Consumption

Trophic Level

Lindemann (1942) introduced the ecological concept of hierarchical trophic structure. The trophic level of an organism (population, taxon, functional group) within a food web refers to its distance to the primary food source of the system (usually autotrophs such as phytoplankton) in terms of trophic steps.
Hence, the autotrophs have the lowest trophic level, usually denoted trophic level 1.
Herbivores feeding on these autotrophs are positioned one tropic level higer, i.e. on trophic level 2.
Carnivores feeding on these herbivores are positioned one tropic level higer, too, i.e. on trophic level 3.
Standard ecology text books as well the ECOPATH homepage provide further information on this concept.

The trophic level of an ecological entity (organism, population, taxon, functional group) can be determined in two ways:

- Trophic levels of food items
The food composition is determined by diet analysis and the tropic level of the consumer is computed as the weighted average trophic level of all food items plus one.

TL_Consumer =

(TL_i * P_i) + 1
where P_i is fraction of item i in diet and

(P_i) = 1

This is the approach used by food web modelling software such as ECOPATH.

- Stable isotope ratios
Metabolic processing of ingested organic matter causes isotopic fractionation of ¹³C:¹²C and ¹⁵N:¹⁴N stable isotope pairs. Therefore the stable isotope ratios of

¹³C and

¹⁵N are proxies of the trophic distance of an organism from the origin of the corresponding food chain. Although the underlying physiological, biochemical and biophysical processes are not yet fully understood, the enrichment of the stable isotopes ¹³C and ¹⁵N across trophic levels is a commonly used tool in studies on organic matter flow and food webs in aquatic and terrestrial ecosystems. On average,

¹³C and

¹⁵N increase by 0.5‰ -1‰ and 2.8 - 3.4‰, respectively, from one trophic level to the next.

Advanced analytical techniques are required for this approach, i.e. C/N-analyzer with coupled mass spectrometer. A number of laboratories around the world offers measurements opportunities on a commercial base.

See Michener & Schell (1994), Lesage et al. (2001) or Jacob et al. (2005) for further information.