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School biology notes: Ecosystems: Pyramids of biomass & transfer efficiency

Food chains, food webs, trophic levels, pyramids of biomass and energy transfer efficiency, food chains, pyramids of numbers

 Doc Brown's school biology revision notes: GCSE biology, IGCSE  biology, O level biology,  ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old students of biology

 This page will help you answer questions such as ...  What is a food chain? How do you construct a food chain?   What is a food web? How do you construct food webs?  How do you construct a pyramid of biomass?   How do you construct a pyramid of numbers?  How do you present and interpret data on biomass transfer?  How do you calculate % efficiency of biomass transfer?

Sub-index for this page

(a) Energy and biomass in food chains

(b) The crucial role of photosynthesis

(c) Food chains, trophic levels and decomposers - examples explained

(d) Food webs and interdependence - examples explained

(e) Pyramids of biomass - construction - examples explained and biomass loss

(f) Calculations of efficiency of biomass/energy transfer

(g) More examples of food chains expressed as pyramids of biomass

(h) Pyramids of numbers - how to construct them - examples explained

See also Ecosystems: biotic & abiotic factors, interactions between organisms, interdependency



(a) Energy and biomass in food chains

Plants and algae make food via sunlight energy and photosynthesis and this increases their biomass - which is a source of energy.

It may seem a bit strange, but animals only use a small percentage of this biomass to actually increase their own biomass.

The reasons for inefficient transfer of biomass and its loss are discussed in section (e).

By observing the numbers and sizes of the organisms in food chains we can find out what happens to energy and biomass (mass of a particular living organism) as it passes along the food chain.

It is important to understand that all living things are interdependent on each other, especially through the pathways of food chains, which are effectively energy chains too.

You need to be able to interpret pyramids of biomass or construct them from appropriate information.

Biomass is measured as the dry material of an organism.


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(b) The crucial role of photosynthesis

Green plants and algae are the initial producers of food, after that its all consumers, including us!

Radiation from the Sun is the initial source of energy for most communities of living organisms.

Green plants and algae absorb a small amount of the light that reaches them.

The transfer from light energy to chemical energy occurs during photosynthesis.

Photosynthesis uses sunlight energy to convert water and carbon dioxide into sugars like glucose, the 'waste product' being oxygen - though plants need oxygen for their respiration at night!

Apparently only ~1% of sunlight energy is transferred via photosynthesis to create biomass.

the simple equation to illustrate photosynthesis is

water + carbon dioxide (+ sunlight) == chlorophyll ==> glucose + oxygen

Some of the glucose is used directly by the plant to power cell chemistry - making all the molecules necessary for life, but some energy is stored in the substances that partly make up the cells of the plants e.g. starch or cellulose - chemical potential energy store. This adds to the biomass of the plant.

Most food chains, and therefore most life-forms, are dependent on the initial input of sunlight energy.

Food chains always start with a producer, which produce their own food.

This is usually a green plant on land, or algae and phytoplankton in water.

Producers (mainly plants) take in the elements they need from their environment.

Molecular compounds like water (roots), carbon dioxide (leaves) and mineral ions e.g. nitrate for nitrogen, phosphate for phosphorus and small quantities of metals like magnesium, iron, potassium and zinc (absorbed by roots).

In producers the energy from sunlight powering photosynthesis produces sugars and other carbohydrates, which in turn provide a source of energy to make fats, glycerol and amino acids for proteins.

From small molecules, plants make long chain carbohydrates, lipids and proteins which form part of their cell structure and tissues e.g. cell membranes/walls and organelles like mitochondria, chloroplasts and ribosomes.

All of these metabolic chemical reactions are catalysed by enzymes.

These biological molecules comprise the organisms biomass - the mass of living material which is passed on up a food chain from one trophic level to another.

Plants are consumed by animals which in turn use the energy in respiration to power physical movement and build their fat and protein body structures.

Similarly, energy is also transferred through organisms in a food chain when consumers eat plant producers and animals eat other animals.

At each stage there are considerable losses of biomass and energy for various reasons (discussed later).


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(c) Food chains, trophic levels and decomposers - examples explained

Food chains are a simple linear way of showing 'what eats what' in an ecosystem.

Food chains must start with a producer, usually a green plant on land, or algae and phytoplankton in water.

You need a source of food to get any food chain going!

Trophic level refers to the stage in a food chain

e.g. pond weed ==> tadpole ==> water beetle ==> perch ==> otter

This food chain involves four stages and five trophic levels.

