Energy Flow through an Ecosystem: Food Chain, Food Web

Functions of Ecosystem

1. Ecological succession or ecosystem development (previous post)
2. Homeostasis (or cybernetic) or feedback control mechanisms (previous post)
3. Energy flow through the food chain
4. Nutrient cycling (biogeochemical cycles) (next post)

Energy Flow Through an Ecosystem – Trophic Levels

(Trophe = Nourishment)

• A trophic level is the representation of energy flow in an ecosystem.
• The trophic level of an organism is the position it occupies in a food chain.
• Trophic level interaction deals with how the members of an ecosystem are connected based on nutritional needs.

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• Energy flows through the trophic levels from producers to subsequent trophic levels is unidirectional.
• Energy level decreases from the first trophic level upwards due to loss of energy in the form of heat at each trophic level.
• This energy loss at each trophic level is quite significant. Hence there are usually not more than four-five trophic levels (beyond this the energy available is negligible to support an organism).
• The trophic level interaction involves three concepts namely

1. Food Chain
2. Food Web
3. Ecological Pyramids

Food Chain

• Transfer of food energy from green plants (producers) through a series of organisms with repeated eating and being eaten link is called a food chain. E.g. Grasses → Grasshopper → Frog → Snake → Hawk/Eagle.
• Each step in the food chain is called trophic level.
• A food chain starts with producers and ends with top carnivores.
• The trophic level of an organism is the position it occupies in a food chain.
• Types of Food Chains: 1) Grazing food chain and 2) Detritus food chain

Grazing food chain

• The consumers which start the food chain, utilising the plant or plant part as their food, constitute the grazing food chain.
• For example, in a terrestrial ecosystem, the grass is eaten by a caterpillar, which is eaten by lizard and lizard is eaten by a snake.
• In Aquatic ecosystem phytoplankton (primary producers) are eaten by zooplanktons which are eaten by fishes and fishes are eaten by pelicans.

Detritus food chain

• This type of food chain starts from organic matter of dead and decaying animals and plant bodies from the grazing food chain.
• Dead organic matter or detritus feeding organisms are called detrivores or decomposers.
• The detrivores are eaten by predators.
• In an aquatic ecosystem, the grazing food chain is the major conduit for energy flow.
• As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the grazing food chain.

Q. With reference to the food chains in ecosystems, which of the following kinds of organism is/are known as decomposer organism/organisms?

1. Virus
2. Fungi
3. Bacteria

Select the correct answer using the codes given below.

1. 1 only
2. 2 and 3 only
3. 1 and 3 only
4. 1, 2 and 3

Explanation:

• Fungi and Bacteria are decomposers. They breakdown organic matter into simple inorganic substances.
• Virus represents dormant life. They are metabolically inactive as long as they are outside a host body. They are not decomposers. They invade host cells and use their nucleus (DNA machinery) to carry out their life processes.
• Protists are unicellular organisms, such as paramecium and euglena. They’re not decomposers either.

Answer: b) 2 and 3 only

Q. Which one of the following is the correct sequence of a food chain?

1. Diatoms-Crustaceans-Herrings
2. Crustaceans-Diatoms-Herrings
3. Diatoms-Herrings-Crustaceans
4. Crustaceans-Herrings-Diatoms

Explanation:

• The food chain starts with a producer and ends with a top consumer.
• Phytoplankton are the primary producers in the oceans. They include:
  • diatoms (unicellular algae),
  • coccolithophores (unicellular, eukaryotic protist),
  • Cyanobacteria (Bluegreen algae)– Synechococcus, Prochlorococcus, Nostoc, spirogyra etc.
  • Dinoflagellates (flagellated protists).
• Crustaceans form a very large group of arthropods which includes crabs, lobsters, crayfish, shrimp, krill and barnacles (Biology NCERT).
• Herrings are a fish, and they eat crustaceans.

Answer: (a) Diatoms-Crustaceans-Herrings. Tough question.

Food Web

• Multiple interlinked food chains make a food web.
• Food web represents all the possible paths of energy flow in an ecosystem.
• If any of the intermediate food chains is removed, the succeeding links of the chain will be affected largely.
• The food web provides more than one alternative for food to most of the organisms in an ecosystem and therefore increases their chance of survival.

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Q. With reference to food chains in ecosystems, consider the following statements:

1. A food chain illustrates the order in which a chain of organisms feed upon each other.
2. Food chains are found within the populations of a species.
3. A food chain illustrates the numbers of each organism which are eaten by others.

Which of the statements given above is/are correct?

1. 1 only
2. 1 and 2 only
3. 1, 2 and 3
4. None

Explanation:

• A food chain illustrates the order in which a chain of organisms feed upon each other. (True)
• Food chains are found within the populations of a species. (Man won’t eat man – so, false)
• A food chain illustrates the numbers of each organism which are eaten by others (food web illustrates the number, not the food chain).

Answer: a) 1 only

Types of Biotic Interactions in a Food Web

‘0’ is no effect; ‘–’ is detrimental; ‘+’ is beneficial.