echoesof.science · Ages 15–16
Biodiversity: measuring it and losing it
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Biodiversity includes the number and kinds of species and their relative abundance. It also exists at other levels: genetic diversity within a species is the raw material for evolution and adaptation, and ecosystem diversity is the number of different ecosystems in an area.
Species counts are uncertain. Sources give about 1.5, 1.7 or 1.75 million described species. One estimate puts the total number of eukaryote species at 8.7 million, so less than 20% may be described, but such totals are best guesses by experts.
The background extinction rate is about one extinction per million species per year (1 E/MSY). Estimates for today are closer to 100 E/MSY, rising to around 500 E/MSY in the coming century, but they are hard to pin down because most extinctions probably happen without anyone observing them.
Experiment: Leaf-litter invertebrates: comparing two habitats
- Collect equal volumes of leaf litter from two contrasting areas. Write a hypothesis about which will be more diverse.
- Place each sample in a funnel above a perforated disc, with a collecting pot containing damp tissue underneath.
- Switch on a lamp above each funnel at the same height. The heat and drying drive the animals down into the pot.
- Leave the funnels for 2–3 days.
- Tip each pot gently into a white tray. Sort the animals into different types with the key and count the individuals of each type.
- Calculate species (or 'type') richness and the relative abundance of each type for both habitats. Compare and evaluate.
- Return the animals and litter to where they were collected.
| Type of invertebrate | Habitat A: number of individuals | Habitat B: number of individuals |
|---|---|---|
| Springtails | ||
| Mites | ||
| Woodlice | ||
| Beetles | ||
| Spiders | ||
| Centipedes / millipedes | ||
| Other | ||
| Total / number of types |
Questions
Explain the difference between species richness and relative abundance, using your own results.
Calculate the percentage of Red List species that were threatened in October 2025 (48,646 of 172,620). Show your working.
Give two reasons why estimates of the total number of species and of extinction rates are uncertain.
Evaluate your Tullgren funnel investigation: give two limitations and one improvement.
Sources for every fact on this sheet: echoesof.science/school/biodiversity/sources/