Every fact in the three biodiversity lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: nobody knows how many species there are. Different sources give about 1.5, 1.7 or 1.75 million described species [3][1][2], and the 8.7 million total is only one estimate; OpenStax calls such numbers best guesses [3]. Extinction rates are also uncertain because most extinctions probably happen without being observed [3]. The Living Planet Index figure (a 73% fall) is the average proportional change in monitored vertebrate populations, not the share of all animals that have been lost [5]. Red List numbers change with every update; we use the update of 10 October 2025 [4].
All sources
[1] What is biodiversity and why does it matter to us?
Natural History Museum, London · Reliability: High
Type: Natural history museum
Biodiversity is the name we give to the variety of all life on Earth. About 1.7 million species have been described by scientists, most of them insects, and it is thought there are millions more. Species richness is the number of different species in an area. Humans rely on biodiversity to survive; it keeps us alive and helps make our lives healthy and safe. Land use change is among the top drivers of biodiversity loss; invasive species can reduce biodiversity, cause extinctions and change ecosystems. Biodiversity is in trouble in the UK and across the globe.
American Museum of Natural History · Reliability: High
Type: Natural history museum (for children)
Biodiversity is the variety of life on Earth and how living things depend on one another. Scientists have identified over 1.75 million species, more than a million of them insects and spiders, and many more remain undescribed. Biodiversity gives us clean air, food, drinking water and products such as medicine, clothing and paper. Threats include habitat loss, invasive species, overconsumption, pollution and climate change. All life is related through DNA.
Biodiversity covers the number and kinds of species and their relative abundance. Genetic diversity is the raw material for evolution and adaptation in a species; ecosystem diversity is the number of different ecosystems in an area. About 1.5 million described eukaryote species are less than 20% of the total, which one estimate puts at 8.7 million; such estimates are best guesses. The background extinction rate is about one extinction per million species per year (E/MSY); current estimates are closer to 100 E/MSY, and around 500 E/MSY in the coming century; extinction-rate estimates are hampered because most extinctions probably happen without observation.
[5] Catastrophic 73% decline in the average size of global wildlife populations in just 50 years
WWF (Living Planet Report 2024) · Reliability: Medium-high
Type: Conservation organisation (press release of its report with ZSL)
The Living Planet Index tracks almost 35,000 vertebrate populations of 5,495 species from 1970 to 2020 and shows a 73% decline in the average size of monitored populations. It reflects the average proportional change in population sizes, not the number of individual animals lost. Habitat loss and degradation and overharvesting, driven mainly by the global food system, are the dominant threats. The steepest decline is in freshwater populations (85%).
Ground vegetation in two contrasting areas of woodland or grassland is compared by random sampling of both areas or along a transect, using quadrats, recording the number of individuals of each species or an estimate of abundance. For many statistical tests, 10–30 samples are needed in each area. Safety: long sleeves and trousers (brambles, nettles, ticks), sturdy footwear, watch for branches and uneven ground, do not work alone.
Used in: Ages 10–11 (experiment); Ages 15–16 (experiment)
In a Tullgren funnel a leaf-litter sample is placed in a funnel above a perforated disc; the rise in temperature and the drying effect from a lamp above encourage the animals to move away from the heat, and they fall into a collecting vessel below. Leave the funnel for at least 2–3 days. Be gentle when handling live animals.
Used in: Ages 15–16 (experiment); Ages 15–16 (quiz)