= Trophic level
{title2=$\mathrm{TL}$}
{wiki}
A <trophic level> describes feeding position relative to <primary producers>. Assign <primary producers> level 1; in the illustrated <food chain>, <zooplankton> occupy level 2, small <fish> level 3, and large <fish> level 4. Following a single feeding path adds one level at each step. The labels make dependence on the initial resource visible, but a branching <food web> can give an <organism> several feeding paths. See https://openstax.org/books/biology-2e/pages/46-1-ecology-of-ecosystems[Clark, Douglas and Choi, Biology 2e, 46.1: feeding positions and branching food webs].
An <omnivore> can obtain food from more than one <trophic level>. As an explicitly assumed diet-weighted example, suppose an <omnivore> gets half its assimilated food <energy> from level-1 <primary producers> and half from level-2 <consumers>. Define its <trophic level> as one plus the weighted <mean> level of its food: $1+(1/2)1+(1/2)2=2.5$. This computed position depends on the specified diet and weighting convention; it is not an immutable property of the <species>. A different life stage or diet can change it.
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