QCE Biology - Unit 3 - Biodiversity and populations

Biodiversity Measures and Population Estimates | QCE Biology

Learn species richness, evenness, percentage cover, percentage frequency, Simpson's diversity index and the Lincoln index.

Updated 2026-05-18 - 7 min read

Biodiversity measures and population estimates is part of the way QCE Biology turns living systems into evidence students can describe, analyse and evaluate. The safest way to study it is to connect each term to a data pattern, a biological mechanism and a limitation.

Core explanation

Richness

Species richness is the number of different species present. It is easy to calculate, but it ignores whether one species dominates the community.

Evenness

Evenness describes how similar the species abundances are. A community with balanced abundances is usually more diverse than one where almost every individual belongs to one species.

Percentage cover and frequency

Percentage cover estimates how much area a species occupies. Percentage frequency records the proportion of samples where the species appears. Together, they help distinguish widespread sparse species from dense local patches.

Lincoln index

Capture-recapture estimates mobile populations by comparing marked individuals in a first sample with marked individuals recaptured in a second sample.

Choosing the right biodiversity measure

Different measures answer different questions. A strong response names the measure, calculates it correctly and explains what the number means biologically.

| Measure | Calculation or method | Best used for | Main weakness | | --- | --- | --- | --- | | Species richness | Count the number of species | Quick comparison of how many species occur at a site | Ignores abundance and evenness | | Relative abundance | Compare each species count with the total count | Seeing whether one species dominates | Depends on reliable counts | | Percentage frequency | (quadrats containing species / total quadrats) x 100 | Widespread plants or sessile organisms | Does not show how much space the species covers | | Percentage cover | Estimate area covered in each quadrat | Grasses, algae, corals or layered vegetation | Observer estimates can be subjective | | Simpson's diversity index | Use species counts to combine richness and evenness | Comparing overall diversity between communities | Can hide which species changed | | Lincoln index | N = (marked first sample x total second sample) / marked recaptured | Mobile animal populations | Assumes marking does not change survival or recapture chance |

Percentage frequency and percentage cover can tell different stories. A grass species might appear in 90 percent of quadrats but cover only 8 percent of the area in each quadrat, meaning it is widespread but sparse. Another plant might appear in 20 percent of quadrats but cover most of those quadrats, meaning it is clumped in favourable microhabitats.

Simpson's diversity index worked table

For QCE Biology, always check the formula given in the question because different courses use different forms of Simpson's index. A common form is:

$D=1-\frac{\sum n(n-1)}{N(N-1)}$

where n is the number of individuals in each species and N is the total number of individuals.

| Species | Individuals, n | n(n-1) | | --- | ---: | ---: | | A | 18 | 306 | | B | 12 | 132 | | C | 7 | 42 | | D | 3 | 6 | | Total | N = 40 | sum = 486 |

$D=1-\frac{486}{40\times39}=1-0.312=0.688$

A value closer to 1 indicates higher diversity in this version of the index. A value closer to 0 indicates lower diversity, usually because the community has low richness, low evenness or both.

Assumptions behind population estimates

Random sampling matters because the sample is being used to represent a larger area. If quadrats are placed by convenience, the estimate may be biased toward visible, accessible or unusually dense patches. Random coordinates, a grid system or a random-number generator make the sampling design easier to defend.

Capture-recapture has stricter assumptions than most students remember. The population should be closed between samples, meaning no major births, deaths, immigration or emigration. Marked individuals should mix back into the population before the second capture. Marks should not fall off, harm the organism, make it more visible to predators or change the chance of recapture. The second sample should be large enough that the number of marked recaptures is meaningful.

If no marked individuals are recaptured, the Lincoln index cannot be used directly because the denominator would be zero. If only one or two marked individuals are recaptured, the estimate is possible but low-confidence because a tiny change in recaptures would greatly change the answer.

For percentage cover, state how cover was estimated. Grid quadrats, point-intercept methods and visual estimates can produce different values. For percentage frequency, remember that a species can have high frequency even when it has low cover.

How to use this in data questions

Start by identifying what has been measured. In Biology, a graph or table is rarely just asking for a trend; it is asking whether you can connect the trend to a process. Quote enough data to show the pattern, then use the concept language from the syllabus. If the evidence is limited, name the limitation precisely: sample size, sampling method, uncontrolled variables, measurement precision, population choice or the time scale of the data.

A useful study habit is to turn each heading into a data prompt. Ask what you would expect to happen if the relevant variable increased, decreased or was removed. For ecology topics, think about abundance, distribution, biodiversity, biomass and carrying capacity. For genetics topics, think about genotype, phenotype, gene expression, allele frequency and inheritance pattern. For evolution topics, think about variation, selection pressure, gene flow, isolation and relatedness.

When a question asks you to evaluate, do not just list problems with the experiment. Link the limitation to the confidence of the conclusion. For example, a small sample size matters because a few unusual individuals can distort the pattern. An uncontrolled abiotic factor matters because it gives another possible explanation for the same biological trend. This is the difference between naming a limitation and using it scientifically.

Worked example

Common exam traps

Other traps to watch for:

• using a general word when a syllabus term is available
• ignoring units, sample size or time scale
• treating a model as a perfect copy of the real ecosystem or cell
• writing a memorised paragraph that does not use the given data

Quick check

Sources

• QCAA Biology subject page
• QCAA Biology 2025 syllabus
• OpenStax Biology 2e: Population Demography
• OpenStax Biology 2e: Community Ecology
• Australian Museum: Biodiversity