What would happen to the size of the herbivore population if the producers population decreased?

If the producer population decreased then the size of the herbivore population would decrease as well. This is because the herbivores have less food overall to eat so herbivores will die off until herbivores all have food. So your answer would be it would decrease

Producers are the plants of the food chain. They use the sun or other inorganic energy sources as a way to make food for themselves.