Every fact in the three food chain 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: real food webs are much more tangled than the short chains shown here, and many animals eat at more than one level. The '10% rule' is an average [3]: measured transfer efficiencies vary between ecosystems, for example about 14.5% from producers to primary consumers at Silver Springs [4]. The energy and pollutant numbers in the 10–11 and 15–16 activities are made-up model values chosen to show the idea; they are not measurements from a real ecosystem, except where a source is cited.
All sources
[1] Food Chain
National Geographic Education · Reliability: Medium-high
Type: Educational publisher (encyclopedic entry)
A food chain describes who eats whom in the wild; energy and nutrients move along it. Producers (autotrophs) make their own food, usually by photosynthesis: plants, algae, phytoplankton and some bacteria. Primary consumers (herbivores) eat producers, e.g. deer, turtles and many birds; secondary consumers eat herbivores, tertiary consumers eat secondary consumers. Carnivores eat meat; omnivores, such as people, eat plants and animals. Apex predators are at the top and have no predators. Detritivores eat dead remains (vultures, dung beetles); decomposers such as fungi and bacteria break down waste and return nutrients to the soil or oceans for producers. Examples: grass → rabbit → fox; grass → grasshopper → rat → snake → hawk; phytoplankton → krill → blue whale.
National Geographic Education · Reliability: Medium-high
Type: Educational publisher (encyclopedic entry)
Each food chain is one possible path for energy and nutrients; all the interconnected, overlapping food chains in an ecosystem make up a food web. Example: a caterpillar eats tree leaves, a sparrow eats the caterpillar, a snake eats the sparrow and an eagle eats the snake. In kelp forests sea otters eat sea urchins, which graze on kelp; if otters decline, urchins can destroy the kelp. Biomass decreases at each trophic level; 80–90% of an organism's energy is lost as heat or waste, so a healthy web has many producers, fewer herbivores and even fewer carnivores.
National Geographic Education · Reliability: Medium-high
Type: Educational publisher
On average only 10% of the energy available at one trophic level is passed on to the next. Energy is lost because much sunlight never reaches producers, producers use energy for respiration, some eaten material is not digested and is excreted, a large share is released as heat in respiration, organisms die without being eaten, and decomposers lose most of the energy as heat. Most ecosystems support at most about five trophic levels.
[4] Biology 2e, 46.2 Energy Flow through Ecosystems
OpenStax, Rice University · Reliability: High
Type: Open university textbook
Net primary productivity is the energy left in producers after respiration and heat loss. Silver Springs: gross primary productivity 20,810 kcal/m²/yr, of which 13,187 kcal/m²/yr is lost to respiration and heat. Trophic level transfer efficiency TLTE = production at present level ÷ production at previous level × 100; about 14.48% from producers to primary consumers at Silver Springs. Ectotherms have a net production efficiency about 10 times higher than endotherms (caterpillar about 18%, squirrel as low as 1.6%). After four to six transfers too little energy remains. Pyramids of numbers and biomass can be inverted (about 200 trees but 150,000 herbivorous insects per 0.1 ha; English Channel phytoplankton 4 g/m², zooplankton 21 g/m², because phytoplankton turn over quickly); pyramids of energy are always upright. Biomagnification: DDT caused eggshell fragility in fish-eating birds and was banned in the US in the 1970s; in Saginaw Bay, walleye had more than four times the PCBs of phytoplankton.