Question

What would happen in an ecosystem without herbivores?

Answer 1

The ecosystem would either collapse or be significantly reduced in the number of levels it has. Carnivores have no way of obtaining plants that are necessary for survival if there are no herbivores for them to eat. If the ecosystem didn't just collapse, you'd just be left with plants that grow, decompose, then grow again. It's more likely though that the system would just collapse because the nutrients from decomposed carnivores and herbivores are missing from the soil, depleting it.