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 don't know if it's all of them, but I know someone who said that veganism is actually more harmful for the planet because plants need pesticides and fertilizer, which kill bugs. It took no effort to challenge them on that at all.
Sounds like my coworker who argued that veganism kills more animals that meat does, due to the pest control/elimination measures that might be necessary to protect crops. As if pest control isn't a thing in the meat industry from the feed crops, to the feed lots, to the barns to the slaughterhouses.
What do they think livestock eat?
pesticides and fertilizer!