(6a) Pyramids of biomass, how to construct them, examples explained, why is biomass lost?

Biomass is mainly organic matter, derived from recently living organisms, and the source chemical energy production for life. Examples include crops, agricultural residues, sewage waste, but the largest biomass residues are from wood. It is specified for a given population of a trophic level in a food chain.

The term biomass refers to the amount of living material you are dealing with e.g. in a trophic level.

Its worth bearing in mind that only ~1% of sunlight energy ends up as plant biomass!

The mass of living material (the biomass) at each stage in a food chain (trophic level) is always less than that of the previous stage.

The amount of biomass transferred from one trophic level to the next higher trophic level is only  ~10%.

I've done some calculations in part 6 to see how biomass transfer works out quantitatively.

The biomass at each stage can be drawn to scale and shown as a pyramid of biomass.

Level 1 is always at the bottom of the pyramid, followed by the other successive trophic levels.

Pyramids of biomass only use the dry mass because wet mass can vary so much in water content.

 

Examples of food chains and biomasses

Each bar on a biomass pyramid represents the relative amount of mass for the organisms of that trophic level in a food chain and because of natural losses (including energy molecules), it always decreases up the pyramid (take care - this is not always so for a pyramid of numbers).

Up the food chain:

producer (1) ==>  primary consumer (2)  ==>  secondary consumer (3)  ==> tertiary consumer (4)

(trophic level)    The producer is usually a photosynthesising plant or algae.

 

How to construct a biomass pyramid - quantitatively to scale on graph paper:

To draw to scale, you must keep the vertical height the same for each level and make the horizontal length of the bar proportional to the biomass of that trophic level in the pyramid.

In a biomass pyramid, each horizontal bar (drawn to scale) is proportional to the relative mass of the living material at that producing level and feeding levels (trophic levels).

Obviously, the bigger the bar, the greater the biomass at the producer/feeding-trophic level.

Above is a biomass pyramid drawn to scale on the graph paper for the food chain:

2000 g of pondweed (plant producer, trophic level 1)

==> 200 g insects (herbivore, primary consumer, trophic level 2)

==> 80 g frogs (carnivore, secondary consumer, trophic level 3)

==> 40 g of bass fish (carnivore, tertiary consumer, trophic level 4)

(Note the vertical height of each bar is the same (1 cm) and the horizontal length is proportional to the biomass of that trophic level)

Why is the biomass diagram a 'pyramid'?

Up the food chain, up the trophic levels and 'up the pyramid', the biomass gets less because of loss of organic material e.g. not digested, excreted waste.

Also, the energy from respiration, although required to sustain life, eventually becomes waste heat energy to the surroundings and waste products of water and particularly carbon dioxide - most biomass in based on carbon compounds like sugars, fats and proteins..

 

Loss of biomass

The amounts of material and energy contained in the biomass of organisms are reduced at each successive stage (each trophic level) in a food chain because:

So, why the reduction trend in biomass?

(i) Organisms don't eat everything they consume

Some materials (biomass) and energy are always lost in the organisms’ waste materials by e.g. animal excretion (urine, faeces-droppings), fallen leaves from trees etc.

This is often plant material that cannot be digested by animals.

Herbivores need to eat large quantities of plant material to obtain sufficient amount of biomass for growth and energy.

In doing so they produce large amounts of faeces containing the indigestible plant material.

Carnivores, including ourselves and most pet! eat less often because meat and processed carbohydrates are easier to digest, and produce less faeces, less biomass loss - though this still contains indigestible material.

The word egestion means getting rid of undigested waste food in an animals faeces - not all food taken in can be digested and used by the organism. This contributes to the wasted mass and energy n a food chain.

Excretion means getting rid of waste products through chemical reactions in the body e.g. breathing out, sweating and urinating.

Urea, excreted in urine, is a waste substance produced in the kidney as a means of removing excess nitrogen from unused amino acids from protein - further loss of biomass.

Also, some parts of plant or animal material are inedible e.g. certain types complex carbohydrates in plants like lignin or the bony parts of animals - bone, horn, hooves or teeth.

Therefore, like the waste, these materials do not get passed up the food chain.

(ii) Part of the biomass is the chemical energy store of the organism

Respiration supplies all the energy needs for living processes, including movement and much of this energy is eventually transferred to the surroundings, particularly with warm blooded mammals where much energy is spent in maintaining their raised body temperature.

In respiration, animals use large amounts of glucose from digested food.

In the process of respiration biomass is converted into other substances which are also lost as waste, e.g. as in the chemistry of respiration.

The overall simplistic equation for respiration (the opposite of photosynthesis)

glucose + oxygen ==> water + carbon dioxide (waste + energy)

This energy is needed for all life processes, energy to do things like movement of any organism, heat to keep mammals and birds warm - which ultimately means large amounts of energy are transferred to the surroundings.

There is even waste, loss of biomass, from the protein animals eat. Any excess protein-amino acids is converted to urea and excreted in urine.

Because of (i) and (ii) ...

A pyramid of biomass will always be a pyramid shape - each successive trophic level contains less mass of living material if you could add up all the masses of the individuals in a population,

AND, it is rare for food chains to have more than five trophic levels because so much biomass is lost at each stage.

The able summarises the ideas explained above

Its worth pointing out that commercial food production is more efficient than the biomass transfers in the food chains of naturally occurring ecosystems because:

There are fewer stages, less energy is lost,

and concentrated food sources can be used to supplement natural sources

e.g. food supplements can be fed to cattle as well as their natural grazing in fields of grass.

 

The 'pros and cons' of pyramids of biomass and pyramids of numbers data analysis

Pyramids of biomass are better at showing the amount of energy in a trophic level than a pyramid of numbers.

Although a simplification, you can think that usable chemical potential energy is proportional to biomass.

Comparing a pyramid of numbers (may/may not be a pyramid) with a pyramid of mass (always a pyramid).

BUT ...

It is much easier to do the counting of organisms to estimate their populations in a habitat.

In order to gather biomass data you must first collect and kill specimens to obtain their dry mass.

It may be difficult to catch and weigh the specimens in the first place - to avoid ethical issues over catching and killing organisms, estimates of biomass are often used.

The biomass of an organism may vary - plants in winter may shed their leaves reducing their biomass compared to summer.

Omnivores may feed at two trophic levels, confusing the biomass data.

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Key points - Summary of ideas

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

A pyramid of biomass is a diagram that represents the relative amount of biological material (biomass) at each trophic level in a food chain.

It shows how biomass decreases as you move up the food chain because energy is lost at each stage through processes like respiration, movement, and waste production.

How to Construct a Pyramid of Biomass

1. Identify the Trophic Levels

   • The producer (plants) is always at the bottom.

   • The primary consumer (herbivores) comes next.

   • The secondary consumer (carnivores that eat herbivores) follows.

   • The tertiary consumer (top predators) is at the top.

2. Measure Biomass

   • Biomass is measured as the dry mass of organisms at each trophic level.

   • It is usually given in units like grams per square meter (g/m²).

3. Draw the Pyramid

   • Each bar should be proportional to the biomass at that level.

   • Bars must be touching and centered around a midpoint.

   • The pyramid should always be pyramid-shaped, as biomass decreases at higher levels.

For example, in a food chain:

Grass → Grasshopper → Frog → Snake

The biomass of grass will be the largest, followed by grasshoppers, then frogs, and finally snakes.

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Summary of learning objectives and key words or phrases

Know how to construct pyramid of biomass for a food chain of plants and animals

Be able to explain why biomass is lost at each stage in a food chain.

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