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Measuring Biodiversity - TCE Biology (Tasmania)

Syllabus dot point

“Define the levels of biodiversity and describe how species richness and evenness are measured.”

TCEBiologyBiodiversity and ecosystems (Level 2…7 min read

Quick answer

Biodiversity is the variety of life, measured at three levels: genetic diversity within a species, species diversity within a community, and ecosystem diversity across a region. Species diversity combines species richness (how many species) and evenness (how balanced their numbers are). Diversity indices and sampling methods let ecologists put a number on it.

Not examined in the current TASC course

Measuring biodiversity is not part of TASC's Biology Level 3 course (BIO315124, current for 2026), so this page is not examined in the Level 3 external exam. It is part of TASC's Biology Level 2 course (BIO215123). Level 3 examines genetic diversity within populations instead: see population genetics.

What this dot point is asking

The three levels of biodiversity

Biodiversity is more than just a count of species. It is measured at three connected levels:

  • Genetic diversity: the variety of alleles and genotypes within a single species. High genetic diversity helps a population adapt to change and resist disease.
  • Species diversity: the variety of different species in a community or area.
  • Ecosystem diversity: the variety of habitats, communities, and ecological processes across a region or the whole planet.

All three matter for the health and resilience of life. A region might have many ecosystems but, if each is dominated by one species, its species diversity could still be low.

Biodiversity

Biodiversity is the variety of all living things, considered at the genetic, species, and ecosystem levels within a defined area.

Species richness and evenness

Species diversity has two components that must be considered together:

  • Species richness is simply the number of different species present.
  • Species evenness describes how evenly the individuals are spread among those species.

A community is more diverse when it has both high richness and high evenness. Two communities can have the same richness but very different diversity if one is dominated by a single species.

Same richness, different diversity

Forest X has 100 birds across four species split 25, 25, 25, 25. Forest Y also has 100 birds across four species but split 97, 1, 1, 1. Both have a richness of four, but Forest X has high evenness and Forest Y is dominated by one species. Forest X is considered more diverse, because a diversity index weights both richness and evenness.

Diversity indices

To capture richness and evenness in one figure, ecologists use a diversity index such as Simpson's index. The index returns a value that rises as both richness and evenness increase, so it is a fairer comparison than a raw species count. The exact formula is less important at this level than the idea: a single number that reflects both how many species are present and how balanced their abundances are. Comparing index values lets ecologists track change over time or compare two sites objectively.

Diversity is richness and evenness combined

A diversity index is more useful than a simple species count because it accounts for both the number of species and how evenly individuals are distributed. A site dominated by one species has lower diversity than a site with the same number of species in balanced numbers.

Sampling to estimate biodiversity

It is rarely possible to count every organism, so ecologists sample. Common methods include:

  • Quadrats: square frames placed (often randomly) to count or estimate the percentage cover of plants and slow-moving animals.
  • Transects: a line across a habitat along which organisms are recorded, useful for showing how communities change along an environmental gradient.
  • Capture-mark-recapture: for mobile animals, a sample is caught, marked, and released, then a second sample is taken to estimate total population size.

Random placement and adequate sample numbers reduce bias and make estimates more reliable.

Why biodiversity matters

High biodiversity tends to make ecosystems more stable and productive, and more able to recover from disturbance. Diverse ecosystems provide services such as pollination, clean water, and climate regulation. Loss of biodiversity, through habitat destruction, climate change, pollution, and introduced species, reduces this resilience. Measuring biodiversity gives scientists a baseline to detect decline and to evaluate conservation efforts.

Endemism and hotspots

Some species are endemic, meaning they are found naturally in only one place, such as many Tasmanian species. Areas with high numbers of endemic species and high overall diversity are called biodiversity hotspots and are conservation priorities, because losing habitat there causes a disproportionate loss of unique life.

Treating biodiversity as just a species count

A common error is to equate biodiversity with the number of species alone. Genuine biodiversity measurement includes genetic and ecosystem levels, and at the species level it weighs evenness as well as richness. Two sites with the same species count can have very different biodiversity.

Exam-style questions

Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

Original6 marks
Two woodland sites each contain 4 plant species. Site A has individuals split 25, 25, 25, 25; Site B has them split 70, 10, 10, 10 (totals of 100 each). Using Simpson's diversity index D=1−∑(pi)2D = 1 - \sum (p_i)^2, where pip_i is the proportion of each species, calculate DD for each site and explain which site is more diverse.
Show worked answer →

A 6 mark answer shows both calculations and interprets them.

Site A
Each pi=0.25p_i = 0.25, so ∑(pi)2=4×0.252=4×0.0625=0.25\sum (p_i)^2 = 4 \times 0.25^2 = 4 \times 0.0625 = 0.25. Then D=1−0.25=0.75D = 1 - 0.25 = 0.75.
Site B
pip_i values are 0.70,0.10,0.10,0.100.70, 0.10, 0.10, 0.10. ∑(pi)2=0.49+0.01+0.01+0.01=0.52\sum (p_i)^2 = 0.49 + 0.01 + 0.01 + 0.01 = 0.52. Then D=1−0.52=0.48D = 1 - 0.52 = 0.48.
Interpretation
Site A has the higher index (0.750.75 versus 0.480.48), so Site A is more diverse. Both sites have the same richness (4 species), but Site A has far greater evenness, and Simpson's index rewards evenness as well as richness.

Markers reward both correct DD values and the explanation that equal richness but higher evenness makes Site A more diverse.

Original4 marks
Define the three levels of biodiversity and explain why a population with low genetic diversity may be more vulnerable to extinction.
Show worked answer →

A 4 mark answer defines the three levels and links low genetic diversity to extinction risk.

Three levels. Genetic diversity (variation in alleles within a species), species diversity (the variety of species in a community), and ecosystem diversity (the variety of habitats and ecosystems in a region).

Vulnerability of low genetic diversity. With few alleles, a population has little variation for selection to act on, so it is less able to adapt to new diseases or environmental change. A single disease or stress could affect all individuals similarly, raising the chance the whole population is wiped out.

Markers reward the three correctly defined levels and the link between reduced variation, reduced adaptability and higher extinction risk.

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Sources & how we know this

Based on the official TASC course document. Not yet individually reviewed by a person; always check TASC and your school's materials for current rules.

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