Apart from the highest trophic level (5), the top predator:

each trophic level provides food and energy for the next higher trophic level,

some food is used immediately in respiration, so energy is lost as heat to the surroundings, and some becomes biomass of the organism including protein, carbohydrates for respiration and an energy store of fat.

The energy resource is used to power all the life processes of the organism, but much energy is lost as heat to the surroundings (before being used to keep a warm bloodied animal warm).

The energy that isn't stored as biomass isn't transferred to the next trophic level and is lost in the food chain - this can be as much as 90% loss i.e. only around 10% may be passed on from one trophic level to another.

For more on this last point see Pyramid of Biomass

There are usually no more than 4 or 5 trophic levels because so much biomass and energy is lost up the food chain at each trophic level that higher levels can't be supported.

 

Producers make their own food usually using sunlight energy in photosynthesis (trophic level 1).

All plants and plankton like algae are examples of food producers.

All the subsequent consumers cannot make their own food, but may become the food for another organism next in the food chain.

 

Producers are initially eaten by the primary consumer (trophic level 2)

Consumers are organisms that eat other organisms (plants or animals) in a food chain.

The primary consumer is the first to eat the producer e.g. a herbivore - plant eater.

Herbivores only eat things like plants or plankton e.g. rabbits eat grass or small fish eat algae

 

Primary consumers are then eaten by a secondary consumer (trophic level 3).

Secondary consumers are the second consumers in the food chain and are carnivores.

Carnivores are 'meat' eaters - here carnivores eat herbivores e.g. fox eats rabbit.

At trophic level 3, carnivores eat the primary herbivore consumers from trophic level 2.

Omnivores eat both plants and animals depending on the availability of food.

 

Secondary consumers are eaten by tertiary consumers (trophic level 4). etc.

Tertiary consumers are the third consumers in the food chain etc.

We now have carnivores eating carnivores!

Consumers that hunt and eat other animals are called predators and their prey are the animals they eat.

Note that if a predator eats all their chosen prey, it will die out for lack of food or move to another habitat.

In stable communities the numbers of predators and preys rises and falls but in a balanced way.

Carnivores that not hunted themselves by another predator are at the top of the food chain, and therefore they are always the highest trophic level - they are known as the apex predator.

 

When you have several food chains interacting/overlapping you get a food web, which is part of a much wider description of an ecosystem.

 

NOTE organisms that don't get eaten, eventually die, and the dead plants and animals are broken down and eaten by decomposers.

Decomposer organisms like bacteria and fungi play are a very important part of ecosystems - they break down dead plants and animal material.

To decompose dead plant or animal material, these organisms secrete enzymes that break the dead organic material down into smaller soluble food molecules which can diffuse into the microorganisms i.e. absorption of nutrients - sometimes referred to as extracellular digestion.

 

Summary of trophic levels

As already mentioned, each stage of a food chain is called a trophic level.

(The modern scientific word trophic comes from 'trophikos' in the late 19th century from the ancient Greek word 'trophe' which meant 'nourishment'.)

A specific trophic level consists of one or more organisms that perform a specific role in one or more food chains - see food web examples later.

The trophic level number increases up the food chain.

The first four trophic levels of a food chain etc. can be shown as ...

producer  ==>  primary consumer  ==>  secondary consumer  ==>  tertiary consumer etc.

trophic level 1  ==>  trophic level 2  ==>  trophic level 3  ==> trophic level 4 etc.

The arrows indicate the direction of biomass transfer, that is also the direction of chemical potential energy transfer up the food chain.

AND the decomposers take of everything in the end!!!!

 

Three examples of food chains and trophic level

(a) One of many food chains in a river or lake ecosystem

Pondweed ==> eaten by tadpoles ==> tadpoles eaten by water beetle ==> perch fish eats water beetle ==> otter eats the perch

This food chain involves five trophic levels.

The pondweed is the photosynthesising producer (trophic level 1).

Tadpoles are the primary consumer (trophic level 2, herbivore).

The water beetle is the secondary consumer (trophic level 3, carnivore).

The perch is the tertiary consumer (trophic level 4, carnivore).

The otter is trophic level 5 at the top of the food chain, which must always be the highest trophic level - known as the apex predator ('top predator' - top carnivore!).

