Editing Ecology (section)

=== Trophic levels ===
{{Main|Trophic level}}

[[File:TrophicWeb.jpg|thumb|upright=2.05|A trophic pyramid (a) and a food-web (b) illustrating [[ecological relationship]]s among creatures that are typical of a northern [[boreal ecosystem|boreal]] terrestrial ecosystem. The trophic pyramid roughly represents the biomass (usually measured as total dry-weight) at each level. Plants generally have the greatest biomass. Names of trophic categories are shown to the right of the pyramid. Some ecosystems, such as many wetlands, do not organize as a strict pyramid, because aquatic plants are not as productive as 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"/>{{rp|598}}]]

A trophic level (from Greek ''troph'', τροφή, trophē, meaning "food" or "feeding") is "a group of organisms acquiring a considerable majority of its energy from the lower adjacent level (according to [[ecological pyramid]]s) nearer the abiotic source."<ref name="Hariston93"/>{{rp|383}} Links in food webs primarily connect feeding relations or [[trophism]] among species. Biodiversity within ecosystems can be organized into trophic pyramids, in which the vertical dimension represents feeding relations that become further removed from the base of the food chain up toward top predators, and the horizontal dimension represents the [[Relative species abundance|abundance]] or biomass at each level.<ref name="Duffy07"/> When the relative abundance or biomass of each species is sorted into its respective trophic level, they naturally sort into a 'pyramid of numbers'.<ref name="Elton27" />

Species are broadly categorized as [[autotrophs]] (or [[primary producers]]), [[heterotrophs]] (or [[consumer (food chain)|consumer]]s), and [[Detritivore]]s (or [[decomposers]]). Autotrophs are organisms that produce their own food (production is greater than respiration) by photosynthesis or [[chemosynthesis]]. Heterotrophs are organisms that must feed on others for nourishment and energy (respiration exceeds production).<ref name="Odum05" /> Heterotrophs can be further sub-divided into different functional groups, including [[primary consumers]] (strict herbivores), [[Trophic dynamics|secondary consumers]] ([[carnivorous]] predators that feed exclusively on herbivores), and tertiary consumers (predators that feed on a mix of herbivores and predators).<ref name="David03"/> Omnivores do not fit neatly into a functional category because they eat both plant and animal tissues. It has been suggested that omnivores have a greater functional influence as predators because compared to herbivores, they are relatively inefficient at grazing.<ref name="Oksanen91"/>

Trophic levels are part of the [[holistic]] or [[complex systems]] view of ecosystems.<ref name="Loehle88"/><ref name="Ulanowicz79"/> Each trophic level contains unrelated species that are grouped together because they share common ecological functions, giving a macroscopic view of the system.<ref name="Li00"/> While the notion of trophic levels provides insight into energy flow and top-down control within food webs, it is troubled by the prevalence of omnivory in real ecosystems. This has led some ecologists to "reiterate that the notion that species clearly aggregate into discrete, homogeneous trophic levels is fiction."<ref name="Polis96"/>{{rp|815}} Nonetheless, recent studies have shown that real trophic levels do exist, but "above the herbivore trophic level, food webs are better characterized as a tangled web of omnivores."<ref name="Thompson07"/>{{rp|612}}