Producers, Decomposers, and Consumers

Producer: an organism capable of producing (making) compounds from energy.
If the population of producers drop, then there will be not as many organisms capable of producing compounds for the other organisms to consume.

Decomposer: an organism that decomposes (breaks down) organic material.
If the population of decomposers drop, then there would not be as many organisms capable of breaking down waste to be used to benefit the other organisms/it's environment.

Consumer: an organism that feeds on plants and animals.
If the population of consumers drops, then there would be no organism that would consume the compounds that we're being made, creating excess waste and overall throwing off the entire routine of the ecosystem.