Maarg Manthan · Topic 2.3

Flow of Energy in an Ecosystem

Environment & Ecology › Ecosystem Structure and Function · Topic 2.3

Energy flow in an ecosystem is the one-way transfer of energy from the Sun through producers, consumers and decomposers, with energy lost as heat at each step. The Sun is the ultimate source. Plants capture only about 1 percent of the solar energy reaching them as chemical energy in photosynthesis. The flow obeys two laws of thermodynamics: energy cannot be created or destroyed, only changed from one form to another, and each change loses some usable energy as heat, so a continuous input from the Sun is needed. Because of these losses, only about 10 percent of the energy at one trophic level passes to the next, according to Lindeman's ten percent law of 1942. Energy flow explains why food chains are short and why there are fewer carnivores than herbivores.

5 min read · UPSC Prelims · Environment & Ecology · Ecosystem Structure and Function

At a Glance

Source Sun
Direction One way
Transfer About 10 percent
Capture by plants About 1 percent of sunlight
Law Lindeman, 1942
Exam link UPSC Prelims, Environment & Ecology: Ecosystem Structure and Function

Where Does It Fit?

This topic follows the account of ecosystem components. It describes how energy moves through them, which leads to the next topics on trophic levels, food webs and pyramids. Previous topic: Structure/Components of an Ecosystem: Abiotic and Biotic Components. Next topic: Trophic Levels, Food Chain and Its Types and Significance.

How Does Energy Flow Through an Ecosystem?

Step or concept	Details
Solar input	About 50 percent of incoming solar radiation is photosynthetically active radiation (PAR), in the 400 to 700 nm wavelength. Of this, plants fix only 2 to 10 percent, so about 1 percent of total sunlight
First law of thermodynamics	Energy is neither created nor destroyed but changes form, from light to chemical to heat
Second law of thermodynamics	In every transfer some energy becomes unusable heat, so systems tend towards disorder and need a constant energy input
Producers	Capture solar energy; the total they fix is gross primary production (GPP). Minus their respiration gives net primary production (NPP)
Consumers	Eat producers or other consumers and use part of the energy for growth and reproduction, and lose part as heat
Decomposers	Obtain energy from dead matter and release the rest as heat
Heat loss	Final fate of all energy, which radiates to space

What Is the Ten Percent Law and Why Does It Matter?

Ten percent law: stated by Raymond Lindeman in 1942: on average only about 10 percent of the energy of one trophic level is converted into the body of the next
Example: if producers have 10,000 units of energy, herbivores get about 1,000, carnivores 100 and top carnivores 10
Where the rest goes: lost as heat in respiration, as undigested waste, in parts not eaten, and in movement
Short chains: limited energy supports usually only four or five trophic levels
Fewer predators: top predators are few in number and need large territories
Biomass: usually decreases up the levels in most ecosystems
Unidirectional flow: energy cannot be reused, unlike nutrients which cycle
Importance: explains the pyramid of energy and why eating lower in the chain feeds more people

What Else Should You Remember?

Gross primary production: total organic matter made by photosynthesis
Net primary production: GPP minus plant respiration, the food available to consumers
Secondary production: energy stored in consumers
Assimilation efficiency: share of food energy that is absorbed
Production efficiency: share of assimilated energy that is converted to new body mass
Ecological efficiency: share transferred between levels, often 5 to 20 percent
Warm-blooded animals: use more energy for body heat, so have lower production efficiency
Cold-blooded animals: convert more energy to growth
Detritus pathway: much of the energy flows through dead matter
Energy flow in ponds: phytoplankton to zooplankton to fish
Standing crop: the amount of living matter at a point in time
Energy subsidy: fuel and fertiliser added to farming
Solar constant: about 1.4 kilowatts per square metre at the top of the atmosphere
Models of flow: single channel, Y-shaped and universal models are in Food Web and Models for Energy Flow
Link: see Trophic Levels, Food Chain and Its Types and Significance
Link to productivity: see Ecological Productivity and Ecological Pyramid
Climate: more sunlight in the tropics gives high productivity
Human diet: plant-based diets use less energy per person
Fisheries: catches at high trophic levels are smaller
Biomagnification: toxins increase while energy decreases up the chain, as in Biomagnification
Odum: E P Odum advanced energy flow as the central idea of ecosystem ecology
Lindeman: his study of Cedar Bog Lake in Minnesota

Why Does It Matter for the Exam?

