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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Energy Transfer
Energy is constantly flowing through ecosystems, starting with the sun and moving through different organisms. Understanding how this energy moves helps us see why ecosystems are structured the way they are and why certain animals eat others!
Key Definitions:
Almost all energy in ecosystems comes from the sun. Plants capture about 1% of the light energy that reaches them through photosynthesis, converting it to chemical energy stored in glucose. This energy then flows through the ecosystem as organisms eat and are eaten.
Energy flows in one direction through ecosystems. It cannot be recycled like nutrients. Once energy is used for life processes (movement, growth, reproduction), most is lost as heat to the environment and cannot be reused by other organisms.
Food chains show the feeding relationships between organisms in an ecosystem. They represent the path of energy from one organism to another.
A food chain always starts with a producer (usually a green plant) and ends with a top predator. The arrows in a food chain show the direction of energy flow, pointing from the organism being eaten to the one doing the eating.
Organisms that make their own food through photosynthesis. They form the first trophic level and include plants, algae and some bacteria.
Organisms that get energy by eating other organisms. Primary consumers eat producers, secondary consumers eat primary consumers and tertiary consumers eat secondary consumers.
Organisms like fungi and bacteria that break down dead organic matter, releasing nutrients back into the ecosystem for producers to use again.
A simple food chain might look like this:
Grass → Rabbit → Fox
However, real ecosystems are much more complex, with many interconnected food chains forming a food web.
The African savanna has a complex food web. Grasses and trees are the producers. Zebras, gazelles and giraffes are primary consumers eating the plants. Lions and cheetahs are secondary consumers eating the herbivores. When these animals die, decomposers like vultures, fungi and bacteria break down the remains. If lions were removed from this ecosystem, zebra populations might increase dramatically, leading to overgrazing and damage to the plant community.
As energy moves from one trophic level to the next, a lot of it is lost. This is why food chains typically have only 4-5 links.
Only about 10% of the energy available at one trophic level is transferred to the next level. This means that if producers have 1,000 joules of energy, primary consumers will only receive about 100 joules, secondary consumers about 10 joules and tertiary consumers just 1 joule.
Energy is lost at each trophic level because:
To calculate the percentage of energy transferred between trophic levels:
% energy transfer = (Energy in higher trophic level ÷ Energy in lower trophic level) × 100
Example: If grass contains 20,000 kJ/m²/year and rabbits contain 1,800 kJ/m²/year, then:
(1,800 ÷ 20,000) × 100 = 9% energy transfer efficiency
Pyramids of energy show the amount of energy stored in the biomass of organisms at each trophic level. They are always pyramid-shaped because energy is lost at each level. The width of each bar represents the amount of energy at that level, measured in kJ/m²/year.
Biomass is the dry mass of all the organisms at a particular trophic level. Like energy, biomass decreases at each trophic level up the food chain.
A pyramid of biomass shows the total mass of organisms at each trophic level, usually measured in g/m². They are usually pyramid-shaped, but there can be exceptions.
For example, in some aquatic ecosystems, the producers (phytoplankton) have a very rapid turnover rate. At any one time, their biomass might be less than the biomass of the primary consumers (zooplankton) that feed on them, creating an inverted pyramid of biomass.
Understanding energy transfer helps explain why eating lower on the food chain is more efficient. If humans eat plants directly (as primary consumers), more of the plants' energy is available to us. If we eat animals that eat plants (acting as secondary consumers), we get only about 10% of the energy that was in the plants. This is why it takes much more land to produce 1 kg of beef than 1 kg of wheat or vegetables. As the global population grows, this understanding becomes increasingly important for sustainable food production.
Scientists use their understanding of energy transfer to:
The efficiency of energy transfer can vary depending on:
Energy flows through ecosystems from the sun to producers and then to various levels of consumers. At each step, a significant amount of energy (about 90%) is lost, primarily as heat. This energy loss limits the number of trophic levels in a food chain and explains why there are fewer organisms at higher trophic levels. Understanding these principles helps us manage ecosystems more effectively and develop sustainable food production systems.
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