View source for Energy flow (ecology)

{{Short description|Flow of energy through food chains in ecological energetics}}
{{Other uses|Energy flow (disambiguation){{!}}Energy flow}}

[[File:TrophicWeb.jpg|thumb|upright=1.35|A food pyramid and a corresponding food web, demonstrating some of the simpler patterns in a food web.]]
[[File:Diagram of Trophic Layers & Energy Transfer in an Ecosystem.svg|thumb|A graphic representation of energy transfer between trophic layers in an ecosystem.]]

'''Energy flow''' is the flow of energy through living things within an ecosystem.<ref name="Lindemanm0©_1942">{{cite journal|vauthors=Lindeman RL|date=1942|title=The Trophic-Dynamic Aspect of Ecology|url=https://www.fcnym.unlp.edu.ar/catedras/ecocomunidades/Lindman_1942.pdf|journal=Ecology|volume=23|issue=4|pages=399–417|doi=10.2307/1930126|jstor=1930126|access-date=2020-12-04|archive-date=2017-03-29|archive-url=https://web.archive.org/web/20170329165523/http://www.fcnym.unlp.edu.ar/catedras/ecocomunidades/Lindman_1942.pdf|url-status=dead}}</ref> All living organisms can be organized into [[Primary producer|producers]] and [[Consumer (food chain)|consumers]], and those producers and consumers can further be organized into a [[food chain]].<ref name="Briand_1987">{{cite journal | vauthors = Briand F, Cohen JE | title = Environmental correlates of food chain length | journal = Science | location = New York, N.Y. | volume = 238 | issue = 4829 | pages = 956–60 | date = November 1987 | pmid = 3672136 | doi = 10.1126/science.3672136 | bibcode = 1987Sci...238..956B }}</ref><ref name="Vander_Zanden_1999">{{cite journal | vauthors = Vander Zanden MJ, Shuter BJ, Lester N, Rasmussen JB | title = Patterns of Food Chain Length in Lakes: A Stable Isotope Study | journal = The American Naturalist | volume = 154 | issue = 4 | pages = 406–416 | date = October 1999 | pmid = 10523487 | doi = 10.1086/303250 | s2cid = 4424697 }}</ref> Each of the levels within the food chain is a trophic level.<ref name="Lindemanm0©_1942" /> In order to more efficiently show the quantity of organisms at each trophic level, these food chains are then organized into trophic pyramids.<ref name="Lindemanm0©_1942" /> The arrows in the food chain show that the energy flow is unidirectional, with the head of an arrow indicating the direction of energy flow; energy is lost as heat at each step along the way.<ref name="Briand_1987" /><ref name="Vander_Zanden_1999" />

The unidirectional flow of energy and the successive loss of energy as it travels up the [[food web]] are patterns in energy flow that are governed by [[thermodynamics]], which is the theory of energy exchange between systems.<ref>{{cite book| vauthors = Sharma JP |url=https://www.worldcat.org/oclc/908431622|title=Environmental studies|date=2009|publisher=University Science Press|isbn=978-81-318-0641-8|edition=3rd|location=New Delhi|oclc=908431622}}</ref><ref>{{cite book| vauthors = Van Ness HC |url=https://www.worldcat.org/oclc/849744641|title=Understanding thermodynamics|publisher=Dover Publications, Inc|year=1969|isbn=978-1-62198-625-6|edition=Dover|location=New York|oclc=849744641}}</ref> Trophic dynamics relates to thermodynamics because it deals with the transfer and transformation of energy (originating externally from the sun via solar radiation) to and among organisms.<ref name="Lindemanm0©_1942" />