Laws: first and second law of thermodynamics
Ten percent: Lindeman, 1942
Percent captured: about 1 percent of sunlight
Current affairs link: solar versus biomass energy
Common errors: saying energy cycles or that 90 percent passes on

As an analytical point, energy flow sets the limits on food chains and on how many animals an ecosystem can support. It also explains why human food security is helped by eating lower in the food chain.

Why Is This Topic Important?

Source Sun
Loss Heat
Rule Ten percent

What Came Next?

The next topic covers trophic levels, food chains and their types. Next topic: Trophic Levels, Food Chain and Its Types and Significance.

Key People and Terms

PAR Photosynthetically active radiation
GPP Gross primary production
NPP Net primary production
Ecological efficiency Transfer efficiency
Standing crop Living matter at a time
Assimilation Absorption of food energy

Exam Corner

Points to Remember

Plants fix about 1 percent of sunlight.
Ten percent law: Lindeman, 1942.
NPP = GPP minus respiration.
PAR wavelength: 400 to 700 nm.
Energy flows one way.

Do Not Confuse With

GPP and NPP Total versus after plant respiration
Energy flow and nutrient cycling One-way versus cyclic
Assimilation and production efficiency Absorbed versus converted to body mass

Memory Hook

Sun gives one, plants keep little, each step tithes ten, heat takes the rest.

Mains Angle

Questions ask for laws, percentages and terms. These points help.

Pathway Sun to heat
Law Ten percent
Use Food chains

A Question You May Face

An original practice question, not a past paper question.

“Explain the flow of energy in an ecosystem and the significance of the ten percent law for food chains.”

How to Answer

Source Sun, PAR.
Laws Thermodynamics.
Ten percent Example.
Impact Short chains.

GS Relevance

Prelims: laws, ten percent law, PAR, energy flow models. GS Paper 3: ecology.

Frequently Asked Questions

How does energy flow in an ecosystem?

Energy enters an ecosystem as sunlight, which green plants and other producers convert into chemical energy by photosynthesis. This energy moves to herbivores and then to carnivores through feeding, and to decomposers from dead matter. At every step part is lost as heat, so the flow is one-directional and needs continuous solar input.

What is the ten percent law?

The ten percent law of Raymond Lindeman states that only about 10 percent of the energy at one trophic level is transferred to the next higher level, as the rest is used in life processes or lost as heat. It explains why food chains have only four or five links and why top predators are few.

Why is energy flow unidirectional?

Energy flow is unidirectional because, under the second law of thermodynamics, every transfer converts part of the energy into heat that organisms cannot use again, and this heat radiates away. Energy therefore cannot be recycled, and ecosystems need a constant new supply of energy from the Sun.

What is the difference between gross and net primary production?

Gross primary production is the total amount of organic matter produced by producers through photosynthesis. Net primary production is what remains after the producers use part for their own respiration, and it is the amount available to herbivores and decomposers. Net primary production equals gross production minus respiration.

What percentage of sunlight do plants capture?

Only about 1 percent of the sunlight reaching the Earth is captured by plants in photosynthesis. About half of incoming radiation is photosynthetically active, and plants fix around 2 to 10 percent of that, depending on the species and conditions.

PYQ Practice — Statement Analysis

1 Only about 10 percent of energy passes to the next trophic level.

True

As per Lindeman's law.

2 Energy cycles in an ecosystem like nutrients.

False

It flows one way.

3 Net primary production equals GPP minus respiration.