School › Biodiversity › Ages 15–16
Biodiversity: measuring it and losing it
Genetic, species and ecosystem diversity; how uncertain species counts and extinction rates are; interpreting the IUCN Red List and the Living Planet Index; and a Tullgren-funnel investigation of leaf-litter invertebrates.
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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.[3]
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.[3][1][2]
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.[3]
The IUCN Red List (October 2025 update) includes 172,620 species, of which 48,646 are threatened with extinction: about 28% of the species assessed.[4]
The Living Planet Index follows almost 35,000 vertebrate populations of 5,495 species. Between 1970 and 2020 their average size fell by 73%, with the steepest fall (85%) in freshwater populations. The figure is an average proportional change, not the number of animals lost.[5]
The dominant threats are habitat loss and degradation and overharvesting, driven mainly by the global food system; invasive species, pollution and climate change add to them.[5][2][1]
Try it: Leaf-litter invertebrates: comparing two habitats
You need
- Two Tullgren funnels (funnel, perforated disc or wire mesh, collecting pot) and two desk lamps
- Leaf litter from two contrasting areas (e.g. under a hedge and under a mown-grass edge or a single-species plantation), same volume each
- Small quadrat or a beaker to measure equal volumes
- Damp tissue for the collecting pots
- White tray, hand lens, simple invertebrate key
- Gloves
Safety
Lamps get hot: keep them at a safe distance from the litter, never leave flammable material touching the bulb, and switch off when unattended overnight if the school's risk assessment requires it. Wear gloves when collecting litter, wash hands afterwards, handle live animals gently and return them. Collect only small amounts and from areas where it is allowed.
What to do
- 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.
What happens, and why?
Invertebrates move away from the rising temperature and drying effect of the lamp and fall into the pot, so a Tullgren funnel extracts them without searching by hand. Counting types gives richness; counting individuals gives relative abundance, which is also part of biodiversity. With only one sample per habitat the result is weak: for statistical tests 10–30 samples per area are usually needed, and some animals (e.g. slow or flying ones) are extracted less well.[7][6][3]
Play: Strong or weak evidence?
Does this make a biodiversity comparison more reliable, or does it weaken the evidence?
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Quiz
Other ages: Ages 6–7 · Ages 10–11