Measuring Biodiversity: Species Richness, Evenness and Simpson's Diversity Index

What Is Biodiversity? Three Levels of Variation

Biodiversity — a contraction of biological diversity — refers to the variety of life on Earth and encompasses all the ways in which living systems differ from one another. The QCAA Biology syllabus requires students to recognise that biodiversity operates simultaneously at three distinct, hierarchical levels: genetic diversity, species diversity, and ecosystem diversity. Each level is measurable, meaningful, and influenced by different ecological and evolutionary processes.

• Genetic diversity describes the range of heritable allelic variation within and between populations of the same species. High genetic diversity equips a population with a broader toolkit of responses to environmental change, disease, and predation. For example, the greater bilby (Macrotis lagotis) populations remaining in isolated refuges across Queensland and the Northern Territory show markedly reduced genetic diversity compared with historical records — a consequence of population fragmentation that increases their vulnerability to disease outbreaks and climatic shifts.
• Species diversity refers to the variety of species within a defined area. It incorporates both how many different species are present (richness) and how evenly those species are distributed across the community (evenness). Species diversity is the level most commonly measured in field surveys and is the primary focus of Simpson's Diversity Index.
• Ecosystem diversity describes the range of different habitat types, ecological communities, and the abiotic and biotic processes that sustain them across a landscape or region. Queensland alone contains an exceptional array of ecosystems — from the wet tropical rainforests of the Daintree, which are listed among the world's oldest continuous rainforests, to the mulga woodlands of the Channel Country, the Great Barrier Reef's coral systems, and the brigalow belt — each harbouring distinct communities of organisms and fulfilling distinct ecosystem functions.

These three levels are interconnected: ecosystem diversity supports species diversity, and species diversity underpins genetic diversity. Conservation strategies that address only one level are rarely sufficient in isolation. Understanding all three levels is therefore fundamental to evaluating community health and designing effective management responses.

Species Richness and Species Evenness: Two Sides of Diversity

When ecologists speak of species diversity in the field, they are almost always referring to two distinct but complementary properties of a community: species richness and species evenness. Students frequently conflate these concepts, yet they can diverge dramatically in real ecosystems, and each tells a different ecological story.

Species richness (S) is simply the total number of different species recorded within a defined sampling area or community. It is a raw count — no weighting is applied for how many individuals of each species are present. Richness is intuitive and easy to compare between sites, but it provides no information about how the organisms are distributed among those species. A sample containing 10 species with 1000 individuals is counted as having a richness of 10, regardless of whether 999 of those individuals belong to a single species.

Relative abundance expresses the proportion of each species' population relative to all individuals recorded in the sample. Formally: relative abundance of species i = (n_i / N) × 100%, where n_i is the number of individuals of species i and N is the total number of individuals across all species. Relative abundance data can be displayed as bar charts, pie charts, or rank-abundance (Whittaker) plots.

Species evenness (E) measures how similar the relative abundances of species are to one another. A perfectly even community is one in which every species has an identical number of individuals. Evenness is maximised when all species are equally represented and minimised when one or a few species dominate overwhelmingly. Evenness matters because a community with low evenness — even one with high richness — may be functionally fragile, since the ecosystem processes it supports depend disproportionately on dominant species.

Both communities above have the same richness (S = 6), yet they are ecologically very different. Site B is clearly dominated by a single species — likely an introduced grass or weed — while Site A represents a balanced, resilient community. This comparison illustrates why richness alone is an insufficient measure of biodiversity.