If the producer population in an ecosystem decreased, what would most likely happen to the size of the herbivore population?

Herbivores will decrease

Explanation:

If the producers will decrease in number then the organisms feeding on them( herbivores) will also decrease.

The food chain starts with the producers, they make food that is used by them and other organisms.

If the number of producers will decrease the organisms feeding on them will also decrease in number. This is so because the herbivores are the organism that only feed on the herbs and plants as a soul source of energy.