Measuring biodiversity (species richness, evenness, Simpson's diversity index): VCE Environmental Science Unit 3

What this dot point is asking

VCAA wants you to define and distinguish species richness and species evenness, and to use a diversity index (most commonly Simpson's) to compare ecosystems quantitatively. You may be asked to calculate an index from a data table and interpret what it means.

Richness and evenness

Species richness is simply the number of different species recorded in a sample or site. A rockpool with 12 species is richer than one with 5. Richness ignores how common each species is.

Species evenness describes how equally individuals are distributed among the species present. If a site has 4 species with roughly equal numbers, evenness is high. If one species dominates (say 95 per cent of individuals) and three are rare, evenness is low.

Both matter. Two sites can have identical richness but very different diversity because one is dominated by a single species. A good measure of biodiversity combines richness and evenness, which is what a diversity index does. Evenness is sometimes reported on its own as a value between $0$ and $1$, where $1$ means every species has exactly the same number of individuals, but in VCE it is most often captured through a combined diversity index rather than calculated separately.

Simpson's diversity index

Simpson's index gives a single number that accounts for both richness and evenness. The form VCAA prints on the exam and expects you to use is:

The term $\frac{n_i(n_i - 1)}{N(N - 1)}$ is the probability that two individuals drawn at random (without replacement) both belong to the same species. Summing this across every species and subtracting from $1$ gives the probability that the two individuals belong to different species, so a higher $SID$ signals a more diverse, more even community. An equivalent proportion-based form, $D = 1 - \sum (n_i / N)^2$, gives a very similar value for large samples, but the $N(N - 1)$ version is the one VCAA supplies, so use it in the exam and show every substitution.

Sampling and comparing ecosystems

Index values are only meaningful when sampling is done consistently. Use the same quadrat or transect size, the same sampling effort, and random or systematic placement to avoid bias. Quadrats suit plants and slow or sessile organisms; transects are used to record how communities change along an environmental gradient such as distance from a creek. Larger or more numerous samples reduce the chance of missing rare species and give a more reliable estimate of richness, because rare species are easy to overlook in a single small quadrat.

Comparing the index between sites (for example, a grazed paddock versus a fenced conservation reserve, or upstream versus downstream of an outfall) lets you draw conclusions about the effect of a management practice or a threat. Because $SID$ is a single standardised value between $0$ and $1$, it allows objective comparison in a way that a raw species list cannot. A falling diversity index measured over several years is an early warning of ecosystem degradation, and a rising index after a restoration project is evidence that management is working.

Why ecologists prefer an index to a raw count

Species richness alone treats a site with one dominant weed and a few survivors the same as a balanced community, as long as the number of species matches. By folding evenness into the calculation, a diversity index rewards communities where individuals are spread across many species and penalises those dominated by one. This matters for management decisions: a reserve might keep the same number of species while a single invasive grass quietly takes over, and only an index that responds to evenness would reveal that the ecosystem is degrading. When you interpret an index in the exam, comment on both richness and evenness, and always read it alongside the species list so you can tell whether the species present are native or introduced.