Energy flows through the ecosystem, but no new energy is created. The sun represents a constant input of new energy to the ecosystem. Autotrophs use this energy to create chemical energy and inorganic matter to make organic compounds, thus making the energy provided by the sun available for the other components of the energy flow chain.
Without "producers" making energy available for all other links, the amount of "new" energy flowing through the system will decrease.
On the other end of the energy flow system, you find the "decomposers", the organisms in this group are in charge of degrading the organic remains of dead organisms, as well as metabolic waste products. These organisms are in charge of making the energy stored in the organic remains available for other groups of the system to use. Without the members of this group, the energy stored in the remains wouldn't be able to reenter the cycle and the energy levels, usually conserved, will drop greatly.
Out of both groups, the "producers" are in charge of producing and recycling energy, and the "decomposers" are in charge of "recycling" the energy stored in organic remains, the one whose absence will cause a greater drop in energy will be the decomposers.