Editing Food web (section)

===Trophic levels===
{{main|Trophic level}}

[[File:TrophicWeb.jpg|thumb|upright=1.8|A trophic pyramid (a) and a simplified community food web (b) illustrating ecological relations among creatures that are typical of a northern [[Boreal ecosystem|Boreal]] terrestrial ecosystem. The trophic pyramid roughly represents each level's biomass (usually measured as total dry weight). Plants generally have the greatest biomass. Names of trophic categories are shown to the right of the pyramid. Like many wetlands, some ecosystems do not organize as a strict pyramid because aquatic plants are less productive than long-lived terrestrial plants such as trees. Ecological trophic pyramids are typically one of three kinds: 1) pyramid of numbers, 2) pyramid of biomass, or 3) pyramid of energy.<ref name="Odum05">{{cite book | last1=Odum | first1=E. P. | last2=Barrett | first2=G. W. | title=Fundamentals of Ecology | edition=5th | publisher=Brooks/Cole, a part of Cengage Learning | year=2005 | isbn=978-0-534-42066-6 | url=http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_isbn=9780534420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag=true&topicName= | url-status=dead | archive-url=https://web.archive.org/web/20110820163059/http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_isbn=9780534420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag=true&topicName= | archive-date=2011-08-20 }}</ref>]]

Food webs have trophic levels and positions. Basal species, such as plants, form the first level and are the resource-limited species that feed on no other living creature in the web. Basal species can be autotrophs or [[detritivores]], including "decomposing organic material and its associated microorganisms which we defined as detritus, micro-inorganic material and associated microorganisms (MIP), and vascular plant material."<ref name="Tavares-Cromar96">{{cite journal | last1=Tavares-Cromar | first1=A. F. | last2=Williams | first2=D. D. | title=The importance of temporal resolution in food web analysis: Evidence from a detritus-based stream | year=1996 | journal=Ecological Monographs | volume=66 | issue=1 | pages=91–113 | doi= 10.2307/2963482 | url=https://tspace.library.utoronto.ca/bitstream/1807/768/2/Importance_of_temporal_resolution_in_food_web_analysis.pdf | jstor=2963482 | bibcode=1996EcoM...66...91T | hdl=1807/768 | hdl-access=free }}</ref>{{rp|94}} Most autotrophs capture the sun's energy in [[chlorophyll]], but some autotrophs (the [[lithotroph|chemolithotrophs]]) obtain energy by the chemical oxidation of inorganic compounds and can grow in dark environments, such as the sulfur bacterium ''[[Thiobacillus]]'', which lives in hot [[sulfur springs]]. The top level has top (or apex) predators that no other species kills directly for their food resource needs. The intermediate levels are filled with omnivores that feed on more than one trophic level and cause energy to flow through several food pathways starting from a basal species.<ref name="Pimm79" />

In the simplest scheme, the first trophic level (level 1) is plants, then herbivores (level 2), and then carnivores (level 3). The trophic level equals one more than the chain length, which is the number of links connecting to the base. The base of the food chain (primary producers or [[detritivore]]s) is set at zero.<ref name="Pimm91" /><ref name="Cousins85">{{cite journal | last1=Cousins | first1=S. | title=Ecologists build pyramids again. | journal=New Scientist | volume=1463 | pages=50–54 | url=https://books.google.com/books?id=NOPpwVvNu44C&q=trophic+level&pg=PA51 | date=1985-07-04 }}{{Dead link|date=May 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Ecologists identify feeding relations and organize species into trophic species through extensive gut content analysis of different species. The technique has been improved through the use of stable isotopes to better trace energy flow through the web.<ref name="McCann07">{{cite journal|last1=McCann |first1=K. |title=Protecting biostructure |pmid=17330028 |doi=10.1038/446029a |journal=Nature |year=2007 |volume=446 |page=29 |issue=7131 |bibcode=2007Natur.446...29M |s2cid=4428058 |doi-access=free }}</ref> It was once thought that omnivory was rare, but recent evidence suggests otherwise. This realization has made trophic classifications more complex.<ref name="Thompson07">{{cite journal |last1=Thompson |first1=R. M. |last2=Hemberg |first2=M. |last3=Starzomski |first3=B. M. |last4=Shurin |first4=J. B. |title=Trophic levels and trophic tangles: The prevalence of omnivory in real food webs. |journal=Ecology |volume=88 |issue=3 |pages=612–617 |doi=10.1890/05-1454 |url=http://myweb.dal.ca/br238551/thompson_hem_star_shur_ecology07.pdf |pmid=17503589 |date=March 2007 |bibcode=2007Ecol...88..612T |url-status=dead |archive-url=https://web.archive.org/web/20110815150110/http://myweb.dal.ca/br238551/thompson_hem_star_shur_ecology07.pdf |archive-date=2011-08-15 |accessdate=2011-06-10 }}</ref>