Which term defines a well tested scientifically supported statement that explains how something works

The term is Hypothesis.

Explanation:

An Hypothesis, is a well posed statement maked by a researcher. It is based on an observation that explains something about  a process or a system and are tested trough experimentation. After that, and based on the results of experimentation, the researcher can makes conclusion

it's anything that aren't producers and consumers.

explanation:

you've got to be clear on what "this" is

a. decomposes use carbon from dead organisms as fuel for cellular respiration, returning carbon dioxide back into the atmosphere.