A food chain or a food web defines who is eaten by whom in a given ecosystem. Body size is a key factor in the feeding behaviour of organisms. In aquatic food webs in particular, individual organisms often change in body size over several orders of magnitude during their life cycle. Therefore, trophic links are mainly dictated by the individual body size rather than species identity .
The Norwegian Sea is a good example of an open Large Marine Ecosystem in which various species or groups of organisms respond to the physical environment and define the food webs (for example via seasonal patterns of plankton production, the trophic couplings between plankton and fish populations). Such food chains / webs are important in understanding productivity issues because they operate through biomass production at various steps and trophic levels and ultimately define ecological efficiency.
Ecological efficiency is the efficiency with which production at one trophic level is converted into production at the next level. This corresponds mainly to assimilation and growth efficiencies.
Assimilation efficiency corresponds to the fraction of food taken up by an organism, and depends on the composition and digestibility of ingested food, typically being around 80% for zooplankton and fish.
Growth efficiency is expressed as the fraction of ingested food required for growth and approximates 20% in the same organisms. This apparently low bio-efficiency reflects the price of speed and simultaneous regulation of biochemical processes operating at various organization levels, and thus far from thermodynamic equilibrium.
These factors contribute therefore to an overall low ecological efficiency, which has been empirically estimated at 10-20% (the part microbial food loops / webs play in the system is ignored here). It follows that on a scale of 100 units, from primary production down to various trophic levels, the production of herbivores is maximum 20, while that of first level carnivores is 2, and so on. A trophic chain of 7 occurs in the Norwegian Sea : Flagellates > Ciliates > Oithona > Euchaeta > Themisto > Herring > Salmon, with salmon production of 0.0002 – 0.02 units.Thus, production falls rapidly as we climb the food chain and with long food chains top trophic level production can be as low as 10-5 to 10-8 of the primary production.
The difficulty here is the fact that it is not obvious to estimate precisely enough the trophic position of the target population, simply because food webs – with the notable exception of agriculture - are not structured. This is particularly the case in marine pelagic ecosystems where the developmental stage, the size etc of a given species influences its position within the food web during the life cycle. In addition, in marine ecosystems, feeding is generally highly diverse and non-selective. As a matter of fact, nutrients that support production come from a common pool, shared by benthic (detritus-based) and pelagic (primary producer based) energy pathways. The coupling of the two pathways increases the range of trophic interactions and recycling, therefore affecting food web stability and resilience. While predators may use energy from both pathways, in the pelagic community predation is more strongly governed by body size.
Furthermore, the carrying capacity (AF8) for the top level predators in an ecosystem is very sensitive to the number of steps in the food chains / webs that support it. Therefore, the condition of the entire ecosystem is at stake when defining harvesting stocks or demographic loads. Long food chains, complex and unstructured food webs are indicators of both resilient ecosystems and rich biodiversity.
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2. Dorst J : La force du vivant. Flammarion ; 1979.
3. Blanchard JL, Law R, Castle MD, Jennings S : Coupled energy pathways and the resilience of size-structured food webs. Theoretical Ecology 2010:1-12.
14 juin 2012