Sun is the endless source of energy, but the chemical materials of the environment are exhaustible.The producers fix solar energy into the chemical energy of organic compounds by utilizing inorganic substance (i.e., CO₂ and water). This stored energy is passed to consumers by repeated eating and being eaten.Decomposers act on dead decaying organic matter and return the locked inorganic substance back to the environment for their reuse by producers.Without this, all life will ultimately cease to exist and earth will be full of dead organisms and waste organic substances.

(1) is a mollusc (a herbivore or primary consumer in aquatic habitat)
(2) is an aquatic plant or producer (producer level.)
(3) crop plants are producers in cropland ecosystem.
(4) is a rat (primary concumer).

If succession occurs in an area devoid of life, it is termed as primary succession. On the other hand, if succession occurs in an area where natural or man made causes have resulted in destruction but area still has organic matter, then it is termed as secondary succession.
Primary succession on a bare rock is very slow as establishment of new communities is slow process and formation of soil takes several hundred years.On the other hand, in areas where natural biotic communities have been destroyed,e.g.., because forests after fire or abondoned land after floods, succession is faster.It because such areas still have soil and sediment rich in organic matter.

Green algae are not likely to be found in the deepest strata of sea because the environment is perpectually dark and sun as source of energy not available.