8.5 - 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.

Efficiency of biomass transfer between trophic levels

• The efficiency of transfer is the percentage of the biomass (or the energy it holds) at one trophic level that is passed on to the next, rather than lost.
• It's worked out by dividing the amount transferred to the next level by the amount available at the level below, then multiplying by 100 to turn the fraction into a percentage.

Formula for efficiency of transfer

The typical efficiency is about 10% - this is the value usually used when no measured masses are given.

Why food chains are short

• Because only about a tenth of the biomass passes on at every step, the amount of biomass and energy available shrinks rapidly up the chain.
• This limits both the number of organisms a food chain can support and how long the chain can be:
  • The number of organisms tends to fall at each higher level.
  • After about four or five levels there's too little biomass left to support any more, so the chain runs out.
• Shorter food chains lose less overall, so more of the original energy reaches the top.

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Worked example - Efficiency from rabbits to snakes

A population of rabbits holds 144 kg of biomass. The snakes that feed on them gain 15 kg. Calculate the efficiency of transfer from rabbits to snakes.

The next step up works out much the same way: if the snakes (15 kg) pass on 1.6 kg to the hawks above them, the efficiency of that transfer is $\frac{1.6}{15}\times 100=10.7\%$.

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.

Pyramids of number

• A pyramid of number represents a food chain as a stack of horizontal bars, one for each trophic level.
• The length of each bar shows the number of organisms at that level.
• The producer is at the bottom, with the consumers stacked above it in food-chain order.

Why the bars usually get smaller going up

• An organism at each level usually has to eat many organisms from the level below to stay alive.
• So there are fewer organisms the higher up the chain you go.
• When the food chain works like this, the bars narrow towards the top and the diagram is a true pyramid shape.

The main weakness - Shape can be irregular

• A pyramid of number is not always a true pyramid - this is its main weakness.
• The shape becomes irregular when an organism is much larger or much smaller than the one that eats it:
  • A single large rosebush can feed twenty caterpillars, so the producer bar is shorter than the bar above it.
  • A few hundred fleas feeding on one fox give an inverted bar at the top.
• Because the count of organisms ignores their size, a number pyramid can be misleading about how the food chain really works.
• This is why a pyramid of biomass, or a pyramid of energy, is often a better representation of the same chain.

Pyramid of energy

• A pyramid of energy represents a food chain as a stack of horizontal bars, one for each trophic level.
• The length of each bar shows the energy at that level - usually the rate at which energy is transferred up to the next level, rather than the amount stored at any one moment.
• The producer bar sits at the bottom, because producers capture energy from the Sun first.
• Each bar above is smaller, so the pyramid always narrows upward.

Why the pyramid narrows

• Energy is lost at every transfer up the chain. Most of the energy taken in at a level is:
  • Lost as heat
  • Used for movement
  • Passed out as undigested material
• Only a very small quantity is stored as new growth, and so made available to the level above.

Always a true upright pyramid

• Because energy always decreases up the chain, a pyramid of energy is always a true upright pyramid - it can never be irregular or inverted.
• This is unlike a pyramid of number, and it avoids the occasional odd shapes of a pyramid of biomass.
• This makes it the most reliable of the three for comparing food chains and judging how efficiently they transfer energy.

Example - Plankton food chain

• Tiny, fast-reproducing plankton have only a small biomass at any instant, so a pyramid of biomass looks wrong.
• However, they transfer energy onward very quickly, so the pyramid of energy comes out as a normal upright pyramid.
• The same chain drawn as an irregular pyramid of biomass can be redrawn as a regular pyramid of energy.