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!