Nothing. If the animal is eating it, then that’s probably because it’s supposed to eat it. Who am I to fuck with nature, and think I can actually settle it? And if it’s not supposed to be eating that particular plant but is doing so because it’s starving well, I still wouldn’t do anything. Either the plant dies and becomes extinct, or the animal dies and becomes instinct. Either way, I’d rather not the entire fate of a species and its demise depend on some direct action I did.
Not that it would really make a difference. If an endangered caterpillar is eating an endangered plant, I can stop that one caterpillar, but it won’t stop all the other caterpillars that are eating the same type of plant in other places. I mean, I’m not some kind of professional caterpillar hunter here.
PS; Vikings riding around on mammoths.