== Energetics and the carbon cycle ==
[[File:Carbon_Cycle-animated_forest.gif|thumb|309x309px|The carbon cycle of a terrestrial ecosystem.<ref>{{cite web |title= Carbon Cycle |url=http://www.nps.gov/olym/hand/process/ccycle.htm|url-status=live|archive-url= https://web.archive.org/web/20060812063958/https://www.nps.gov/olym/hand/process/ccycle.htm |archive-date= 12 August 2006 }}</ref> Beginning with [[photosynthesis]], water (blue) and carbon dioxide (white) from the air are taken in with solar energy (yellow), and are converted into plant energy (green).<ref name="Whitmarsh_1999" /> <q>100×10<sup>15</sup> grams of carbon/year fixed by photosynthetic organisms, which is equivalent to 4×10<sup>18</sup> kJ/yr = 4×10<sup>21</sup> J/yr of free energy.</q> [[Cellular respiration]] is the reverse reaction, wherein energy of plants is taken in and carbon dioxide and water are given off. The carbon dioxide and water produced can be recycled back into plants.]]The first step in energetics is [[photosynthesis]], wherein water and carbon dioxide from the air are taken in with energy from the sun, and are converted into [[Dioxygen in biological reactions|oxygen]] and glucose.<ref name="Whitmarsh_1999" /> [[Cellular respiration]] is the reverse reaction, wherein oxygen and sugar are taken in and release energy as they are converted back into carbon dioxide and water. The carbon dioxide and water produced by respiration can be recycled back into plants.

Energy loss can be measured either by efficiency (how much energy makes it to the next level), or by biomass (how much living material exists at those levels at one point in time, measured by standing crop).<ref name="Lindemanm0©_1942" /> Of all the net primary productivity at the producer trophic level, in general only 10% goes to the next level, the primary consumers, then only 10% of that 10% goes on to the next trophic level, and so on up the food pyramid.<ref name="Lindemanm0©_1942" /> Ecological efficiency may be anywhere from 5% to 20% depending on how efficient or inefficient that ecosystem is.<ref>{{cite journal| vauthors = Teal JM |date=1962|title=Energy flow in the salt marsh ecosystem of Georgia|url=https://www.jstor.org/stable/1933451|journal=Ecology|volume=43|issue=4|pages=614–624|doi=10.2307/1933451|jstor=1933451}}</ref><ref name="Lindemanm0©_1942" /> This decrease in efficiency occurs because organisms need to perform cellular respiration to survive, and energy is lost as heat when cellular respiration is performed.<ref name="Lindemanm0©_1942" /> That is also why there are fewer tertiary consumers than there are producers.<ref name="Lindemanm0©_1942" />

== Primary production ==
{{Main|Primary production}}
A [[Primary producer|producer]] is any organism that performs [[photosynthesis]].<ref>{{cite book | vauthors = Morris J, Hartl DL, Knoll AH, Lue R, Michael M | date = 2019 | title = Biology: How Life Works | edition = 3rd | publisher = W. H. Freeman | isbn = 978-1319017637 }}</ref> Producers are important because they convert energy from the sun into a storable and usable chemical form of energy, glucose,<ref name="Lindemanm0©_1942" /> as well as oxygen. The producers themselves can use the energy stored in glucose to perform cellular respiration. Or, if the producer is consumed by herbivores in the next trophic level, some of the energy is passed on up the pyramid.<ref name="Lindemanm0©_1942" /> The glucose stored within producers serves as food for [[Consumer (food chain)|consumers]], and so it is only through producers that consumers are able to access the sun’s energy.<ref name="Lindemanm0©_1942" /><ref name="Whitmarsh_1999">{{cite book | vauthors = Whitmarsh J, Govindjee | date = 1999 | chapter = The photosynthetic process | veditors = Singhal GS, Renger G, Sopory SK, Irrgang KD, Govindjee | title = Concepts in photobiology: photosynthesis and photomorphogenesis | location = Boston | publisher = [[Kluwer Academic Publishers]] | pages = 11–51 | isbn = 978-0-7923-5519-9 }}. <q>100×10<sup>15</sup> grams of carbon/year fixed by photosynthetic organisms, which is equivalent to 4×10<sup>18</sup> kJ/yr = 4×10<sup>21</sup> J/yr of free energy stored as reduced carbon.</q></ref> Some examples of primary producers are algae, mosses, and other plants such as grasses, trees, and shrubs.<ref name="Lindemanm0©_1942" />