 

(b) One of many food chains involving a garden and field ecosystem

Cabbage ==> butterflies lays eggs - resulting caterpillars feed off cabbage ==> butterflies eaten by blue/great tit birds ==> bird of prey eg kestrel, catches smaller birds eg blue/great tits

This food chain involves four trophic levels.

The cabbage is the producer.

The caterpillar is the primary herbivore consumer.

The blue tit (carnivore) is the secondary consumer .

The bird of prey is the tertiary consumer - the top of this food chain (the apex predator).

The word apex refers to the top of the 'triangle' of the pyramid of biomass e.g. like the one for four trophic levels shown below.  More on the pyramid of biomass later.

In this pyramid of biomass: 1 = cabbage, 2 = caterpillar, 3 = blue tit bird, 4 = bird of prey (raptor)

 

(c) food chain: green plants ===> rabbits ==> foxes

This food chain involves three trophic levels.

The photosynthesising grass is the producer.

The rabbits are the primary herbivore consumers.

The foxes are the secondary carnivore consumers - the top of this food chain.

In terms of numbers we are talking ....

thousands of blades of grass ===> dozens of rabbits ===> a few foxes

 

(d) food chain:  grass ===> grass hopper ===> rat ===> snake

producer ==> primary consumer (herbivore) ===> secondary consumer (carnivore) ===> tertiary consumer (carnivore)

trophic level 1 ===> trophic level 2 ===> trophic level 3  ==> trophic level 4

In this pyramid of biomass: 1 = grass, 2 = grass hopper, 3 = rat, 4 = snake


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(d) Food webs - interdependence - examples explained

Interdependence means organisms depend on other organisms for resources such as food or shelter in order to survive and reproduce. Therefore, any change in the population of one species, can have knock on effect on another species in the same community. See also notes on ecosystems.

Because of all the different species that co-exist in an environment - the ecosystem, it means there are lots of possible interconnected food chains, though all must start with a producer - usually a photosynthesising plant.

A food web links many, if not all, of the different food chains together.

All the species in a food web are interdependent.

In a community each species depends on others for many things e.g. food (plants eaten by animals, prey hunted by predator, parasites on hosts), mutualism - pollination of flowers by insects, plant seed dispersal by animals, shelter - nests made of twigs and leaves and beaver dams.

This unfortunately means that if one organism population changes, then others populations are affected too - I've called these 'disruptions' - 'what affects what', but not all have negative results - there are often 'gainers' as well as 'losers'. Summed up as 'knock on' effects.

 

Examples of food webs and interdependence

The trophic levels are indicated i.e. producer, primary/secondary/tertiary consumers AND examples of what may happen if there is a sudden increase or decrease in the population of a particular organism.

 

Food web example 1

1. Part of a typical complex food web in a lake or river.

Trophic level 1: The photosynthesising producers are the algae and pond weed.

Trophic level 2: The primary consumers are the tadpoles and aquatic insects.

Trophic level 3: The secondary consumers can be the perch, stickleback and frogs

Trophic level 4: Tertiary consumers at the top of the food chain are the pike, otter and heron.

(some organisms can be in different trophic levels depending on the food chain sequence)

'Disruptions'

(i) Suppose the lake becomes overgrown with weed reducing the light entering the water. The weeds under the water decay and use up all the oxygen (eutrophication effect). The reduction of oxygen affects all the consumers and their numbers are considerably reduced, often to zero in the water.

(ii) Suppose the number of aquatic insects is reduced due to adverse weather conditions. There is less food for the stickleback and perch fishes and frogs so their numbers decline. In turn there is less food for the pike, otter and heron further up the food chain, so their numbers decline too. The heron can easily fly to new waters in search of food, but fishes might not be able to move out of the particular lake or river system. At the same time there will be more food for the tadpoles (less competition for the algae and pondweed), so more frogs survive, but the perch population may increase too because there are more tadpoles to eat.

Things can get very complicated and often can only be quantitatively described and understood by a computer programme whose algorithms describe the rise and fall of the different populations of the species in the habitat.

 

Food web example 2

2. Part of a typical complex food web in a park or garden.

Trophic level 1: The photosynthesising producers are the fruit bushes (or any other plants).

Trophic level 2: The primary consumers are the caterpillars, insects like the greenfly, snails, worms and blue tits.

Trophic level 3: The secondary consumers can be the blue tit, ladybird, thrush and sparrow hawk.

Trophic level 4: Tertiary consumer at the top of the food chain is the sparrow hawk.

