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Food web

From Tilcayo Cats, the lean encyclopaedia by Tilcayo Knowledge Systems
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A food web is a network of linked food chains that shows who eats whom in an ecosystem. It shows how energy and nutrients flow through a whole community of living things.[1][2] In most ecosystems, the feeding links between organisms form a complex web rather than a simple chain.[2][3]

Food chains

A food chain is a sequence of organisms in which each one is a source of food and energy for the next.[2] For example, in an oak wood a clover plant may be eaten by a snail, the snail by a thrush, and the thrush by a sparrowhawk.[4] When food chains are drawn, the producer goes at the bottom. The arrows point from the organism that is eaten to the one that eats it.[4] Food chains rarely have more than five levels.[5] One common explanation is that energy is lost at each step, so there is not enough left to support animals at very high levels.[5]

Trophic levels

Ecologists place each organism in an ecosystem at a feeding level, or trophic level, depending on where it gets its nutrients.[6][7]

  • Producers, such as green plants, make their own food. Through photosynthesis they use energy from sunlight to turn carbon dioxide and water into glucose, giving off oxygen.[6][6] On land most producers are green plants. In open water the main producers are tiny floating organisms called phytoplankton.[8]
  • Primary consumers, or herbivores, are animals that eat mainly plants or algae.[9]
  • Secondary consumers are carnivores that eat herbivores, such as spiders and lions. Tertiary consumers, such as tigers, hawks and orcas, are at a higher level still. Omnivores, such as pigs, rats and humans, eat both plants and animals.[9][5]
  • Decomposers, mostly bacteria and fungi, break down the wastes and remains of plants and animals. Detritus feeders, such as earthworms, hyenas and vultures, also feed on dead matter.[9][9][10]

Decomposers return nutrients to the soil, water and air so that producers can use them again.[9] Without decomposers and detritus feeders, the planet would be buried in plant litter, animal wastes and dead bodies.[9]

Some ecologists find the idea of trophic levels unhelpful, because many animals feed at several different levels.[5] Food webs can show these animals, such as omnivores, more realistically than food chains can.[1]

Energy in food webs

As energy flows through a food web, the amount of high-quality chemical energy available to organisms falls at each level.[2] At every transfer, some energy is lost as heat.[11] A pyramid of energy shows how much energy flows through each level. It can never be upside down.[12] The share of energy passed from one level to the next is often said to be about 10 per cent, but it actually varies from less than 0.1 per cent to more than 20 per cent.[11][12][12]

The ecologist Elton noticed that animals at the bottom of a food chain are usually common, while those at the top are few. He called this pattern a pyramid of numbers.[13][13] A pyramid of numbers can be upside down, however. A single large tree, for example, can support many small animals.[14]

Energy cannot be recycled, because it is lost as heat, so living things need fresh energy from the Sun every day. Matter, on the other hand, is recycled.[15] A single atom of a nutrient may pass through one food chain after another.[15][1]

Changes in food webs

Because no predator and prey exist in isolation, a change to one species can affect many others.[3] A trophic cascade happens when a predator reduces the numbers of its prey, so that the prey's food, usually plants or phytoplankton, increases.[16] Off Nova Scotia, overfishing of cod led to more small fish and bottom-living invertebrates, fewer zooplankton and more phytoplankton.[16] Removing one predator can even harm the animals it seemed to threaten. Removing cats, for example, can allow rats to increase and put birds at greater risk.[16]

