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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
SL

Exam analysis

Chance of appearing8%of analysed past papers
Latest appearanceNovember 2024
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

November 2024Paper3 ["SL"] · TZ112(a)[ 1 ]A4.2.1—Biodiversity definition
May 2024Paper3 ["SL"] · TZ214[ 4 ]A4.2.1—Biodiversity definition
May 2023Paper3 ["SL"] · TZ215(b)[ 2 ]A4.2.1—Biodiversity definition
May 2023Paper3 ["SL"] · TZ215(a)[ 1 ]A4.2.1—Biodiversity definition
November 2020Paper3 ["SL"] · TZ014(a)(ii)[ 1 ]A4.2.1—Biodiversity definition

Coverage 2010–2024 · Updated 15 Jul 2026

Biodiversity Exists at Three Linked Levels

Biodiversity is the variety of life at genetic, species and ecosystem levels. A community can have the same richness as another yet lower diversity if one species dominates.

Two beetle communities have equal species richness but different evenness; the more even community has greater species diversity.

D=N(N−1)∑n(n−1)D = \frac{N(N-1)}{\sum n(n-1)}

N is the total number of organisms and n is the number in each species. For comparable samples, a larger D indicates greater species diversity.

Summary: Turn Biodiversity Evidence into Action

Focus Measure the state or trend Diagnose the driver Match the action Monitor recovery
genetic diversity allelic variation, heterozygosity, population size isolation, bottleneck or inbreeding reconnect populations or manage breeding genetic diversity and effective population size
species diversity richness, evenness, abundance, extinction risk exploitation, invasive species or habitat change protect, control threats, breed or reintroduce abundance, recruitment and distribution
ecosystem diversity/function habitat extent, condition and process indicators conversion, fragmentation, pollution or climate stress protect, restore and reconnect habitat structure, processes and resilience

Define the level → establish a comparable trend → identify the causal driver → choose an action at the same scale → test whether biological recovery follows.

No single index or intervention is sufficient: species counts can miss evenness, genetic erosion and ecosystem dysfunction, while a reserve can fail without connectivity, enforcement or threat removal.

Biodiversity definition

[ 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.

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.