[[Chemosynthetic]] bacteria perform a process similar to photosynthesis, but instead of energy from the sun they use energy stored in chemicals like hydrogen sulfide.<ref name="Kellermann_2012">{{cite journal | vauthors = Kellermann MY, Wegener G, Elvert M, Yoshinaga MY, Lin YS, Holler T, Mollar XP, Knittel K, Hinrichs KU | title = Autotrophy as a predominant mode of carbon fixation in anaerobic methane-oxidizing microbial communities | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 47 | pages = 19321–6 | date = November 2012 | pmid = 23129626 | pmc = 3511159 | doi = 10.1073/pnas.1208795109 | bibcode = 2012PNAS..10919321K | doi-access = free }}</ref><ref name="Cavenaugh_1981">{{cite journal | vauthors = Cavanaugh CM, Gardiner SL, Jones ML, Jannasch HW, Waterbury JB | title = Prokaryotic Cells in the Hydrothermal Vent Tube Worm Riftia pachyptila Jones: Possible Chemoautotrophic Symbionts | journal = Science | location = New York, N.Y. | volume = 213 | issue = 4505 | pages = 340–2 | date = July 1981 | pmid = 17819907 | doi = 10.1126/science.213.4505.340 | bibcode = 1981Sci...213..340C }}</ref> This process, referred to as [[chemosynthesis]], usually occurs deep in the ocean at hydrothermal vents that produce heat and chemicals such as hydrogen, hydrogen sulfide and methane.<ref name="Kellermann_2012" /> Chemosynthetic bacteria can use the energy in the bonds of the hydrogen sulfide and oxygen to convert carbon dioxide to glucose, releasing water and sulfur in the process.<ref name="Cavenaugh_1981" /> Organisms that consume the chemosynthetic bacteria can take in the glucose and use oxygen to perform cellular respiration, similar to herbivores consuming producers.

One of the factors that controls primary production is the amount of energy that enters the producer(s), which can be measured using productivity.<ref name="Amthor_2001">{{cite journal | vauthors = Amthor JS, Baldocchi DD | title = Terrestrial higher plant respiration and net primary production. | journal = Terrestrial Global Productivity | date = 2001 | pages = 33–59 | doi =  10.1016/B978-012505290-0/50004-1| isbn = 9780125052900 }}</ref><ref name="Sigman_2012">{{cite journal | vauthors = Sigman DM, Hain MP | title = The biological productivity of the ocean. | journal = Nature Education Knowledge. | date = 2012 | volume = 3 | issue = 6 | pages = 1–6 | url = https://sigman.princeton.edu/sites/default/files/pdfs/Sigman_and_Hain_2012_NatureEdu.pdf }}</ref><ref name="Lindemanm0©_1942" /> Only one percent of solar energy enters the producer, the rest bounces off or moves through.<ref name="Sigman_2012" /> Gross primary productivity is the amount of energy the producer actually gets.<ref name="Sigman_2012" /><ref name="Cebrian_1999">{{cite journal | vauthors = Cebrian J | title = Patterns in the Fate of Production in Plant Communities | journal = The American Naturalist | volume = 154 | issue = 4 | pages = 449–468 | date = October 1999 | pmid = 10523491 | doi = 10.1086/303244 | s2cid = 4384243 }}</ref> Generally, 60% of the energy that enters the producer goes to the producer’s own respiration.<ref name="Amthor_2001" /> The net primary productivity is the amount that the plant retains after the amount that it used for cellular respiration is subtracted.<ref name="Sigman_2012" /> Another factor controlling primary production is organic/inorganic nutrient levels in the water or soil that the producer is living in.<ref name="Cebrian_1999" />

== Secondary production ==
Secondary production is the use of energy stored in plants converted by consumers to their own biomass. Different ecosystems have different levels of consumers, all end with one top consumer. Most energy is stored in organic matter of plants, and as the consumers eat these plants they take up this energy. This energy in the herbivores and omnivores is then consumed by carnivores. There is also a large amount of energy that is in primary production and ends up being waste or litter, referred to as detritus. The detrital food chain includes a large amount of microbes, macroinvertebrates, meiofauna, fungi, and bacteria. These organisms are consumed by omnivores and carnivores and account for a large amount of secondary production.<ref name="Allan_2007">{{cite book| vauthors = Allan JD, Castillo MM |url=https://www.worldcat.org/oclc/144222191|title=Stream ecology: structure and function of running waters|date=2007|publisher=Springer|isbn=978-1-4020-5582-9|edition=2nd |location=Dordrecht|oclc=144222191}}</ref> Secondary consumers can vary widely in how efficient they are in consuming.<ref name="Smith_2015">{{cite book| vauthors = Smith TM, Smith RL | date = 2015|url=https://www.worldcat.org/oclc/914328590|title=Elements of ecology |isbn=978-1-292-07741-3|edition=9th|location=Boston|oclc=914328590}}</ref> The efficiency of energy being passed on to consumers is estimated to be around 10%.<ref name="Smith_2015" /> Energy flow through consumers differs in aquatic and terrestrial environments.

