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A4.2.1—Biodiversity definition

Biodiversity describes variety within and between ecosystems, species, and genes, with species richness, evenness, and Simpson index enabling quantitative comparison.

Syllabus
First assessment 2025
Objective
A4.2.1
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper3
Typical marks1–2

Common command terms

  • State
  • Discuss
  • Calculate
  • Analyse
  • Compare
  • Evaluate
  • Distinguish
  • Suggest

Scoring notes

Common mistake
Equating richness with overall biodiversity when evenness also affects diversity.

Recent exam appearances

May 2023Paper3 ["HL"] · TZ117(c)[ 1 ]A4.2.1—Biodiversity definition
November 2019Paper3 ["HL"] · TZ016(c)[ 1 ]A4.2.1—Biodiversity definition
May 2017Paper3 ["HL"] · TZ115(c)[ 2 ]A4.2.1—Biodiversity definition
May 2015Paper3 ["HL"] · TZ211(a)(ii)[ 2 ]A4.2.1—Biodiversity definition
May 2015Paper3 ["HL"] · TZ211(a)(i)[ 2 ]A4.2.1—Biodiversity definition
Practice this objective

Coverage 2012–2023 · Updated 15 Jul 2026

Biodiversity Has Several Levels

Biodiversity is the variety of life in all its forms, levels and combinations. It includes genetic diversity within populations, species diversity in communities and ecosystem diversity across habitats and processes.

Species diversity depends on both richness (the number of species) and evenness (how evenly individuals are distributed). A community with many species can still have low diversity if nearly all individuals belong to one species.

D=N(N−1)/Σn(n−1),whereNisthetotalnumberoforganismssampledandnisthenumberbelongingtoeachspecies;alargerDindicatesgreaterdiversityforcomparablesamples.D = N(N - 1) / Σn(n - 1), where N is the total number of organisms sampled and n is the number belonging to each species; a larger D indicates greater diversity for comparable samples.

Two fields may have the same richness, but the field whose individuals are distributed more evenly among species has the higher Simpson's reciprocal index and greater measured species diversity.

Species count alone does not measure genetic or ecosystem diversity. Compare index values only when sampling method and effort are sufficiently comparable.

Biodiversity definition

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using State / Discuss / Calculate.

Command terms

State / Discuss / Calculate / Analyse / Compare / Evaluate / Distinguish / Suggest

What earns marks

Build the answer around this relationship: Biodiversity includes ecosystem diversity, species diversity and genetic diversity.

Watch for

Equating richness with overall biodiversity when evenness also affects diversity.

Representative question

Question 1

[Maximum number: 4]

Entire communities need to be conserved in order to preserve biodiversity. Suggest different quantitative methods to measure changes in biodiversity in a community over time.

Build A Conservation Argument

A strong conservation answer follows a chain: define biodiversity, measure evidence of decline, identify human drivers, then justify conservation action. Biodiversity includes ecosystem, species, and genetic diversity; species diversity uses richness and evenness. Evidence comes from estimates, fossils, repeated surveys, IPBES reports, and indices. Drivers include extinction pressure, ecosystem conversion, population-driven resource demand, pollution, and invasive species. Responses include in situ, ex situ, and EDGE prioritization.

  • Define biodiversity across ecosystem, species, and genetic levels.
  • Measure species diversity using richness, evenness, and Simpson’s reciprocal index.
  • Use evidence cautiously: modern estimates, fossil gaps, repeated surveys, IPBES reports, and reliability checks.
  • Explain drivers using named examples: moa, Caribbean monk seal, Borneo, Great Barrier Reef, lionfish, water hyacinth.
  • Choose conservation responses: in situ, ex situ, and EDGE priorities.

Concept essentials

  • Biodiversity includes ecosystem diversity, species diversity and genetic diversity.
  • Species richness is the number of different species present in a community.
  • Species evenness describes how similar the population sizes of different species are.
  • Simpson reciprocal index allows quantitative comparison of biodiversity between communities or over time.