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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Pyramids of Biomass
When we look at feeding relationships in ecosystems, we need ways to measure and visualise how energy and materials move through food chains. Pyramids of biomass are one of the key tools ecologists use to understand these relationships.
Key Definitions:
Biomass is the total dry mass of living material. We typically dry organisms before measuring their mass to remove water content, which can vary significantly. This gives us a more accurate measurement of the actual organic material present. Biomass is usually measured in grams per square metre (g/m²) or kilograms per hectare (kg/ha).
Measuring biomass allows ecologists to quantify the actual amount of living material at each trophic level. Unlike pyramids of numbers, which simply count organisms, biomass measurements account for the size differences between organisms, giving a more accurate picture of energy distribution in an ecosystem.
A pyramid of biomass shows the total mass of living organisms at each trophic level in an ecosystem. Each bar represents a trophic level, with producers at the bottom and top consumers at the peak.
To construct a pyramid of biomass, follow these steps:
The resulting diagram typically looks like a pyramid, with the largest bar at the bottom (producers) and progressively smaller bars for each higher trophic level (consumers).
In a typical woodland ecosystem, the biomass distribution might look like this:
Notice how the biomass decreases by approximately 90% at each trophic level, creating a classic pyramid shape.
In most terrestrial ecosystems, pyramids of biomass have a broad base that narrows towards the top. This shape reflects the fundamental principle that energy is lost at each trophic level through various processes:
Organisms use energy for their life processes, releasing heat that leaves the ecosystem.
Energy is lost in waste materials that may not be immediately recycled within the ecosystem.
Animals use energy for movement, which is released as heat and cannot be passed on to the next trophic level.
Not all pyramids of biomass follow the classic pyramid shape. In some aquatic ecosystems, we can observe inverted pyramids of biomass, where the producers have less biomass than the consumers.
Inverted pyramids typically occur in aquatic ecosystems for these reasons:
In an ocean ecosystem, you might find this biomass distribution:
This creates an inverted pyramid at the base, even though the energy flow still follows the same principles. The key difference is that phytoplankton reproduce extremely quickly, sometimes doubling their population in a single day, supporting a larger biomass of zooplankton despite their smaller standing crop.
Pyramids of biomass help us understand energy transfer in ecosystems. The decrease in biomass at each trophic level reflects the inefficiency of energy transfer between levels.
Only about 10% of the energy available at one trophic level is transferred to the next. This is why pyramids of biomass typically show a 90% reduction in biomass at each level. This inefficiency explains why food chains rarely have more than 4 or 5 trophic levels - there simply isn't enough energy left to support another level.
You can calculate the efficiency of energy transfer between trophic levels using this formula:
Efficiency (%) = (Biomass at higher level Ă· Biomass at lower level) Ă— 100
For example, if producers have 1000g/m² and primary consumers have 100g/m², the efficiency is (100 ÷ 1000) × 100 = 10%
Understanding pyramids of biomass has important implications for ecology and human food production:
The inefficiency of energy transfer explains why eating lower on the food chain (more plants, fewer animal products) can feed more people using the same amount of land. Plant-based diets require less land because energy isn't lost through multiple trophic transfers.
For example, it takes approximately 10kg of plant material to produce 1kg of beef. This means that growing crops directly for human consumption can feed more people than using the same land to grow crops for livestock.
The concept of biomass pyramids helps explain why carnivores are rarer than herbivores in natural ecosystems. Top predators require vast territories to support their energy needs because of the cumulative energy losses through the food chain. This is why large predators like tigers and polar bears need extensive habitats and are particularly vulnerable to habitat fragmentation.
While pyramids of biomass provide valuable insights into ecosystem structure, they have some limitations:
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