===In aquatic environments===

Heterotrophs contribute to secondary production and it is dependent on primary productivity and the net primary products.<ref name="Smith_2015" /> Secondary production is the energy that herbivores and decomposers use and thus depends on primary productivity.<ref name="Smith_2015" /> Primarily herbivores and decomposers consume all the carbon from two main organic sources in [[aquatic ecosystem]]s, autochthonous and allochthonous.<ref name="Smith_2015" /> Autochthonous carbon comes from within the ecosystem and includes aquatic plants, algae and phytoplankton. Allochthonous carbon from outside the ecosystem is mostly dead organic matter from the terrestrial ecosystem entering the water.<ref name="Smith_2015" /> In stream ecosystems, approximately 66% of annual energy input can be washed downstream. The remaining amount is consumed and lost as heat.<ref name="Fisher_1973">{{cite journal| vauthors = Fisher SG, Likens GE |date= February 1973 |title=Energy Flow in Bear Brook, New Hampshire: An Integrative Approach to Stream Ecosystem Metabolism |journal=Ecological Monographs|language=en|volume=43|issue=4|pages=421–439|doi=10.2307/1942301|jstor= 1942301 }}</ref>

===In terrestrial environments===

Secondary production is often described in terms of [[trophic level]]s, and while this can be useful in explaining relationships it overemphasizes the rarer interactions. Consumers often feed at multiple trophic levels.<ref name="Hairston_1993">{{cite journal| vauthors = Hairston Jr NG, Hairston Sr NG |date= September 1993 |title=Cause-Effect Relationships in Energy Flow, Trophic Structure, and Interspecific Interactions |journal=The American Naturalist |volume=142|issue=3|pages=379–411|doi=10.1086/285546|s2cid= 55279332 |hdl=1813/57238 |hdl-access=free }}</ref>  Energy transferred above the third trophic level is relatively unimportant.<ref name="Hairston_1993" /> The assimilation efficiency can be expressed by the amount of food the consumer has eaten, how much the consumer assimilates and what is expelled as feces or urine. While a portion of the energy is used for respiration, another portion of the energy goes towards biomass in the consumer.<ref name="Smith_2015" /> There are two major food chains: The primary food chain is the energy coming from autotrophs and passed on to the consumers; and the second major food chain is when carnivores eat the herbivores or decomposers that consume the autotrophic energy.<ref name="Smith_2015" /> Consumers are broken down into primary consumers, secondary consumers and tertiary consumers. Carnivores have a much higher assimilation of energy, about 80% and herbivores have a much lower efficiency of approximately 20 to 50%.<ref name="Smith_2015" /> Energy in a system can be affected by animal emigration/immigration. The movements of organisms are significant in terrestrial ecosystems.<ref name="Fisher_1973" /> Energetic consumption by herbivores in terrestrial ecosystems has a low range of ~3-7%.<ref name="Fisher_1973" /> The flow of energy is similar in many terrestrial environments. The fluctuation in the amount of net primary product consumed by herbivores is generally low. This is in large contrast to aquatic environments of lakes and ponds where grazers have a much higher consumption of around ~33%.<ref name="Fisher_1973" /> Ectotherms and endotherms have very different assimilation efficiencies.<ref name="Smith_2015" />

== Detritivores ==
Detritivores consume organic material that is decomposing and are in turn consumed by carnivores.<ref name="Smith_2015" /> Predator productivity is correlated with prey productivity. This confirms that the primary productivity in ecosystems affects all productivity following.<ref name="Wallace_1999">{{cite journal| vauthors = Wallace JB, Eggert SL, Meyer JL, Webster JR | title = Effects of resource limitation on a detrital‐based ecosystem. | journal = Ecological Monographs | date = November 1999 | volume = 69 | issue = 4 | pages = 409–42 | doi = 10.1890/0012-9615(1999)069[0409:eorloa]2.0.co;2}}</ref>

Detritus is a large portion of organic material in ecosystems. Organic material in temperate forests is mostly made up of dead plants, approximately 62%.<ref name="Hairston_1993" />

