Ecosystems possess a natural tendency to persist. This is made possible by a variety of functions performed by the structural components.
The key functions of the ecosystem are: (1) Productivity (2) Energy flow (3) Nutrient cycling.

Robert Constanza et al tried to put a price tag ecosystem services.
It is about 33 billion US dollars/years.

(i) As phospholipid, it forms a major constituent of membranes.
(ii) It is also a constituent of nucleic acids and energy currency of cells, i.e.ATP.
(iii) It is necessary for the formation of teeth and bones in vertebrates and shells by molluscs.

The decomposers are the microorganisms that can feed or operate at any of the trophic level in ecosystem. They decompose the organic matter of both the autotrophs as well as the heterotrophs. Therefore they are not given any fixed trophic level in an ecosystem.

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.

Humus is a dark-coloured, amorphous substance accumulated by the process of humification during decomposition of detritus.
Properties:
(i) It is highly resistant to microbial action and undergoes decomposition at an extremely slow rate.
(ii) Being colloidal in nature ,it serves as a reservoir of nutrient.
(iii) It improves the water-holding capacity of soil.

All organisms/animals which depend directly or indirectly on plants for their food are called consumers.
Consumers are of the following types:
(i) Primary consumers
They are the organisms/animals which depend directly on producers for their food;they are also called as herbivores,e.g. deer, rabbit, zooplanktons.
(ii)Secondary consumers
Secondary consumers are those animals which depend on the primary consumers for their food; they are primary carnivores, e.g frogs,hyena.
(iii)Tertiary consumers
Tertiary consumers are those animals which depend on the secondary consumers for their food;they are the secondary carnivores, e.g. lion, eagle, kite, etc.

(i) The pesticides enter the fish through food chain and slow biomagnification. The birds which eat such fish accumulate the increasing amount of pesticides in bodies which is toxic and as such result in the death of the such fish eating birds.
(ii) Biocontrolling agent should be searched out to remove the pests.

David Tilman's experiments with more species in a plot showed less year to year variation in total biomass, he also showed that increased diversity contributed to higher productivity, so stable community should not show much variation in productivity, it should be resistant to occasional disturbances (natural or man made), it must also be resistant to invasion by alien species.

Pond - an Ecosystem
1. Pond is a fairly self-sustainable unit that shows even complex interactions of an aquatic ecosystem.
2. It is a shallow water body in which all the major/basic components of an ecosystem are well exhibited.
3. The abiotic components ar:
(i) Water with all the dissolved inorganic substance.
(ii) The rich soil deposit at the bottom.
(iii) The solar input.
(iv) The cycle of temperature.
(v) day length.
(vi) Climatic factor.
1. The producers or autotrophic components include phytoplanktons, some algae, floating and submerged and marginal plants.
2. The consumers are represented by
(i) zooplanktons,
(ii) f ree-swimming animals,
(iii) bottom dwelling animals.
1. The decomposers are the bacteria and fungi that are found at the bottom.
2. This system performs all the function of an ecosystem.
(i) Conversion of inorganic materials into organic compounds with the help of radiant energy of sun.
(ii) Consumption of autotrophs  by heterotrophs, (food chain/food web), decomposition and mineralisation (nutrient cycling), etc.

Role of organisms
1. The roots of plants absorb the minute quantities of phosphates found dissolved in the soil solution.
2. The consumers obtain this element either directly (herbivores) or indirectly (carnivores) from plants.
3. The organic wastes from the living organisms and the dead organisms are decomposed and the phosphate-solubilising bacteria release phosphorus.