Every fact in the three habitat 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: woodlice results vary with the species, the time of year and how long the animals were kept before the test; the Nuffield guidance suggests damp matters more to them than darkness [5], but a small class sample can show something different. Quadrat results are estimates: percentage cover is quicker but more subjective than presence/absence or frequency [7], and too few quadrats give unreliable results. The habitat-loss figure (a main threat to 85% of threatened species on the IUCN Red List) is quoted by WWF [8]; it is a broad estimate, and the WWF page also gives older forest figures that we have not used.
All sources
[1] Habitat (OLogy card)
American Museum of Natural History · Reliability: High
Type: Natural history museum (for children)
A habitat is a place where an animal or plant lives and grows: the place and the resources it uses to survive. Habitats provide food, water and shelter. Types include tropical rain forest, savanna, desert, tundra, taiga, open ocean and coral reefs. An Amazon river fish couldn't survive in the freezing Arctic Ocean. Polar bears, seals and walruses spend much of their lives on the ice; the Antarctic is home to penguins and leopard seals. The Arctic fox's ears and tail are very short, so it does not lose too much body heat. When people chop down forests they cut down a bird's nest or a monkey's source of food.
National Geographic Education · Reliability: Medium-high
Type: Educational publisher (encyclopedic entry)
A habitat is a place where an organism makes its home and meets all the environmental conditions it needs to survive. The main components are shelter, water, food and space. Dromedary camels can travel 161 km without a drink of water; the prickly pear cactus is adapted to sandy soil, dry climates and bright sunlight. For a caterpillar, shelter might be the underside of a leaf. An animal can lose space when humans build homes and businesses.
Natural History Museum, London · Reliability: High
Type: Natural history museum
Camel humps are fat, not water; the fat is used for energy. Long eyelashes keep dust out and a third eyelid sweeps out debris. Camels can close their nostrils to keep out sand, and their noses recover moisture from breath. Wide two-toed feet spread the weight so they don't sink in sand. Camels can lose up to 30% of their body weight in water, rarely sweat, let their body temperature rise during the day and fall at night, and produce thick, syrupy urine and very dry faeces.
San Diego Zoo Wildlife Alliance · Reliability: Medium-high
Type: Zoo and conservation organisation
Polar bears have water-repellent fur and a thick layer of fat under the skin that insulates them in sub-zero temperatures. They spend most of their lives travelling between sea ice floes and hunt seals. Disappearing sea ice threatens the species.
Nuffield Foundation (Practical Biology) · Reliability: High
Type: Teaching practical from an educational foundation
Woodlice are usually found in damp, dark places. They lose water quickly in dry air, and their breathing system stops working if they dry out. In a choice chamber with damp and dry sides, woodlice settle within about 5–10 minutes; damp is more important to woodlice than dark. They move more in dry conditions and slow down in damp ones, a reflex response that results in them collecting in damp areas. A 9 cm Petri dish is too small for 20 woodlice, so use fewer animals; teachers must do their own risk assessment.
Practical Biology (Nuffield Foundation / Royal Society of Biology) · Reliability: High
Type: Teaching practical from educational and professional bodies
Random sampling allows you to estimate the populations of different species in an area. Lay two tape measures at right angles and use random numbers to choose coordinates; place the quadrat (0.25 m × 0.25 m or 0.5 m × 0.5 m) or drop it over the shoulder rather than throw it. Each group should assess around 10 quadrats. Record presence or absence, or frequency using a quadrat divided into 100 squares. Safety: risk assessment, hand washing whenever plants and soil are handled, cover cuts, no eating outdoors, sun protection, watch for stings, allergies and hazards such as broken glass.
A frame quadrat is a square frame used to outline a sample area. The simplest method records each species as present or absent. Percentage frequency is the probability of finding a species in a single quadrat: quadrats containing the species ÷ total quadrats × 100. Percentage cover estimates the percentage of the quadrat area occupied by each species; it is faster but more subjective, tends to overestimate flowering plants and underestimate low-growing ones. In practice 10 quadrats is the absolute minimum for each sample area.
Habitat loss is probably the greatest threat to the variety of life on the planet today; it is a main threat to 85% of the threatened species on the IUCN Red List. Tropical forests hold at least half of Earth's species.