Measuring and mapping biodiversity at local, national and global scales

Defining Biodiversity: Three Interlocking Levels

Biodiversity is far richer than a simple count of species. The Convention on Biological Diversity (CBD, 1992), to which Australia is a signatory, defines biodiversity as the variability among living organisms from all sources including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part. For HSC Geography, you must distinguish and apply three hierarchical levels:

• Genetic biodiversity — variation in DNA sequences, alleles and genotypes within a single species. This underpins a species' capacity to adapt to environmental change. A striking Australian example is the northern hairy-nosed wombat (Lasiorhinus krefftii): with fewer than 300 individuals confined to Epping Forest National Park (Qld), the species exhibits critically low genetic diversity, making it acutely vulnerable to disease and climate shifts. Conserving genetic diversity is therefore the foundation of long-term species resilience.
• Species biodiversity — the number (richness) and relative abundance (evenness) of species within a defined area. This is the most commonly measured level and the focus of most biodiversity indices (see Section 2). Australia harbours over 570,000 described species, with approximately 85% of flowering plants, 84% of mammals and 45% of bird species found nowhere else on Earth — an extraordinary rate of endemism reflecting the continent's long Gondwanan isolation.
• Ecosystem biodiversity — the variety of habitat types, ecological communities and ecosystem processes across a landscape or region. It includes diversity of biomes (rainforest, desert, coral reef, alpine heath) and the functional relationships that bind them. Australia spans seven major biome types, from tropical savannas in the north to temperate cool-temperate rainforests in Tasmania, providing globally significant ecosystem diversity.

These three levels are interdependent: low genetic diversity can lead to species extinction, which in turn degrades ecosystem function. Understanding all three is essential for designing effective conservation strategies and for interpreting biodiversity data at any scale.

Measuring Biodiversity: Indices, Richness and Evenness

Measuring biodiversity requires moving beyond simple species lists. Ecologists use quantitative indices that capture both how many species are present and how evenly individuals are distributed among those species. Two indices dominate HSC and university-level study:

1. Simpson's Diversity Index (D)

Simpson's Index measures the probability that two randomly selected individuals belong to different species. The most exam-relevant form is:

D = 1 - [sum of n(n-1) / N(N-1)], where n = number of individuals of each species, N = total individuals across all species.

D ranges from 0 (no diversity — all individuals belong to one species) to values approaching 1 (high diversity). Simpson's is relatively insensitive to rare species and is weighted towards dominant species, making it useful for comparing community structure in disturbed versus undisturbed sites.

Worked example — Coastal heathland survey, Ku-ring-gai Chase NP (NSW):

D = 1 - [216 / (30 × 29)] = 1 - [216 / 870] = 1 - 0.248 = 0.752

A D value of 0.752 indicates moderately high diversity — unsurprising for a structurally complex sandstone heathland.

2. Shannon-Wiener Diversity Index (H')

Shannon-Wiener is borrowed from information theory and treats each species as a source of information. The formula is:

H' = -sum of [p_i × ln(p_i)], where p_i = proportion of individuals belonging to species i, and ln = natural logarithm.

H' values typically range from 0 (one species) to around 4-5 (very high diversity). Shannon-Wiener gives more weight to rare species than Simpson's does, making it sensitive to sampling completeness — you must survey thoroughly to capture rare species.

Worked example (same heathland data):

H' = -(-1.429) = 1.429

This moderate H' value (out of a possible ~1.61 for perfectly even distribution across 5 species) reflects that Banksia serrata dominates the community. Had all 5 species occurred with equal abundance (n = 6 each), H' would equal ln(5) = 1.609.

Species richness vs. evenness: A site with 20 species but 95% of individuals belonging to one species has high richness but very low evenness — and would score a low H' or D. Both components matter for ecological health.