A4.2.2—Current vs. past biodiversity
Current biodiversity is estimated from living species records, while past biodiversity is reconstructed from incomplete fossil evidence and dating methods.
- Syllabus
- First assessment 2025
- Objective
- A4.2.2
- Level
- HL
Biodiversity Changes through Time
Millions of living species have been discovered, named and described, and many more remain undiscovered. Fossil evidence suggests that more species may be alive now than at any earlier time in Earth's history.
Present-day surveys sample living organisms directly, whereas past biodiversity is reconstructed from fossils and dated rock layers. Fossilization is rare and uneven, so soft-bodied, small or poorly sampled organisms are underrepresented.
A fair comparison states the time interval, separates number of species from abundance, considers changes in taxonomic definitions and acknowledges unequal sampling. Speciation raises diversity through time; extinction lowers it.
A diverse fossil assemblage can establish that many lineages existed in one period, but missing fossils cannot establish that no additional species were present.
The fossil record supports a comparison but is incomplete. 'Currently more species' is an evidence-based inference, not an exact census of all past and present species.
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
- Many living species have not yet been described, so current biodiversity is estimated rather than fully known.
- Fossils provide evidence of past biodiversity but preserve organisms unevenly.
- Radiometric dating helps place fossil evidence into a geological time sequence.
- Comparisons of current and past biodiversity are limited by gaps in both living surveys and fossil records.