measures of biodiversity including species richness, species evenness and the use of diversity indices to compare ecosystems

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.

Simpson's diversity index

Simpson's index gives a single number that accounts for both richness and evenness. One common form used in VCE is:

The term (n / N) is the proportion of the community made up by one species. Squaring it and summing across all species gives the probability that two randomly chosen individuals belong to the same species. Subtracting from 1 turns it into the probability they belong to different species, so higher values mean more diversity.

Sampling and comparing ecosystems

Index values are only meaningful when sampling is done consistently. Use the same quadrat or transect size, the same effort, and random or systematic placement to avoid bias. Larger or more numerous samples reduce the chance of missing rare species and give a more reliable estimate of richness.

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. A falling diversity index over time is an early warning of ecosystem degradation.