Question

What would happen in an ecosystem without herbivores?

Answer 1

It would die out quickly. Carnivores rely on herbivores since at some point down the food chain the only source of meat is a herbivore.
So say the apex predator in a system is an eagle;
The eagle eats smaller birds, which eat caterpillars, which eat leaves. Without caterpillars, smaller birds cannot find food so will die out. Without smaller birds, eagles will also die out because their food source cannot survive.

Answer 2

Food chain will collapse.

plants are taking sunlight and making that energy avilable to primary consumer. this enegy will be further propagated to top of food chain.
if herbivores not their then energy cycle will stop and all other animals will die .