Energy Flow, Trophic Level & Food Chain - Biology Class 12 - NEET PDF Download

Introduction

• The chemical energy of food is the main source of energy required by all living organisms.
• The transfer of energy from one trophic level to another trophic level is called energy flow.
• The flow of energy in an ecosystem is unidirectional: it proceeds from the producer level to successive consumer levels and does not reverse.
• Energy can be used only once in the ecosystem and is ultimately lost as heat; by contrast, minerals and nutrients are recycled within the ecosystem.

Thermodynamic basis of energy flow

• First law of thermodynamics: Energy can neither be created nor destroyed; it can only change from one form to another. This means the total energy entering an ecosystem (primarily as solar radiation) is conserved but is transformed into other forms.
• Second law of thermodynamics: When energy is transformed, part of it becomes unavailable to do useful work (in biological systems, this appears as heat). Therefore, as energy is transferred through trophic levels, more and more of it is lost, reducing the amount available at higher trophic levels.

Trophic level

The organisms in an ecosystem can be arranged into feeding levels called trophic levels. Each trophic level contains organisms that obtain energy in similar ways.

1. Producers (autotrophs): Green plants and other photosynthetic organisms form the first trophic level. They convert solar energy into chemical energy (organic compounds).
2. Primary consumers (herbivores): Animals that feed directly on producers form the second trophic level.
3. Secondary consumers (primary carnivores): Animals that feed on herbivores form the third trophic level.
4. Tertiary consumers (top carnivores): Predators that feed on secondary consumers occupy higher trophic levels; these are often the last levels in a food chain.

Energy flow in an ecosystem

• A large amount of energy is lost at each trophic level; approximately 90% of the energy is lost when it is transferred from one trophic level to the next. Consequently, the amount of usable energy decreases step by step up the food chain.
• Only about 10% of the biomass (and roughly 10% of chemical energy) is transferred from one trophic level to the next. This observation is known as the 10% law of Lindeman (1942).
• Because of this loss, longer food chains provide less energy to their top consumers than shorter chains.
• Energy lost between trophic levels is used for metabolic processes (respiration), released as heat, or lost as undigested material (faeces).
• The energy that producers store through photosynthesis is called primary production. The energy that consumers store in their tissues from consuming other organisms is called secondary production.
• Only about 10-20% of the primary production is converted into secondary production; the remaining 80-90% is lost by consumers as faeces, respiration or other metabolic processes.

Primary production: GPP and NPP

• Gross primary productivity (GPP) is the total rate at which solar energy is converted into chemical energy by photosynthesis in the producers.
• Net primary productivity (NPP) is the rate of storage of energy as organic matter after accounting for the energy used by producers for respiration.
• Relationship: NPP = GPP - R, where R is the energy used in respiration by producers.
• NPP represents the energy available to primary consumers and is therefore an important measure of the energy entering the consumer levels.

Food chain and food web

A food chain is a linear sequence showing the transfer of energy and nutrients from one organism to another, beginning with a producer. A food web is a network of interconnected food chains that better represents feeding relationships in a real ecosystem.

• At each step in a food chain, part of the energy captured by producers is lost as heat during the chemical breakdown of food by respiration (metabolic heat).
• Thus, energy flows through the ecosystem in a single direction and is not recycled. In contrast, nutrients and minerals cycle between organisms and the physical environment.
• The transfer of food energy from one organism to another always involves loss of energy as heat due to metabolic activities.
• The amount of solar radiation reaching the surface of the earth is given in older literature as approximately 2 cal/cm²/min. This solar input is more or less constant and is sometimes called the solar constant or solar flux. A large proportion (about 95-99%) of the incident radiation is lost by reflection or does not reach photosynthetic tissues; Plants capture only 2-10 per cent of the PAR and this small amount of energy sustains the entire living world.
• The energy trapped by producers (primary production) is utilised by consumers in sequence: producers → herbivores → primary carnivores → secondary carnivores.
• The energy stored by consumers in their tissues (secondary production) is only a fraction of the energy they consume; typically 10-20% of the primary production is converted to secondary production and the remainder is lost as faeces, respiration and other metabolic losses.

Types of chains: grazing and detritus

• Grazing food chain: Starts with living green plants (producers) and proceeds to herbivores and carnivores (e.g., grass → grasshopper → frog → snake).
• Detritus food chain: Begins with dead organic matter and detritivores/decomposers (e.g., leaf litter → earthworms → fungi → consumer). This chain is important for recycling nutrients and is coupled to the grazing chain in most ecosystems.

Energy Transfer and Trophic Levels

•  Energy decreases as it moves up each trophic level. When an organism dies, it becomes detritus or dead biomass, which is a food source for decomposers. Organisms at each trophic level depend on those below them for their energy needs. 
•  Each trophic level has a specific amount of living material at any given time, known as the standing crop. This can be measured by the biomass (mass of living organisms) or the number of organisms in a unit area. 
•  Biomass can be measured in terms of fresh or dry weight, with dry weight being more accurate. Dry weight provides a better measurement of actual biomass because it eliminates variability caused by water content. 
•  The number of trophic levels in a grazing food chain is limited by the 10 percent law, which states that only 10 percent of the energy from one trophic level is transferred to the next. 
•  In nature, a grazing food chain can include multiple levels, such as producers, herbivores, primary carnivores, and secondary carnivores. In contrast, a detritus food chain may not have the same limitation on the number of trophic levels. 

Examples and simple calculation using the 10% rule

Example food chain: grass → rabbit → fox.

Suppose producers (grass) fix 10 000 kJ of energy as NPP in a time period.

Energy available to herbivores (rabbit) ≈ 10% of 10 000 kJ = 1 000 kJ.

Energy available to primary carnivores (fox) ≈ 10% of 1 000 kJ = 100 kJ.

Thus, each trophic step retains approximately one order of magnitude less energy, explaining why large numbers of producers are required to support a small number of top predators.

Key implications

• Because energy flow is unidirectional and much energy is lost at each transfer, ecosystems can support more biomass at lower trophic levels than at higher levels.
• Top carnivores require large areas or high primary productivity to meet their energy needs because only a small fraction of original energy reaches them.
• Conservation of top predators and overall ecosystem function requires maintaining primary productivity and the integrity of lower trophic levels.

Summary

Energy enters ecosystems as solar radiation and is captured by producers through photosynthesis. Energy flows through trophic levels in a unidirectional manner and decreases at each transfer because of metabolic losses, respiration and undigested material. The 10% law (Lindeman, 1942) is a useful rule of thumb: roughly 10% of energy at one trophic level is transferred to the next. Understanding energy flow, trophic levels and food chains helps explain ecosystem structure, limits to food chain length, and the importance of conserving primary productivity.