See also

References

  1. ↑ 1.0 1.1 1.2 Chapman, J. L.; Reiss, M. J. (1999). Ecology: Principles and Applications (2nd ed.). Cambridge: Cambridge University Press. p. 128. ISBN 978-0-521-58802-7. {{cite book}}: |access-date= requires |url= (help)
  2. ↑ 2.0 2.1 2.2 2.3 Miller, G. Tyler; Spoolman, Scott E. (2019). Environmental Science (16th ed.). Boston, MA: Cengage Learning. p. 53. ISBN 978-1-337-56961-3. {{cite book}}: |access-date= requires |url= (help)
  3. ↑ 3.0 3.1 Begon, Michael; Howarth, Robert W.; Townsend, Colin R. (2014). Essentials of Ecology (4th ed.). Hoboken, NJ: Wiley. p. 269. ISBN 978-0-470-90913-3. {{cite book}}: |access-date= requires |url= (help)
  4. ↑ 4.0 4.1 Chapman, J. L.; Reiss, M. J. (1999). Ecology: Principles and Applications (2nd ed.). Cambridge: Cambridge University Press. p. 126. ISBN 978-0-521-58802-7. {{cite book}}: |access-date= requires |url= (help)
  5. ↑ 5.0 5.1 5.2 5.3 Chapman, J. L.; Reiss, M. J. (1999). Ecology: Principles and Applications (2nd ed.). Cambridge: Cambridge University Press. p. 127. ISBN 978-0-521-58802-7. {{cite book}}: |access-date= requires |url= (help)
  6. ↑ 6.0 6.1 6.2 Miller, G. Tyler; Spoolman, Scott E. (2019). Environmental Science (16th ed.). Boston, MA: Cengage Learning. p. 48. ISBN 978-1-337-56961-3. {{cite book}}: |access-date= requires |url= (help)
  7. ↑ Begon, Michael; Howarth, Robert W.; Townsend, Colin R. (2014). Essentials of Ecology (4th ed.). Hoboken, NJ: Wiley. p. 325. ISBN 978-0-470-90913-3. {{cite book}}: |access-date= requires |url= (help)
  8. ↑ Miller, G. Tyler; Spoolman, Scott E. (2019). Environmental Science (16th ed.). Boston, MA: Cengage Learning. pp. 48–49. ISBN 978-1-337-56961-3. {{cite book}}: |access-date= requires |url= (help)
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 Miller, G. Tyler; Spoolman, Scott E. (2019). Environmental Science (16th ed.). Boston, MA: Cengage Learning. p. 49. ISBN 978-1-337-56961-3. {{cite book}}: |access-date= requires |url= (help)
  10. ↑ Begon, Michael; Howarth, Robert W.; Townsend, Colin R. (2014). Essentials of Ecology (4th ed.). Hoboken, NJ: Wiley. p. 331. ISBN 978-0-470-90913-3. {{cite book}}: |access-date= requires |url= (help)
  11. ↑ 11.0 11.1 Miller, G. Tyler; Spoolman, Scott E. (2019). Environmental Science (16th ed.). Boston, MA: Cengage Learning. pp. 53–54. ISBN 978-1-337-56961-3. {{cite book}}: |access-date= requires |url= (help)
  12. ↑ 12.0 12.1 12.2 Chapman, J. L.; Reiss, M. J. (1999). Ecology: Principles and Applications (2nd ed.). Cambridge: Cambridge University Press. p. 144. ISBN 978-0-521-58802-7. {{cite book}}: |access-date= requires |url= (help)
  13. ↑ 13.0 13.1 Chapman, J. L.; Reiss, M. J. (1999). Ecology: Principles and Applications (2nd ed.). Cambridge: Cambridge University Press. p. 129. ISBN 978-0-521-58802-7. {{cite book}}: |access-date= requires |url= (help)
  14. ↑ Chapman, J. L.; Reiss, M. J. (1999). Ecology: Principles and Applications (2nd ed.). Cambridge: Cambridge University Press. pp. 129–130. ISBN 978-0-521-58802-7. {{cite book}}: |access-date= requires |url= (help)
  15. ↑ 15.0 15.1 Begon, Michael; Howarth, Robert W.; Townsend, Colin R. (2014). Essentials of Ecology (4th ed.). Hoboken, NJ: Wiley. p. 333. ISBN 978-0-470-90913-3. {{cite book}}: |access-date= requires |url= (help)
  16. ↑ 16.0 16.1 16.2 Begon, Michael; Howarth, Robert W.; Townsend, Colin R. (2014). Essentials of Ecology (4th ed.). Hoboken, NJ: Wiley. p. 270. ISBN 978-0-470-90913-3. {{cite book}}: |access-date= requires |url= (help)
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