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?
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