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As originally conceived, in the context of a food chain rather than a web, the trophic level of a species was simply the integral count of the number of consumption steps between primary producers and that species, where primary producers were and are by definition TL1. In a pelagic marine case it might look like this:

 

TL Example species 'Functional group' Trophic class
5 dolphinfish, tuna, wahoo Piscivorous fish Top predator
4 scads, herring, flyingfish Planktivorous fish Secondary consumer
3 predatory copepods, jellyfish, amphipods Carnivorous zooplankton Secondary consumer
2 Rotifers, filter-feeding copepods Herbivorous zooplankton Primary consumer
1 Diatoms, dinoflagellates Phytoplankton Primary producer


This idea of a food chain might make sense in a few very simple systems but in fact virtually all ecosystems are quite a bit more complex than that. A food web means that many organisms feed on prey of different trophic levels and a simple linear increase in trophic level does not occur. In fact the trophic level must be estimated from the trophic levels of all the prey consumed.

 

where TLi of species i is the mean TLj of its' prey, weighted by Cij, the proportion of prey j in the diet of consumer i.

In the marine case, there are few strict herbivores as even the 'herbivorous' zooplankton are very likely consuming smaller zooplankton with their phytoplankton diet e.g. rotifers included in copepod diet.

 

    Predator (consumer group) diet composition
Prey
TL
Tuna 5.94                     20%
Dolphin 5.92                   30% 10%
Wahoo 5.25                   10% 10%
Leatherback turtle 5.23                      
Flyingfish 4.63                 10%
40%
20%
Scads, Herring 4.29         10%       80% 20%
40%
Jellyfish 4.23               100%      
Amphipods 3.92       2%
30%
40% 60%
  5%
   
Predatory copepods 3.2     5%
70%
30%
40% 40%
  5%
   
Filter-feeding copepods 2.2  
70% 20%
30%
20%
         
Rotifers 2   20%
25%
8%
             
Phytoplankton 1 100% 80%
                 

Highlighted cells indicate cannibalism, i.e. predation within the same functional group, not necessarily within the same species.

 
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