Cool article Burra, appreciate getting my brain moving this morning.
For anyone interested in a more in depth scientific paper on this you can read it
here.
From what I gather, in a nutshell, A food web is laid out in linearish fashion, this chain of predator/ prey relationships represents the path that biomass will travel through the system.
"The trophic level of a species can be defined as the average trophic level of its prey plus one. "
Essentially trophic level is a measure of how far a species is from the sources of biomass in a system.
In the paper they say trophic distance is defined as the difference between the trophic level of predator and that of the prey. The mean of all the trophic distances in a system will always equal one, and the degree of homogeneity between these distances is what is referred to as a networks trophic coherence.
So for example they would lay out something like
Plants>herbavore>omnivore>carnivore.
The plants are assigned a base level, purely herbivores have a lower trophic level than purely carnivores.
The trophic distance between carnivore to herbivore is greater than the trophic distance between carnivore and omnivore. ( I think both these values would be negative since you going back in the chain toward plants. )
The coherence of the system would depend on the variation of trophic levels between all the elements.