3.9 - Biomass Transfer & Pyramids

Biomass and energy transfer

• Biomass is the mass of living material in an organism. At each level of a food chain it carries the energy originally captured from sunlight.
• Producers (plants and algae) sit at the base of every chain - they capture light energy and store it as biomass through photosynthesis.
• Only about 1% of the light reaching producers is transferred into new biomass this way.
• Biomass, and the energy it holds, then passes up the chain as one organism eats another.

The transfer is very inefficient

• Only about 10% of the biomass at any trophic level is passed on to the level above.
• The other 90% is lost on the way.

Three reasons so much is lost at each step

1. Not all of the food is eaten - Bones, teeth, claws, roots, and tough plant fibres are usually left uneaten, so their biomass never enters the next level.
2. Not all that is eaten is absorbed - Indigestible parts pass straight through the gut and are egested as faeces.
3. Most absorbed material is used in respiration - This releases energy for movement, warmth, and other life processes rather than being built into new body tissue.
   • Respiration returns this material to the surroundings as waste products such as carbon dioxide and water.
   • The breakdown of proteins adds urea, lost in urine.

Consequences for the food chain

• Because so much is used up keeping each organism alive rather than stored, each trophic level holds less biomass and less energy than the one below it.
• There are usually fewer organisms higher up the chain.
• This is why food chains rarely have more than four or five trophic levels - after a few steps there is too little biomass and energy left to support another level.

Pyramid of biomass

• A pyramid of biomass represents a food chain as a stack of horizontal bars, one for each trophic level.
• The length of each bar shows the total biomass at that level - the combined mass of all the organisms there if you weighed them together.
• So the producer bar represents the total mass of all producers, the next bar the total mass of all primary consumers, and so on.

Drawing the pyramid

• The producer always forms the large bar along the bottom as trophic level 1.
• Bars are drawn in food-chain order, each one labelled with its trophic level, such as producer or primary consumer.
• Bars are scaled to the actual biomass values so the pyramid is accurate.

Why the bars get shorter going up

• Each higher trophic level holds less biomass than the one below it.
• This is because biomass is lost at every transfer up the chain, so only a small fraction of the biomass at one level ends up as biomass at the next.
• As a result, a pyramid of biomass is almost always a true pyramid shape - a wide base narrowing to the top - even in food chains where the numbers of organisms don't get smaller up the chain.