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Biomass is lost because not all material is absorbed, glucose is used in respiration, and waste such as urea is lost in urine. 11 questions answered.
Why Biomass is Lost Between Trophic Levels is part of Trophic levels in an ecosystem (biology only) in GCSE AQA Higher Biology.
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Biomass is lost because not all material is absorbed, glucose is used in respiration, and waste such as urea is lost in urine. 11 questions answered.
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Use this lesson to stay inside Trophic levels in an ecosystem (biology only) while connecting the detail back to the wider Ecology topic.
Read the core explanations from Why Biomass is Lost Between Trophic Levels before testing yourself from memory.
Picture a massive oak tree weighing several tonnes. Now imagine all the caterpillars feeding on its leaves - they weigh much less. The birds eating those caterpillars? Even lighter. Why does biomass shrink so dramatically as we move up food chains?
This isn't just about size - it's about energy and efficiency. Understanding biomass loss explains why there are millions of grass plants, thousands of rabbits, but only a few foxes in an ecosystem.
When a rabbit eats grass, it doesn't magically transform 100% of that plant material into rabbit tissue. Most of the grass biomass is lost through several key processes that we need to understand.
Think of it like a leaky bucket - biomass enters at the top but drains out through multiple holes before reaching the next level. Let's examine each of these 'holes' in detail.
Biomass disappears through four main routes. Each one represents energy that cannot be passed to the next trophic level.
Loss 1: Not All Material is AbsorbedWhen animals eat, they don't absorb everything. Indigestible parts like cellulose in plant cell walls, bones, and fur pass straight through. This unabsorbed material is egested as faeces.
Loss 2: Respiration Removes GlucoseLarge amounts of glucose are used in respiration to release energy for movement, growth, and maintaining body temperature. This glucose is converted to carbon dioxide and water, which are lost from the organism.
Loss 3: Metabolic Waste ProductsAbsorbed material is lost as waste products. Urea (from protein breakdown) and excess water are removed in urine. Carbon dioxide from respiration is exhaled.
Loss 4: Heat LossRespiration releases heat energy to maintain body temperature in warm-blooded animals. This heat energy cannot be transferred to the next trophic level.
We can calculate exactly how efficient biomass transfer is between trophic levels. This helps us understand why food chains are limited in length.
Worked example: Calculating Transfer Efficiency
In a grassland ecosystem, grass has biomass 2000 kg/m². Rabbits have biomass 200 kg/m². Calculate the transfer efficiency. [3 marks]
Answer: 10%
The massive biomass losses between levels create a fundamental limit on food chain length. Let's see why mathematically.
Starting with 10,000 kg of producer biomass and 10% efficiency at each transfer:• Producers: 10,000 kg• Primary consumers: 1,000 kg• Secondary consumers: 100 kg• Tertiary consumers: 10 kg• Quaternary consumers: 1 kg
By the fifth level, there's barely enough biomass to support a single large predator. This is why apex predators like eagles and sharks are rare - there isn't enough energy reaching their trophic level to support large populations.
Biomass transfer questions appear regularly in GCSE exams. Here's how to tackle them successfully and avoid the most common pitfalls.
Exam depth extension: In why biomass is lost between trophic levels, connect the biomass mystery to what happens to biomass using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.
Trap Alert: Many students think animals 'use up' biomass like fuel in a car. Actually, biomass is converted into different forms - CO₂, urea, heat - rather than disappearing completely. The mass is conserved but becomes unavailable to the next trophic level.
Understanding biomass loss explains the structure of all ecosystems. It's why there are always more producers than consumers, and why protecting habitats at the base of food webs is so crucial for maintaining biodiversity at higher levels.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Egestion is the loss of unabsorbed material, such as cellulose, bones and fur, that passes straight through the gut as faeces. Excretion is the loss of metabolic waste products from absorbed material, such as urea from protein breakdown and excess water in urine, and carbon dioxide from respiration.
Biomass is lost between trophic levels because not all material is absorbed, so indigestible parts like cellulose, bones and fur are egested as faeces; large amounts of glucose are used in respiration and lost as carbon dioxide and water; waste such as urea is lost in urine; and heat is lost.
Large amounts of glucose are used in respiration to release energy for movement, growth and maintaining body temperature. This glucose is converted to carbon dioxide and water, which are lost from the organism, and the heat released cannot be transferred to the next trophic level.
Apex predators like eagles and sharks are rare because so much biomass is lost at each transfer. Starting with 10,000 kg of producers at 10% efficiency leaves 1,000 kg, 100 kg, 10 kg and then just 1 kg at the fifth level, not enough to support large populations.
Some food is egested as faeces because animals cannot absorb everything they eat; indigestible parts such as cellulose in plant cell walls, bones and fur pass straight through. This unabsorbed material is one of the four main routes of biomass loss.
Heat loss reduces biomass transfer because respiration releases heat energy to maintain body temperature in warm-blooded animals, and this energy cannot be passed to the next trophic level.
Lost biomass does not disappear completely; it is converted into other forms such as CO₂, urea and heat. The mass is conserved but becomes unavailable to the next trophic level, so animals do not 'use up' biomass like fuel.
Waste products cause biomass loss because absorbed material is lost as urea from protein breakdown and excess water in urine, and as carbon dioxide from respiration, which is exhaled.
Starting with 10,000 kg of producer biomass and 10% efficiency at each transfer, there would be 1,000 kg of primary consumers, 100 kg of secondary consumers, 10 kg of tertiary consumers and just 1 kg of quaternary consumers.
If grass has a biomass of 2000 kg/m² and rabbits have 200 kg/m², the transfer efficiency is 200 ÷ 2000 × 100% = 10%. The formula is biomass at higher level ÷ biomass at lower level × 100%.
There are always more producers than consumers because biomass is lost at every transfer between trophic levels. This is why protecting habitats at the base of food webs is so crucial for maintaining biodiversity at higher levels.