- Define and explain Pyramids of Biomass in your own words
- Use key terms such as ecosystem accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Pyramids of Biomass: construct pyramids of biomass and explain why energy transfer is inefficient.
Construct pyramids of biomass and explain why energy transfer is inefficient.
Key ideas
Pyramids of biomass and energy loss
A pyramid of biomass shows the dry mass at each trophic level, and it is almost always pyramid-shaped because biomass is lost between levels. Only about 10% of energy is transferred: the rest is lost through respiration and movement, in uneaten parts, and in waste such as faeces and urine. This is why food chains rarely have more than five trophic levels — there is too little energy left.
The carbon cycle
Carbon dioxide is removed from the air by photosynthesis and locked into biomass. It returns through respiration by plants, animals and decomposers, through combustion of wood and fossil fuels, and through decay. Burning fossil fuels releases carbon stored millions of years ago, raising atmospheric carbon dioxide faster than natural processes can reabsorb it.
Key term — ecosystem: A community of organisms together with their physical environment, interacting as a system.
The grass in a field contains 20 000 kJ of energy. The rabbits that feed on the grass contain 2 400 kJ. Calculate the percentage efficiency of energy transfer from grass to rabbits.
Efficiency = (energy transferred ÷ energy available) × 100. Divide: 2 400 ÷ 20 000 = 0.12. Multiply by 100: 0.12 × 100 = 12.
Answer: 12% efficiency. The remaining 88% is lost through the rabbits' respiration and movement, in parts of grass left uneaten, and in waste.
- Drawing a pyramid of biomass with the widest bar at the top Biomass decreases up the chain because energy is lost at each transfer, so the producer bar at the bottom is always the widest.
- Saying decomposers are not needed Without decomposers, nutrients would stay locked in dead bodies and waste. Decomposers recycle them into the soil for producers to reuse.
Practice
Only about 10% of energy transfers at each level, so after several transfers too little energy remains to support another trophic level.
Most organisms eat several different species and are eaten by several predators, so a web shows the true feeding relationships and how species depend on one another.
Any three of: respiration (released as heat); movement; uneaten parts such as roots or bones; waste products such as faeces and urine.
Burning fossil fuels for energy and transport, and deforestation (which also removes photosynthesis). Reduction: any one of using renewable energy, improving energy efficiency, or reforestation.
Quick check
Which of these best defines "ecosystem"?
Describe two processes that return carbon dioxide to the atmosphere.
In the food chain grass → rabbit → fox → flea, identify the producer, a primary consumer and a secondary consumer.
- Pyramids of Biomass: construct pyramids of biomass and explain why energy transfer is inefficient.
- Pyramids of biomass and energy loss: A pyramid of biomass shows the dry mass at each trophic level, and it is almost always pyramid-shaped because biomass is lost between levels.
- trophic level: A feeding level in a food chain or web, such as producer, primary consumer or secondary consumer.
- Watch out for: drawing a pyramid of biomass with the widest bar at the top