Editing Food web (section)

=== Ecological pyramids ===
[[File:EcologicalPyramids.jpg|thumb|upright=1.5|Illustration of a range of ecological pyramids, including '''top''' pyramid of numbers, '''middle''' pyramid of biomass, and '''bottom''' pyramid of energy. The terrestrial forest (summer) and the [[English Channel]] ecosystems exhibit inverted pyramids.''Note:'' trophic levels are not drawn to scale and the pyramid of numbers excludes microorganisms and soil animals. ''Abbreviations:'' P=Producers, C1=Primary consumers, C2=Secondary consumers, C3=Tertiary consumers, S=Saprotrophs.<ref name="Odum05" />]]
[[File:Trophiclevels.jpg|thumb|A four level trophic pyramid sitting on a layer of soil and its community of decomposers.]]
[[File:TrophicEnergy.jpg|thumb|A three layer trophic pyramid linked to the biomass and energy flow concepts.]]

In a pyramid of numbers, the number of consumers at each level decreases significantly, so that a single [[top consumer]], (e.g., a [[polar bear]] or a [[human]]), will be supported by a much larger number of separate producers. There is usually a maximum of four or five links in a food chain, although food chains in [[aquatic ecosystems]] are more often longer than those on land. Eventually, all the energy in a food chain is dispersed as heat.<ref name="Odum05"/>

[[Ecological pyramid]]s place the primary producers at the base. They can depict different numerical properties of ecosystems, including numbers of individuals per unit of area, biomass (g/m<sup>2</sup>), and energy (k cal m<sup>−2</sup> yr<sup>−1</sup>). The emergent pyramidal arrangement of trophic levels with amounts of energy transfer decreasing as species become further removed from the source of production is one of several patterns that is repeated amongst the planets ecosystems.<ref name="Proulx05" /><ref name="Pimm91">{{Cite journal |last1=Pimm |first1=S. L. |last2=Lawton |first2=J. H. |last3=Cohen |first3=J. E. |title=Food web patterns and their consequences |journal=Nature |volume=350 |issue=6320 |pages=669–674 |year=1991 |url=http://www.nicholas.duke.edu/people/faculty/pimm/publications/pimmreprints/71_Pimm_Lawton_Cohen_Nature.pdf |doi=10.1038/350669a0 |url-status=dead |archive-url=https://web.archive.org/web/20100610135513/http://nicholas.duke.edu/people/faculty/pimm/publications/pimmreprints/71_Pimm_Lawton_Cohen_Nature.pdf |archive-date=2010-06-10 |bibcode=1991Natur.350..669P |s2cid=4267587 |accessdate=2011-06-13 }}</ref><ref name="Raffaelli02">{{cite journal|last = Raffaelli|first = D. |s2cid = 177263265 |title =From Elton to mathematics and back again|year = 2002| journal = Science|volume = 296|issue=5570|pages=1035–1037|doi=10.1126/science.1072080|pmid = 12004106}}</ref> The size of each level in the pyramid generally represents biomass, which can be measured as the dry weight of an organism.<ref name="Ricklefs96"/> Autotrophs may have the highest global proportion of biomass, but they are closely rivaled or surpassed by microbes.<ref name="Whitman98">{{Cite journal |last1 = Whitman |first1 = W. B. |last2 = Coleman |first2 = D. C. |last3 = Wieb |first3 = W. J. |title = Prokaryotes: The unseen majority |journal = Proc. Natl. Acad. Sci. USA |volume = 95 |pages=6578–83 |year = 1998 |doi = 10.1073/pnas.95.12.6578 |pmid = 9618454 |issue = 12 |pmc = 33863|bibcode = 1998PNAS...95.6578W |doi-access = free }}</ref><ref name="Groombridge02">{{Cite book |last1 = Groombridge |first1 = B. |last2 = Jenkins |first2 = M. |title = World Atlas of Biodiversity: Earth's Living Resources in the 21st Century |publisher = World Conservation Monitoring Centre, United Nations Environment Programme |year = 2002 |url = https://books.google.com/books?id=_kHeAXV5-XwC|isbn = 978-0-520-23668-4}}</ref>

Pyramid structure can vary across ecosystems and across time. In some instances biomass pyramids can be inverted. This pattern is often identified in aquatic and coral reef ecosystems. The pattern of biomass inversion is attributed to different sizes of producers. Aquatic communities are often dominated by producers that are smaller than the consumers that have high growth rates. Aquatic producers, such as planktonic algae or aquatic plants, lack the large accumulation of [[secondary growth]] as exists in the woody trees of terrestrial ecosystems. However, they are able to reproduce quickly enough to support a larger biomass of grazers. This inverts the pyramid. Primary consumers have longer lifespans and slower growth rates that accumulates more biomass than the producers they consume. Phytoplankton live just a few days, whereas the zooplankton eating the phytoplankton live for several weeks and the fish eating the zooplankton live for several consecutive years.<ref>{{cite book | last= Spellman| first= Frank R.| title= The Science of Water: Concepts and Applications| year= 2008| publisher= CRC Press| page= 167| url= https://books.google.com/books?id=Grivqd7tLuAC&q=However,+biomass+pyramids+can+sometimes+be+inverted.&pg=PA167| isbn= 978-1-4200-5544-3}}</ref> Aquatic predators also tend to have a lower death rate than the smaller consumers, which contributes to the inverted pyramidal pattern. Population structure, migration rates, and environmental refuge for prey are other possible causes for pyramids with biomass inverted. Energy pyramids, however, will always have an upright pyramid shape if all sources of food energy are included and this is dictated by the [[second law of thermodynamics]].<ref name="Odum05" /><ref name="Wang09">{{cite journal | last1=Wang | first1=H. | last2=Morrison | first2=W. | last3=Singh | first3=A. | last4=Weiss | first4=H. | title=Modeling inverted biomass pyramids and refuges in ecosystems | journal=Ecological Modelling | volume=220 | issue=11 | pages=1376–1382 | doi=10.1016/j.ecolmodel.2009.03.005 | year=2009 | bibcode=2009EcMod.220.1376W | url=http://people.math.gatech.edu/~weiss/pub/General_Mechanisms_Final.pdf | url-status=dead | archive-url=https://web.archive.org/web/20111007214333/http://people.math.gatech.edu/~weiss/pub/General_Mechanisms_Final.pdf | archive-date=2011-10-07 | accessdate=2011-07-05 }}</ref>