In an aquatic ecosystem, leaf matter that falls into streams gets wet and begins to leech organic material. This happens rather quickly and will attract microbes and invertebrates. The leaves can be broken down into large pieces called coarse particulate organic matter (CPOM).<ref name="Allan_2007" /> The CPOM is rapidly colonized by microbes. [[Meiofauna]] is extremely important to secondary production in stream ecosystems.<ref name="Allan_2007" /> Microbes breaking down and colonizing this leaf matter are very important to the detritovores. The detritovores make the leaf matter more edible by releasing compounds from the tissues; it ultimately helps soften them.<ref name="Allan_2007" /> As leaves decay nitrogen will decrease since cellulose and lignin in the leaves is difficult to break down. Thus the colonizing microbes bring in nitrogen in order to aid in the decomposition. Leaf breakdown can depend on initial nitrogen content, season, and species of trees. The species of trees can have variation when their leaves fall. Thus the breakdown of leaves is happening at different times, which is called a mosaic of microbial populations.<ref name="Allan_2007" />

Species effect and diversity in an ecosystem can be analyzed through their performance and efficiency.<ref>{{cite journal | vauthors = Cardinale BJ, Srivastava DS, Duffy JE, Wright JP, Downing AL, Sankaran M, Jouseau C | title = Effects of biodiversity on the functioning of trophic groups and ecosystems | journal = Nature | volume = 443 | issue = 7114 | pages = 989–92 | date = October 2006 | pmid = 17066035 | doi = 10.1038/nature05202 | bibcode = 2006Natur.443..989C | s2cid = 4426751 }}</ref> In addition, secondary production in streams can be influenced heavily by detritus that falls into the streams; production of benthic fauna biomass and abundance decreased an additional 47–50% during a study of litter removal and exclusion.<ref name="Wallace_1999" />

== Energy flow across ecosystems ==
Research has demonstrated that primary producers [[Carbon fixation|fix carbon]] at similar rates across ecosystems.<ref name="Cebrian_1999" /> Once carbon has been introduced into a system as a viable source of energy, the mechanisms that govern the flow of energy to higher trophic levels vary across ecosystems. Among aquatic and terrestrial ecosystems, patterns have been identified that can account for this variation and have been divided into two main pathways of control: top-down and bottom-up.<ref name="Shurin_2006">{{cite journal | vauthors = Shurin JB, Gruner DS, Hillebrand H | title = All wet or dried up? Real differences between aquatic and terrestrial food webs | journal = Proceedings. Biological Sciences | volume = 273 | issue = 1582 | pages = 1–9 | date = January 2006 | pmid = 16519227 | pmc = 1560001 | doi = 10.1098/rspb.2005.3377 }}</ref><ref name="La_Pierre_2015">{{cite book| vauthors = La Pierre K, Hanley T |title=Trophic Ecology: Bottom-Up and Top-Down Interactions Across Aquatic and Terrestrial Systems|publisher=Cambridge University Press|year=2015|isbn=9781316299692|pages=55–85}}</ref> The acting mechanisms within each pathway ultimately regulate community and trophic level structure within an ecosystem to varying degrees.<ref name="Gruner_2008">{{cite journal | vauthors = Gruner DS, Smith JE, Seabloom EW, Sandin SA, Ngai JT, Hillebrand H, Harpole WS, Elser JJ, Cleland EE, Bracken ME, Borer ET, Bolker BM | display-authors = 6 | title = A cross-system synthesis of consumer and nutrient resource control on producer biomass | journal = Ecology Letters | volume = 11 | issue = 7 | pages = 740–55 | date = July 2008 | pmid = 18445030 | doi = 10.1111/j.1461-0248.2008.01192.x | doi-access =  }}</ref> Bottom-up controls involve mechanisms that are based on resource quality and availability, which control primary productivity and the subsequent flow of energy and biomass to higher trophic levels.<ref name="La_Pierre_2015" /> Top-down controls involve mechanisms that are based on consumption by consumers.<ref name="Gruner_2008" /><ref name="La_Pierre_2015" /> These mechanisms control the rate of energy transfer from one trophic level to another as herbivores or predators feed on lower trophic levels.<ref name="Shurin_2006" />

