How do you define omnivory? And what do you teach about trophic levels?

As I wrote about last week, sometimes teaching forces me to think harder about concepts than I would otherwise. Last week’s example was the niche. This week’s is omnivory.

First, there is the question of how to define omnivory. What do you use as your working definition?


And what do you teach your students?

My understanding is that the generally accepted definitions are that an omnivore (without any qualifyiers) is an organism that feeds on plants and animals, whereas a “trophic omnivore” is an organism that feeds at multiple trophic levels. The textbook we use (Morris et al.’s How Life Works) gives the definition for omnivore (“eating both plants and animals”), but doesn’t cover trophic omnivory. But, in my opinion, trophic omnivory is a more useful concept to discuss. I’m a little conflicted on whether to introduce both definitions, or just go with one or the other. Right now, I’m planning on giving students both definitions.

Let’s assume for now that we will work with the trophic omnivory definition in class, which is what I did last year. As students grappled with the concept in their discussion sections, they came up with some interesting questions: what if an animal eats two omnivores? Is that also an omnivore? In one discussion group, they tried working this through with an example of barnacles and mussels, who are both omnivores, since they eat phytoplankton and zooplankton. Assuming both barnacles and mussels eat 50% phytoplankton and 50% zoops (and, yes, we’re lumping lots of things together there, but work with me here), they’re both feeding at a trophic level of 1.5. The students were wrestling with whether a sea star feeding on barnacles and mussels is then an omnivore (since it fed on two omnivores) or something else (since it’s feeding on one trophic level). Clearly the students were thinking well about the concepts! So, even if they couldn’t reach a resolution, I was happy to hear about this discussion. They also spent time trying to figure out what trophic level a venus fly trap is at. I told their discussion instructor that she could blow their minds by teaching them about the pitcher plant-mammal mutualism, where shrews provides a substantial amount of nitrogen to the plant by defecating in the pitcher. I have no idea what trophic level that plant is at! (David Attenborough has a video on the “toilet” pitcher plant. Clearly I need to work that into lecture somehow!)

All of this really comes down to problems with us trying to simplify concepts and apply discrete labels to things that are really pretty messy out in nature. Things like intraguild predation are really common, and lots of organisms do things like feed on other (live) organisms and also on detritus. Then again, while real food webs are messy, there is evidence that most trophic positions do fall around integer values. So, I don’t plan to just throw up my hands and say things are complicated so we can’t use any terms. At the Intro Bio level, I think there’s value in setting the foundation of having students consider trophic levels. For now, I think I will continue teaching trophic levels and then point out that the reality is more complicated. But I would be interested in hearing what others do!

10 thoughts on “How do you define omnivory? And what do you teach about trophic levels?

  1. Don’t forget that fungi are not plants, so saying plants and animals might leave out a common food source. Also, I think it is always good to think about how any concept applies to microbes. What is an omnivore or specialist bacterium? If most ecology is entirely microbial, shouldn’t the concepts be robust for microbes?

  2. That’s an interesting question. The problem with overly simplistic labels, such as “omnivore” and “specialist”, is that they are poor representations of the complexity we observe in natural systems. If you take bats as an example, most species might fit the broad category “omnivore”, as all bats came from an insectivore ancestor and most of them retained this diet at least in part. Nevertheless, this label conceals the huge diversity of trophic strategies found among bats, even some highly specialized frugivorous and granivorous strategies. Furthermore, there are at least those two definitions of omnivore you mentioned, so one never knows what you mean with “omnivore” in a paper, unless you add at least one explanatory sentence. So what is the advantage of using those labels?

  3. to properly define it I suggest you have to work with the three classes of primary producers (TL=0), primary consumers (TL=1) and secondary consumers (TL>1). an omnivore consumes at least from two of these three classes. then tghe concept will also hold for soil-litter systems where you have detritivores that make everything even more complicated. further, this definition will let you easily include microbes and fungi into your rough sketch of the (meta-)food web out there😉
    (this is certainly also not a perfect solution but I feel they are better (or at least more accurate) than your examples)

    • Yeah, I had the same thought. It gets to the linkages between the two definitions of omnivore. The first two trophic levels of most food webs tend to be reasonably well-defined, but above that is where you start getting lots of trophic omnivory. Presumably that’s ultimately because for biological reasons that often make it difficult or maladaptive to be both an autotroph and heterotroph, or to consume and digest both animal and plant tissues.

  4. I’ve often distinguished between “single-life-stage” omnivory, “life-history” omnivory, and “inducible trophic polymorphism” (which don’t fit into either of the previous categories because they are triggered by environmental cues).

  5. This reminds me of a paper in our journal club series, “On the failure of modern species concepts”, by Jody Hey (Trends in Ecology and Evolution, Vol.21, No.8, 2006). It served as as precursor to a long discussion on the species concept debate and useful for distinguishing definitions versus concepts.🙂
    “A general concept can, in turn, be defined more specifically, and in different ways at different times, depending on the context.

  6. I use the “feeds at multiple trophic levels” definition in my teaching (more generally I prefer variance in prey trophic position as a continuous measure of omnivory). I’m teaching it as part of a refresher on food web theory in a second year applied ecology class, so the focus is more on what it means for food web dynamics (e.g. how will the success of top-down biomanipulation depend on the extent of omnivory in the food web) than on categorizing species.

  7. I have trouble thinking about whether to define phenomena that may not capture appropriate complexity of life and which definitions to engage without a clearer sense of what the payoff will be in the curriculum. If talking about trophic omivory allows students to engage literature about foodweb stability, or propose hypotheses and ways to test them, that seems like a really valuable definition to engage. Pitcher plant-shrew mutualisms can capture interest and imagination, and discussions of whether the starfish meet a definition of omnivory shows that students are appreciating the definition in the first place, but is either one central to thinking like an ecologist?

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