despite equal richness.

Q3. Refer to a wetland survey for an IA1 report. (a) Identify two sampling limitations. (b) Predict the effect of doubling sample size on richness and Simpson's index. (c) Justify which index is more sensitive to rare species: Simpson's or species richness. [2+2+2 marks]

• Cue. (a) Sample area, observer bias, season. (b) Richness rises; Simpson's stabilises. (c) Richness more sensitive; Simpson weights common species.