● An `color{brown}("ecosystem")` can be visualised as a `color{violet}("functional unit of nature")`, where `color{violet}("living organisms")` interact among themselves and also with the surrounding `color{violet}("physical environment")`.

● `color{violet}("Ecosystem")` varies greatly in size from a `color{violet}("small pond")` to a `color{violet}("large forest")` or a `color{violet}("sea.")`

● `color{violet}("Many ecologists")` regard the entire `color{brown}("biosphere")` as a `color{violet}("global ecosystem,")` as a composite of all `color{violet}("local ecosystems")` on `color{violet}("Earth.")`

● Since this system is too much big and complex to be studied at one time, it is `color{violet}("convenient")` to `color{violet}("divide")` it into two basic categories, namely the `color{brown}("terrestrial")` and the `color{brown}("aquatic")`.

● `color{violet}("Forest, grassland")` and `color{violet}("desert")` are some examples of `color{brown}("terrestrial ecosystems")`; `color{violet}("pond, lake, wetland, river")` and `color{violet}("estuary")` are some examples of `color{brown}("aquatic ecosystems")`.

● `color{violet}("Crop fields")` and an `color{violet}("aquarium")` may also be considered as `color{brown}("man-made ecosystems.")`

● Important factors to study are the structure of the `color{violet}("ecosystem")`,the input `color{brown}("(productivity)")`, transfer of
energy `color{brown}("(food chain/web, nutrient cycling)")` and the output (`color{violet}("degradation")` and `color{violet}("energy loss")`).

● There are also the `color{violet}("relationships – cycles, chains, webs")` – that are created as a result of these `color{violet}("energy flows")` within the system and their `color{violet}("inter- relationship.")`

● `color{violet}("Interaction of biotic and abiotic")` components result in a `color{violet}("physical structure")` that is characteristic for each type of `color{violet}("ecosystem.")`

● `color{violet}("Identification")` and `color{violet}("enumeration")` of `color{violet}("plant")` and `color{violet}("animal")` species of an `color{violet}("ecosystem")` gives its species composition.

● `color{violet}("Vertical distribution")` of different species occupying different levels is called `color{brown}("stratification")`.

● For example, trees occupy `color{violet}("top vertical strata")` or `color{violet}("layer of a forest, shrubs")` the second and herbs and grasses occupy the bottom layers.

● The `color{violet}("components of the ecosystem")` are seen to `color{violet}("function")` as a `color{violet}("unit")` when consider the following aspects are considered:

(i) `color{brown}("Productivity")`.

(ii) `color{brown}("Decomposition")`.

(iii) `color{brown}("Energy flow")`

(iv) `color{brown}("Nutrient cycling")`

● To understand the `color{violet}("ethos")` of an `color{violet}("aquatic ecosystem")`,a `color{violet}("small pond")` can be taken as an example.

● This is fairly a self-sustainable unit and rather simple example that explain even the `color{violet}("complex interactions")` that exist in an `color{violet}("aquatic ecosystem.")`

● A `color{violet}("pond")` is a shallow water body in which all the above mentioned `color{violet}("four basic components")` of an ecosystem are well exhibited.

● The `color{violet}("abiotic component")` is the water with all the dissolved `color{violet}("inorganic")` and `color{violet}("organic")` substances and the rich soil deposit at the bottom of the `color{violet}("pond.")`

● The `color{violet}("solar")` input, the cycle of temperature, day-length and other `color{violet}("climatic conditions regulate")` the rate of function of the entire pond.

● The `color{brown}("autotrophic")` components include the `color{violet}("phytoplankton,")` some algae and the floating, submerged and `color{violet}("marginal plants")` found at the edges.

● The `color{brown}("consumers")` are represented by the `color{violet}("zooplankton,")` the free swimming and bottom dwelling forms.

● The `color{brown}("decomposers")` are the `color{violet}("fungi, bacteria")` and `color{violet}("flagellates")` especially abundant in the bottom of the `color{violet}("pond.")`

● This system performs all the `color{violet}("functions of any ecosystem")` and of the `color{violet}("biosphere")` as a whole, which are repeated over and over again. These are

`star` `color{brown}("Conversion")` of `color{violet}("inorganic")` into `color{violet}("organic")` material with the help of the radiant energy of the sun by the `color{violet}("autotrophs;")`

`star` `color{brown}("Consumption")` of the `color{violet}("autotrophs")` by `color{violet}("heterotrophs;")`

`star` `color{brown}("Decomposition")` and `color{brown}("mineralisation")` of the `color{violet}("dead matter")` to release them back for reuse by the `color{violet}("autotrophs,")`

● There is unidirectional movement of energy towards the higher `color{violet}("trophic")` levels and its `color{violet}("dissipation")` and `color{violet}("loss")` as `color{violet}("heat")` to the environment.