● Except for the deep sea `color{violet}("hydro-thermal ecosystem, sun")` is the only `color{violet}("source of energy")` for all `color{violet}("ecosystems on Earth.")`

● Of the incident `color{violet}("solar radiation")` less than 50% of it is `color{brown}("photosynthetically active radiation")` (PAR).

● We know that `color{violet}("plants")` and `color{violet}("photosynthetic")` and `color{violet}("chemosynthetic bacteria (autotrophs),")` fix `color{violet}("suns’ radiant energy")` to make food from simple inorganic materials.

● Plants capture only 2-10 % of the PAR and this `color{violet}("small amount of energy")` sustains the entire living world.

● So, it is very important to know how the `color{violet}("solar energy captured")` by `color{violet}("plants flows")` through different `color{violet}("organisms")` of an `color{violet}("ecosystem.")`

● All `color{violet}("organisms")` are dependent for their `color{violet}("food on producers,")` either directly or indirectly.

● So there is a `color{brown}("unidirectional flow of energy")` from the `color{violet}("sun")` to producers and then to consumers.

● Further, `color{violet}("ecosystems")` are not exempt from the `color{violet}("Second Law of thermodynamics")`.

● They need a constant supply of energy to `color{violet}("synthesise the molecules")` they require, to counteract the universal tendency toward increasing `color{violet}("disorderliness.")`

● The `color{violet}("green plant")` in the `color{violet}("ecosystem-terminology")` are called `color{brown}("producers.")`

● In a `color{brown}("terrestrial ecosystem,")` major producers are `color{violet}("herbaceous")` and `color{violet}("woody plants.")`

● Likewise, primary producers in an `color{brown}("aquatic ecosystem")` are various species like `color{violet}("phytoplankton, algae")` and `color{violet}("higher plants.")`

● Starting from the `color{violet}("plants (or producers) food chains")` or rather webs are formed such that an `color{violet}("animal feeds")` on a `color{violet}("plant")` or on another `color{violet}("animal")` and in turn is food for another.

● The `color{violet}("chain or web")` is formed because of this `color{violet}("interdependency.")`

● No energy that is `color{violet}("trapped")` into an `color{violet}("organism")` remains in it for ever.

● The `color{violet}("energy trapped")` by the `color{violet}("producer")`, hence, is either passed on to a `color{violet}("consumer")` or the `color{violet}("organism dies.")`

● `color{violet}("Death of organism")` is the beginning of the `color{brown}("detritus food chain/web.")`

● All animals depend on `color{violet}("plants (directly or indirectly)")` for their food needs.

● They are hence called `color{brown}("consumers")` and also `color{violet}("heterotrophs.")`

● If they feed on the producers, the plants, they are called `color{brown}("primary consumers")`, and if the animals eat other animals which in turn eat the plants (or their produce) they are called `color{violet}("secondary consumers")`.

● Likewise, you could have `color{brown}("tertiary consumers")` too.

● Obviously the `color{violet}("primary consumers")` will be `color{brown}("herbivores.")`

● Some common `color{violet}("herbivores")` are insects, `color{violet}("birds")` and `color{violet}("mammals")` in `color{violet}("terrestrial ecosystem")` and `color{violet}("molluscs")` in aquatic ecosystem.

● The consumers that feed on these `color{violet}("herbivores")` are carnivores, or more correctly `color{brown}("primary carnivores")` (though secondary consumers).

● Those animals that depend on the `color{violet}("primary carnivores")` for food are labeled `color{brown}("secondary carnivores.")`

● A simple `color{brown}("grazing food chain (GFC)")` is depicted below:

`color{violet}("Grass Goat Man")`
(Producer) (Primary Consumer) (Secondary consumer)

● The `color{brown}("detritus food chain (DFC)")` begins with dead organic matter.

● It is made up of `color{brown}("decomposers")` which are `color{violet}("heterotrophic organisms,")` mainly fungi and bacteria.

● They meet their `color{violet}("energy")` and `color{violet}("nutrient requirements")` by `color{violet}("degrading dead")` organic matter or `color{violet}("detritus.")`

● These are also known as `color{brown}("saprotrophs")` (sapro: to decompose).

● `color{violet}("Decomposers secrete digestive enzymes")` that breakdown `color{violet}("dead")` and `color{violet}("waste materials")` into simple, inorganic materials, which are subsequently absorbed by them.

● In an `color{brown}("aquatic ecosystem,")` GFC is the major conduit for energy flow.

● As against this, in a `color{brown}("terrestrial ecosystem,")` a much `color{violet}("larger fraction ")` of energy flows through the `color{violet}("detritus food chain")` than through the GFC.

● `color{violet}("Detritus food chain")` may be connected with the `color{violet}("grazing food chain")` at some levels: some of the organisms of DFC are prey to the GFC animals, and in a `color{violet}("natural ecosystem")`, some animals like cockroaches, crows, etc., are omnivores.

● These natural interconnection of `color{violet}("food chains")` make it a `color{brown}("food web.")`

● Organisms occupy a place in the `color{violet}("natural surroundings")` or in a community according to their `color{violet}("feeding relationship")` with other organisms.

● Based on the source of their `color{violet}("nutrition")` or `color{violet}("food, organisms")` occupy a specific place in the food chain that is known as their `color{brown}("trophic level.")`

● Producers belong to the first `color{violet}("trophic")` level, `color{violet}("herbivores (primary consumer)")` to the second and `color{violet}("carnivores (secondary consumer)")` to the third.

● The important point to note is that the amount of `color{violet}("energy decreases")` at successive trophic levels.

● When any `color{violet}("organism dies")` it is converted to `color{violet}("detritus")` or `color{violet}("dead biomass")` that serves as an energy source for `color{violet}("decomposers.")`

● `color{violet}("Organisms")` at each trophic level depend on those at the lower trophic level for their `color{violet}("energy demands.")`

● Each trophic level has a certain `color{violet}("mass of living")` material at a particular time called as the `color{brown}("standing crop.")`

● The `color{violet}("standing crop")` is measured as the `color{violet}("mass of living organisms")` ( `color{brown}("biomass")`) or the number in a unit area.

● The `color{violet}("biomass")` of a species is expressed in terms of `color{brown}("fresh")` or `color{brown}("dry weight.")`

● `color{violet}("Measurement of biomass")` in terms of `color{violet}("dry weight")` is more accurate.

● The number of trophic levels in the `color{violet}("grazing food chain")` is restricted as the transfer of energy follows `color{brown}("10 % law")` – only 10 % of the energy is transferred to each trophic level from the lower trophic level.

● In nature, it is possible to have so many levels – `color{violet}("producer, herbivore, primary carnivore, secondary carnivore")` in the `color{violet}("grazing food chain.")`