=== Aquatic vs terrestrial ecosystems ===
Much variation in the flow of energy is found within each type of ecosystem, creating a challenge in identifying variation between ecosystem types. In a general sense, the flow of energy is a function of primary productivity with temperature, water availability, and light availability.<ref name="Ricklefs_2000">{{cite book| vauthors = Ricklefs RE, Miller GL |  title=Ecology|date=2000|publisher=W.H. Freeman & Co |isbn=0-7167-2829-X|edition=4th |location=New York|oclc=40734932}}</ref> For example, among aquatic ecosystems, higher rates of production are usually found in large rivers and shallow lakes than in deep lakes and clear headwater streams.<ref name="Ricklefs_2000" /> Among terrestrial ecosystems, marshes, swamps, and tropical rainforests have the highest primary production rates, whereas tundra and alpine ecosystems have the lowest.<ref name="Ricklefs_2000" /> The relationships between primary production and environmental conditions have helped account for variation within ecosystem types, allowing ecologists to demonstrate that energy flows more efficiently through aquatic ecosystems than terrestrial ecosystems due to the various bottom-up and top-down controls in play.<ref name="La_Pierre_2015" />

==== Bottom-up ====
The strength of bottom-up controls on energy flow are determined by the nutritional quality, size, and growth rates of primary producers in an ecosystem.<ref name="Cebrian_1999" /><ref name="Shurin_2006" /> [[Photosynthesis|Photosynthetic]] material is typically rich in [[nitrogen]] (N) and [[phosphorus]] (P) and supplements the high herbivore demand for N and P across all ecosystems.<ref name="Cebrian_2004">{{cite journal| vauthors = Cebrian J, Lartigue J |date=2004|title=Patterns of Herbivory and Decomposition in Aquatic and Terrestrial Ecosystems |journal=Ecological Monographs |volume=74|issue=2|pages=237–259|doi=10.1890/03-4019 }}</ref> Aquatic primary production is dominated by small, single-celled [[phytoplankton]] that are mostly composed of photosynthetic material, providing an efficient source of these nutrients for herbivores.<ref name="Shurin_2006" /> In contrast, multi-cellular terrestrial plants contain many large supporting [[cellulose]] structures of high carbon but low nutrient value.<ref name="Shurin_2006" /> Because of this structural difference, aquatic primary producers have less biomass per photosynthetic tissue stored within the aquatic ecosystem than in the forests and grasslands of terrestrial ecosystems.<ref name="Shurin_2006" /> This low biomass relative to photosynthetic material in aquatic ecosystems allows for a more efficient turnover rate compared to terrestrial ecosystems.<ref name="Shurin_2006" /> As phytoplankton are consumed by herbivores, their enhanced growth and reproduction rates sufficiently replace lost biomass and, in conjunction with their nutrient dense quality, support greater secondary production.<ref name="Shurin_2006" />

Additional factors impacting primary production includes inputs of N and P, which occurs at a greater magnitude in aquatic ecosystems.<ref name="Shurin_2006" /> These nutrients are important in stimulating [[plant growth]] and, when passed to higher trophic levels, stimulate consumer biomass and growth rate.<ref name="La_Pierre_2015" /><ref name="Ricklefs_2000" /> If either of these nutrients are in short supply, they can limit overall primary production.<ref name="Allan_2007" /> Within lakes, P tends to be the greater limiting nutrient while both N and P limit primary production in rivers.<ref name="La_Pierre_2015" /> Due to these limiting effects, nutrient inputs can potentially alleviate the limitations on net primary production of an aquatic ecosystem.<ref name="Gruner_2008" /> Allochthonous material washed into an aquatic ecosystem introduces N and P as well as energy in the form of carbon molecules that are readily taken up by primary producers.<ref name="Allan_2007" /> Greater inputs and increased nutrient concentrations support greater net primary production rates, which in turn supports greater secondary production.<ref name="Cebrian_2004" />

==== Top-down ====
Top-down mechanisms exert greater control on aquatic primary producers due to the roll of consumers within an aquatic food web.<ref name="Gruner_2008" /> Among consumers, herbivores can mediate the impacts of trophic cascades by bridging the flow of energy from primary producers to predators in higher trophic levels.<ref name="Schmitz_2008">{{cite journal| vauthors = Schmitz OJ | date = December 2008 |title=Herbivory from Individuals to Ecosystems |journal=Annual Review of Ecology, Evolution, and Systematics |volume=39|issue=1|pages=133–152|doi=10.1146/annurev.ecolsys.39.110707.173418 }}</ref> Across ecosystems, there is a consistent association between herbivore growth and producer nutritional quality.<ref name="Cebrian_2004" /> However, in aquatic ecosystems, primary producers are consumed by herbivores at a rate four times greater than in terrestrial ecosystems.<ref name="Shurin_2006"/> Although this topic is highly debated, researchers have attributed the distinction in herbivore control to several theories, including producer to consumer size ratios and herbivore selectivity.<ref name="Whitmarsh_1999" />

