What is Energy Flow?

Energy flow is the continuous and directional flow of energy through living beings within an ecosystem. All living beings can be segregated into producers and consumers. These producers and consumers can be further divided in a food chain. The term ‘trophic level’ is assigned to each level of the food chain.

What are the characteristics of energy flow?

The following are the characteristics of energy flow:

• Energy in an ecosystem constantly flows from the sun to plants and from the plant-eating animals to the flesh-eating animals in the form of food. This flow is unidirectional.
• There is a decrease in useful energy at each successive level of nutrition due to the loss of energy as heat.
• The entire solar energy that enters the system returns to the non-living world as heat, not light.
• Ecosystems have a limited number of trophic levels as all the organisms of one trophic level are not preyed on by the animals of the subsequent trophic level. Some of the energy is squandered. Also, all of the food consumed by animals is not useful.

Significance of energy flow

Every organism requires energy to carry out its vital life activities as well as to build and repair body tissues. The ultimate source of energy for all living things is the sun. Solar energy received by the ecosystem depends on various factors such as latitude, snow, cloud cover, and air pollution. Photosynthesis is a biological process via which plants convert solar energy into chemical energy. In this process, only about 1% of the light energy striking the green plants is converted into the potential energy of organic substances, and the rest is dissipated as heat. This loss of energy is a result of the operation of the second law of thermodynamics. This law explains that whenever energy is transformed from one form to another, the amount of useful energy decreases as some of it dissipates as heat. When an animal eats a plant, the organic substances of the plant are oxidized in the animal’s body for synthesizing nutrients.

The energy liberated by the animal is equal to that used in synthesizing organic substances by the plant. This complies with the first law of thermodynamics. Some of the energy released by the animal dissipates as heat, and only a part is used in building the tissues. When another animal eats this animal, it oxidizes the organic substance to release energy for synthesizing its cellular constituents.

The decomposition of dead animals releases chemical energy into the environment, which the plants trap through photosynthesis. The plants, using the help of chlorophyll, trap the sunlight energy. The sunlight is converted into chemical energy, from which the carbon dioxide and the water combine to form glucose, and it is the food for the plants. Glucose has many carbon-carbon bonds that store energy.

The sunlight is converted to chemical energy (food) and stored by the producers. The consumers eat the producers. The consumers use the energy of the producers (chemical energy) for the release of active energy, which is used for their biochemical processes. The ultimate source of energy for all the living beings present on the planet is the sun. Plants, along with few exceptional organisms like cyanobacteria, are capable of synthesizing organic compounds through photosynthesis. Autotrophs are the basic underlying trophic of the ecosystem. All the ecosystems present on this earth rely on autotrophs from energy. The only exception to this is the deep ocean hydrothermal vent.

Ten percent law

Energy follows the ten percent law which was given by Linderman. It explains that only about ten percent of it remains at the higher trophic level during the energy transfer between consecutive trophic levels. The other 90% is lost in the form of heat during respiration. According to this rule, if 1,000 joules of solar energy are incident on the green plants, ten joules are entrapped and converted into chemical energy through photosynthesis, and the remaining is lost to the environment. The herbivores that feed on producers get only ten percent or one joule of their energy. Further, carnivores get only 0.1 joules of energy.

Trophic levels

Trophic levels can be defined as the positions of organisms in the food chain or food web. Every organism consumes and is devoured by another organism in the food chain.

Food chains

A food chain can be defined as the network formed through the sequential interlinking of organisms through the transfer of energy, starting from the producer and ending at the secondary or tertiary consumer. The flow of energy from the producers to the consumers is always unidirectional.

Types of food chains

In nature, two types of food chains are observed: the grazing food chain and the detritus food chain.

• In a grazing food chain, green plants are present as producers in the first step. The plant biomass, in turn, is consumed by herbivores, which are subsequently consumed by carnivores. The grazing food chain directly depends on solar energy, which is utilized at the producer level during photosynthesis.
• The detritus food chain depends upon the dead organic matter available in the form of fallen leaves or dead animals. It is not directly dependent on solar radiation. This dead organic matter is acted upon by decomposers such as bacteria, fungi, and protozoa. These organisms have the saprotrophic mode of nutrition and are regarded as the primary consumer in this chain. These detritivores are ingested by consumers such as nematodes, earthworms, and insects that lie at the next trophic level. The main purpose of these organisms is to release inorganic content into nature.

Food web

In an ecosystem, food chains do not operate in isolation. They are interconnected to form a network called the food web. This interlocking occurs at various levels to establish more than one connection amongst different organisms of biotic communities.

Ecological Pyramids

An ecological pyramid is the graphical representation of the trophic levels of a food chain. They are also called Eltonian pyramids as Charles Elton developed them.

There are three types of ecological pyramids:

Pyramid of numbers

In a pyramid of numbers, the number of organisms at the lowest trophic level is always more than the organisms present at the upper trophic level. Plants hold the bottom position in most ecosystems, as the number of producers is always at its highest. During the energy transfer at any trophic level, only 10% of energy present at one trophic level becomes part of the next trophic level. Ninety percent of energy is either lost in the form of waste or is broken down during cellular respiration. As a result, producers can only support a few carnivores and herbivores (heterotrophs). Thus, the top carnivore is rare and is not preyed on by other organisms. The pyramid of numbers is upright in most cases.

Pyramid of biomass

Biomass refers to the amount of living matter present at a specific trophic level at a given time. Plants have higher biomass than other species since their number is greater, and this amount of biomass decreases with an increase in trophic levels. The amount of biomass produced by the top carnivore is the least. It is upright for tree and grassland ecosystems but is inverted for aquatic ecosystems.

Pyramid of energy

This ecological pyramid represents the transfer of energy from one trophic level to another. The pyramid of energy is always upright as energy always flows from the producer to the consumer. The amount of energy reduces with each trophic level, and a significant amount of that energy is lost as heat. The tertiary consumers or the secondary consumers receive the least amount of energy.

Common Mistakes

Students may assume food webs to be several interconnected food chains. However, food webs describe the flow of energy within the components of an ecosystem through a set of complex relationships and are not interconnected food chains.

Context and Applications

This topic is significant in the professional exams for undergraduate, graduate, and postgraduate courses, especially for the following:

• Bachelor of Science in Environmental Science
• Master of Science in Environmental Science
• Doctor of Philosophy in Environmental Science
• Master of Philosophy in Environmental Science

Related Concepts

• Ecology
• Photosynthesis
• Plants

Practice problems

Q1: Which process occurs when dead animals release chemical energy into the environment that the plants trap through photosynthesis?

(a) Decomposition
(b) Deforestation
(c) Desertification
(d) Devolution

Correct choice: (a)

Q2: Which pyramid is upright for the tree and the grassland ecosystems?

(a) Pyramid of biomass
(b) Pyramid of energy
(c) Pyramid of number
(d) All of the above

Correct choice: (d)

Q3: Which pyramid is inverted for the aquatic ecosystem?

(a) Pyramid of gender
(b) Pyramid of energy
(c) Pyramid of number
(d) Pyramid of biomass

Correct choice: (d)

Q4: What are the producers in the first step in a grazing food chain?

(a) Lions
(b) Green plants
(c) Earthworms
(d) Rabbit

Correct choice: (b)

Q5: What is the interconnection of the different food chains?

(a) Food decomposition
(b) Food flow
(c) Food web
(d) Food radiation

Correct choice: (c)