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Home›Ecology and environment

Food webs

In Silver Springs, Florida, the plants made 20,819 kilocalories per square metre in a year and the top predators got 21 of them. Follow energy from producers to predators, take species out of a food web, run a rabbit and fox population model, and see why poisons pile up at the top.

Progress

About this course

In Silver Springs, Florida, the plants made 20,819 kilocalories per square metre in a year and the top predators got 21 of them, about 0.1%. In Lake Ontario, the energy in a Chinook salmon has passed through only three meals since green algae captured it from sunlight. This course follows energy from producers to predators and asks what happens when a species is removed.

One of the earliest written descriptions of a food web comes from al-Jahiz, a scholar in Baghdad in the early 800s, who observed that mosquitoes feed on the blood of elephants and hippos yet are eaten by flies and small birds. The six units start with producers, consumers and decomposers, then move from food chains to food webs. In a made-up meadow web, the mouse eats three things and is eaten by three, so taking it out leaves every other species with something to eat. The energy pyramid unit uses the 10% rule. The predator and prey unit shows snowshoe hares and lynx cycling about every 10 years with the lynx 1 to 2 years behind. Volterra published predator and prey equations in 1926, and Alfred Lotka had reached the same ones in 1925, so they carry both names. Keystone species recounts Robert Paine prying sea stars off a Washington shore, after which the number of species fell from 15 to 8. The last unit finds walleye in Saginaw Bay carrying more than four times as much PCB as the phytoplankton below them. The lab lets you take out species one at a time and run a rabbit and fox model.

Before you start

Food webs start with plants turning sunlight into food, so Plants gives useful background. The energy pyramid unit turns kilocalories into percentages, which Decimals and percent covers, and the predator and prey model uses a simple growth rate.

When you finish

You can sort living things into producers, consumers and decomposers, build a food web from food chains, and calculate what share of the energy reaches the top predators at Silver Springs. You can read the rise and fall of hares and lynx, explain what Robert Paine meant by a keystone species, and describe how PCBs build up toward the top of a web.

  1. 1Producers, consumers and decomposersIn Lake Ontario the energy in a Chinook salmon has passed through only three meals since it was captured from sunlight by green algae.
  2. 2From food chains to food websIn the made-up meadow below, the mouse eats three things and is eaten by three, so taking it out leaves every other species with something to eat.
  3. 3The energy pyramid and the 10% ruleOf the 20,819 kcal that the plants of Silver Springs made on each square metre in a year, 21 kcal, about 0.1%, reached the top predators.
  4. 4Predators and prey: rabbits and foxesTrapping records from North American forests show snowshoe hares and lynx rising and falling in a cycle of about 10 years, with the lynx 1 to 2 years behind.
  5. 5Keystone species and the Yellowstone wolvesWhen Robert Paine pried the sea stars off a strip of rocky shore in Washington State, the number of species living there fell from 15 to 8.
  6. 6Poisons and invaders in food websIn Saginaw Bay on Lake Huron, walleye at the top of the food web carried more than four times as much PCB as the phytoplankton at the bottom.

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Sources for this course (16)
  • OpenStax Biology 2e 46.1 Ecology of Ecosystems: food chains and webs, Lake Ontario chain, grazing and detrital webs, Silver Springs 20,819 / 3,368 / 383 / 21 kcal/m²/yr (openstax.org/books/biology-2e/pages/46-1-ecology-of-ecosystems)
  • 46.2 Energy Flow through Ecosystems: chemoautotrophs at hydrothermal vents (NW Eifuku, 1,535 m)
  • TLTE 14.48% (1,103 of 7,618)
  • four to six transfers, NPE caterpillar 18% and squirrel 1.6%, English Channel 4 and 21 g/m², energy pyramids always upright, biomagnification, DDT and Silent Spring, PCBs banned 1979, Saginaw Bay walleye more than four times the PCBs of phytoplankton, EPA fish advice (.../pages/46-2-energy-flow-through-ecosystems)
  • 45.6 Community Ecology: lynx and snowshoe hare cycle about 10 years, lag 1 to 2 years, nearly 200 years of trapping data, keystone Pisaster and banded tetra, foundation species kelp and coral, Asian carp 95% of biomass in parts of the Illinois River (.../pages/45-6-community-ecology)
  • National Geographic Education: Food Web (al-Jahiz, early 800s) and Energy Flow and the 10 Percent Rule (education.nationalgeographic.org/resource/food-web/, /resource/energy-flow-and-10-percent-rule/)
  • NOAA Education, Aquatic food webs (noaa.gov/education/resource-collections/marine-life/aquatic-food-webs)
  • Kingsland 2015, Alfred J. Lotka and the origins of theoretical population ecology, PNAS 112:9493 (pmc.ncbi.nlm.nih.gov/articles/PMC4534218): Lotka 1925 Elements of Physical Biology, Volterra 1926
  • Bacaër 2011, A Short History of Mathematical Population Dynamics ch. 13 (D'Ancona, Adriatic fish markets, First World War)
  • Paine 1966, Food web complexity and species diversity, Am. Nat. 100:65-75 (Mukkaw Bay, 15 to 8 species)
  • Paine 1969 Am. Nat. 103:91 (keystone)
  • Lafferty and Suchanek 2016, Am. Nat. 188:365 (pubs.usgs.gov/publication/70175674)
  • NPS Yellowstone: History of Wolf Management (nps.gov/yell/learn/historyculture/wolf-management.htm: last pack killed 1926, 14 wolves January 1995, 17 in January 1996, 10 from Montana 1997)
  • Wolf Restoration in Yellowstone: Reintroduction to Recovery (nps.gov/articles/wolf-restoration-in-yellowstone-reintroduction-to-recovery.htm: 41 wolves, peak 174 in up to 16 packs, about 100 in 10 packs, about 10% a year)
  • Elk (nps.gov/yell/learn/nature/elk.htm: about 17,000 in 1995, 3,915 in 2013, 6,673 in 2022)
  • The Big Scientific Debate: Trophic Cascades (nps.gov/articles/the-big-scientific-debate-trophic-cascades.htm).

Checked: 2026-10-17