Why would the loss of producers have the most dramatic impact on an ecosystem?

Producers are the base of the ecosystem. Producers are the things that create their own food, e.g. grass, algae, and other photosythetic organisms. Herbivores eat these producers and in turn carnivores eat the herbivores. Scavengers and decomposers will consume the dead remains of both the herbivores and carnivores. So if producers ceased to exist the entire food chain would crumble. Herbivores would no longer be able to eat which would cause them to die out. This would cause the carnivores to starve to death. Scavengers and decomposers would live the longest but eventually their food sources would cease to exist causing them to starve as well.