= Energy flow
{disambiguate=ecology}
{wiki}
= Energy flow
{synonym}
<Energy flow> tracks <energy> entering an <ecosystem>, passing through <organisms> and leaving its boundary. To see its quantitative effect, assume net production at one <trophic level> is $P_1=1000\,\mathrm{kJ\,m^{-2}\,day^{-1}}$ and assume a transfer efficiency $\varepsilon=0.10$ at each of two successive steps. Here $P_i$ measures the <energy> incorporated into new living material per unit <area> per day, after that level's own <cellular respiration>; it is not a count of <organisms> or a standing stock. The <joule> supplies the <energy> unit, with $1\,\mathrm{kJ}=1000\,\mathrm J$.
$$
P_{i+1}=\varepsilon P_i,\qquad P_2=100,\qquad P_3=10\quad\mathrm{kJ\,m^{-2}\,day^{-1}}.
$$
\Image[/trophic-energy-budget.png]
{title=An assumed <energy flow> budget: 1000 → 100 → 10 kJ per square metre per day}
{description=Original toy calculation with 10 percent transfer efficiency assumed at both steps. Bars use one linear scale; the labels retain visibility of the smallest bar. The differences 900 and 90 are production not transferred along the selected <food chain>, not measurements of <heat> alone. The example is separate from the four-position illustrative <food web>.}
Only $10/1000=0.01$, or one percent, of the initial production reaches level 3 in this toy <food chain>. The first step leaves $1000-100=900$ and the second $100-10=90$ outside the next level's new production. These differences can include uneaten material, wastes, and <energy> released as <heat> during <cellular respiration>; their separate amounts have not been assigned. <Detritus> can still carry <energy> to <decomposers>. The 10 percent efficiency is an assumption, not a universal law: transfer efficiencies vary with the <organisms>, food and conditions. The production ratio $P_{i+1}/P_i$ also differs from the fraction of an individual meal converted into growth. See https://openstax.org/books/biology-2e/pages/46-2-energy-flow-through-ecosystems[Clark, Douglas and Choi, Biology 2e, 46.2: production and transfer efficiency].
The shrinking budget explains why supporting another <trophic level> requires enough production below it. It does not prove an exact maximum length for every <food chain>, particularly when a <food web> imports resources or has branching diets. Lindeman's 1942 paper grouped feeding relationships into <trophic levels>, compared transfers and <cellular respiration>, and emphasized the <ecosystem> as a unit connecting living and nonliving components. That viewpoint made different <organisms> comparable through a common <energy> budget. See https://doi.org/10.2307/1930126[Lindeman, The Trophic-Dynamic Aspect of Ecology (1942)].
Material recycling and <energy flow> describe different bookkeeping. <Carbon>, <nitrogen> and <phosphorus> can be reused through <chemical reactions> and transfers between <organisms> and their surroundings. <Energy> transferred as <heat> is conserved, but is no longer the same store of chemical <energy> available to a <consumer>; photosynthetic <primary producers> need continuing <light> input. A <decomposer> helps recycle material and also releases <heat> through <metabolism>. Thus loss from one feeding budget means transfer into other forms or routes, never destruction of <energy>. See https://openstax.org/books/biology-2e/pages/46-3-biogeochemical-cycles[Clark, Douglas and Choi, Biology 2e, 46.3: material cycles]. <Conservation of energy> applies to the whole accounting.
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