[[File:Lake Superior Food Web.pdf|thumb|upright=1.3|A freshwater food web demonstrating the size differences between each trophic level. Primary producers tend to be small algal cells. Herbivores tend to be small macro-invertebrates. Predators tend to be larger fish.<ref>{{cite journal | vauthors = Krause AE, Frank KA, Mason DM, Ulanowicz RE, Taylor WW | title = Compartments revealed in food-web structure | journal = Nature | volume = 426 | issue = 6964 | pages = 282–5 | date = November 2003 | pmid = 14628050 | doi = 10.1038/nature02115 | bibcode = 2003Natur.426..282K | hdl = 2027.42/62960 | s2cid = 1752696 | hdl-access = free }}</ref>]][[Modelling biological systems|Modeling]] of top-down controls on primary producers suggests that the greatest control on the flow of energy occurs when the size ratio of consumer to primary producer is the highest.<ref name="Shurin_2005">{{cite journal| vauthors = Shurin JB, Seabloom EW |date=2005|title=The strength of trophic cascades across ecosystems: predictions from allometry and energetics |journal=Journal of Animal Ecology|language=en|volume=74|issue=6|pages=1029–1038|doi=10.1111/j.1365-2656.2005.00999.x|issn=1365-2656|doi-access=free}}</ref> The size distribution of organisms found within a single trophic level in aquatic systems is much narrower than that of terrestrial systems.<ref name="Shurin_2006"/> On land, the consumer size ranges from smaller than the plant it consumes, such as an insect, to significantly larger, such as an [[ungulate]], while in aquatic systems, consumer body size within a trophic level varies much less and is strongly correlated with trophic position.<ref name="Shurin_2006" /> As a result, the size difference between producers and consumers is consistently larger in aquatic environments than on land, resulting in stronger herbivore control over aquatic primary producers.<ref name="Shurin_2006" />

Herbivores can potentially control the fate of [[organic matter]] as it is cycled through the food web.<ref name="Schmitz_2008" /> Herbivores tend to select nutritious plants while avoiding plants with structural [[Defense mechanism (biology)|defense mechanisms]].<ref name="Shurin_2006"/> Like support structures, defense structures are composed of nutrient poor, high carbon cellulose.<ref name="Schmitz_2008" /> Access to nutritious food sources enhances herbivore metabolism and energy demands, leading to greater removal of primary producers.<ref name="Cebrian_1999" /> In aquatic ecosystems, phytoplankton are highly nutritious and generally lack defense mechanisms.<ref name="Schmitz_2008" /> This results in greater top-down control because consumed plant matter is quickly released back into the system as labile [[Biodegradable waste|organic waste]].<ref name="Allan_2007" /><ref name="Schmitz_2008" /> In terrestrial ecosystems, primary producers are less nutritionally dense and are more likely to contain defense structures.<ref name="Shurin_2006" /> Because herbivores prefer nutritionally dense plants and avoid plants or plant parts with defense structures, a greater amount of plant matter is left unconsumed within the ecosystem.<ref name="Schmitz_2008" /> Herbivore avoidance of low-quality plant matter may be why terrestrial systems exhibit weaker top-down control on the flow of energy.<ref name="Shurin_2006" />

== See also ==
* [[Food web]]
* [[Ecological stoichiometry]]
* [[Energy]]

== References ==
{{Reflist}}

== Further reading ==
{{refbegin}}
* {{cite journal | vauthors = Podolinsky S |year=2004|title=Socialism and the Unity of Physical Forces|journal=[[Organization & Environment]]|volume=17|issue=1|pages=61–75|doi=10.1177/1086026603262092|s2cid=144332347}}
* {{cite book| vauthors = Weiner DR |year=2000|title=Models of Nature: Ecology, Conservation and Cultural Revolution in Soviet Russia|publisher=[[University of Pittsburgh Press]]|location=U.S.}}
{{refend}}

{{modelling ecosystems|state=expanded}}

{{DEFAULTSORT:Energy Flow}}
[[Category:Ecology terminology]]
[[Category:Energy]]
[[Category:Environmental science]]
[[Category:Ecological economics]]