Energy flows through the community from basal species to top predators. We envision this flow using food chains and webs illustrating the directionality of energy flow from basal organisms to top predators (see the humorous fish food chain below).
Energy in the cartoon system above flows from the producers through a herbivore, and two predators. This simple food chain demonstrates one very useful organizational tool in the study of ecological communities; trophic levels. Above, we clearly see 4 trophic levels, (1) the plant, (2) the herbivore, (3) primary carnivore, and (4) secondary carnivore. I think that ecology has benefited from this simplification of trophic relationships, but how well does this simplification represent reality? Do organisms really separate into recognizable trophic levels?
In the simple fish system above each trophic level consumes only the fish from the level directly beneath it. When organisms consume species from multiple trophic levels, it is called omnivory. In the past there has been a large debate over whether or not omnivory is prevalent in natural communities. Some have said that omnivory destabilizes communities, others say that while it is more likely to be destabilizing, int he instances when it is found in real communities, it tends to have a stabilizing impact. All I want to say about stability here is that yes, it does appear that omnivory reduces the probability of a system being stable (maybe more on that later) it tends to reduce variability in the return time of the system.
More important I think is the question of the prevalence of omnivorous interactions. One paper that I think is really fascinating on this topic is “Trophic levels and trophic tangles: the prevalence of omnivory in real food webs,” a paper in Ecology by Thompson et al. (2007). The basic finding is that most species cannot be assigned to an integer trophic level (1, 2, 3, 4, etc). Furthermore, those that could be assigned an integer trophic level were primarily found in the first and second levels (plants and herbivores). What this means is that as you go up the food chain higher level species are more likely to be omnivorous. This makes sense, in that the higher a species level is, the more levels it has to choose prey from. Moreover, higher trophic level species (much as in our cartoon food chain) tend to be larger than their lower trophic level food. When you are larger, you can basically eat whatever you want to (at least morphologically, e.g. their gape size is large enough).
Now, I want to demonstrate this finding that omnivory increases in prevalence higher up the food chain. Here is the food web from Otago Harbor, NZ (which I have shown previously) plotted according to trophic level (y-axis). I should note that trophic level was calculated with the NetIndices package in R.
In the picture you can clearly see a defined first and second trophic level for the plants and herbivores. Once you look above the second level, it becomes MUCH more difficult to distinguish between different levels. It truly represents a ” tangled bank” of consumers.
When trophic level is calculated with the NetIndices package, it also calculates an index of omnivory as well. We can then look at the correlation between trophic level and omnivory.
The top left plot is for the Otago Harbor food web pictured above. Up next is 22 webs from the Interaction Web Database.
So there is a clear positive correlation between trophic level and omnivory index that appears to be a general pattern. I won’t show it here, but the correlation coefficients are all significant for the webs I have shown you (although I admit I did not do those statistics properly, without correcting for multiple comparisons). Of course, this just makes logical sense, because there are more opportunities for omnivorous interactions to occur the higher up in the food chain you are.
Nonetheless I think that this shows that when considering energy flow through communities it is important to realize that the upper trophic levels are a tangled bank rather than separate levels.