Conservation of mass?
Do they understand that producing energy and fertilizer using the bodies of animals is less efficient than producing the same number of calories or mass of nitrates from plants+less-energy-than-is-required-to-raise-the-animal-in-question?
This was one of the first things we were taught in school during units on nutrient cycles - the closer to the source of energy, the sun, an organism is in the cycle, the more efficient their utilization of the energy is. I don't know if I'm taking knowledge of this concept for granted but it's so simple in my own mind.
Literally junior high science
I grew up in a poor rural area in the south and first learned trophic levels in 7th grade. I'm certain almost everybody has been exposed to this concept at least by the end of high school
Back in my day we didn't get participation trophics 😤