Without predators, the herbivores would consume all edible plants like a plague of locusts, which are themselves herbivores. Predators are nature's way of keeping balance. Too many herbivores, not enough plants producing oxygen, u;timately leading to the death of all oxygen breathing life via asphyxiation. To many predators leads to too many plants, leads to too much oxygen, resulting in a toxic and highly ignitable atmosphere[and really big bugs].