(some organisms like blue tits or the sparrow hawk can be in two different trophic levels depending on the food chain sequence e.g. blue tits might feed directly on both greenfly and ladybirds)

'Disruptions'

(i) Suppose the population of particular organism like the ladybird is decreased by some bacterial or viral infection. This will affect several food chains and hence the populations of other organisms. With less ladybirds to eat the greenfly, the greenfly population can increase.

(ii) Although the greenfly increase in population, there are less ladybirds for the blue tits, so the blue tit population might decline, but their numbers might remain the same because the greenfly population increases - an alternative food for the blue tits.

 

Food web example 3

3. Part of a typical complex food web in grassy woodland

Trophic level 1: The photosynthesising producers are the varieties of plants and their fruits and nuts etc.

Trophic level 2: The primary consumers are the aphids, beetles, woodlice, worms, slugs, squirrels and rabbits.

Trophic level 3: The secondary consumers can be the insect eating birds, mouse, badger, weasel and owl.

Trophic level 4: Tertiary consumers at the top of the food chain are the owl and fox (bar the fleas!).

(some organisms like the owl can be in two different trophic levels depending on the food chain sequence)

'Disruptions'

(i) If there was a decline in rabbit population, with less food for the foxes, the fox population would begin to decline, but with less foxes around, the rabbit population begins to rise, but then there is more food for the foxes, so its population can rise.

This leads to complex cycles in the rise and fall of the populations of the species involved.

A classic example is the relationship between rabbit (the prey) and fox (predator) populations in a particular community. Predator-prey cycles are always out of phase with each other because it takes some time for one population to respond to changes in the other population.

Note that if a predator eats all their chosen prey, it will die out for lack of food or move to another habitat.

In stable communities the numbers of predators and preys rises and falls but in a balanced way.

 

Food web example 4

4. Part of a typical complex marine food web in the sea.

Trophic level 1: The photosynthesising producers are the plankton, algae and seaweed.

Trophic level 2: The primary consumers are fishes like herring, mussels, crabs and limpets.

Trophic level 3: The secondary consumers are the dolphin, seal, crabs, seagull and starfish.

(some organisms like the crab can be in two different trophic levels depending on the food chain sequence)

'Disruptions'

(i) Every so often you get a massive increase in algae (an 'algal bloom') which provides lots of extra food for the primary consumers and so on. Every species benefits up the food chains.


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(e) Pyramids of biomass - how to construct them - examples explained - efficiency and why biomass is lost

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 further down 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 use the dry mass because wet mass can vary so much in water content.

 

Food chains and biomasses

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.

how to construct a pyramid of biomass on graph paper gcse physics igcse pondweed insects frogs fish bass trout

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 organismsí 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.

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

plant

producers

 sunlight energy

=========>

photosynthesis

food

overall biomass increase

available food consumed by animals

========= food chain =========>

(i) digestion and (ii) respiration at each stage

BIOMASS LOSS plant respiration ↓ overall ↓ biomass decrease
H2O, CO2 losses (i) undigested food loss, (ii) H2O, CO2 losses

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 data analysis

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

 

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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(f) Calculations of the efficiency of biomass/energy transfer up food chains

The fact of the matter is, that up a food chain/biomass pyramid, only a small percentage of the mass is passed on.

Reminder: Only ~1% of sunlight energy ends up as plant biomass.

e.g. To make the numbers easy, lets assume only 10% of biomass is passed on at each stage.

For the food chain 1 of 4 trophic levels, we would get in terms of percentages ...

plant producers (100%) ==> primary consumers (caterpillars, 10%) ==> secondary consumers (small birds 1%) ==> tertiary consumer birds of prey (0.1%).

Instead of percentages the biomass might be expressed as mass in kg or g.

plant producers (100 g) == stage 1 ==> primary consumers (caterpillars, 10 g) == stage 2 => secondary consumers (small birds 1 g) == stage 3 ==> bird of prey (0.1 g)

The typical mass of a common kestrel is 180g, so lets repeat the 10% exercise above ..

180000g of cabbage ==> 18000 g of cabbage white butterflies ==> 1800g of blue tits==> 180g kestrel

So it takes 1080 kg of cabbage to make one 0.18 kg kestrel !!!

 

You can work out how much biomass is lost by subtracting the mass left at one stage from the previous stage.  Suppose we start with 1000 g of producer (cabbage) and on pass on 10% of biomass at each stage from one trophic level to the next up the food chain - using the example above.

at stage 1 the mass loss is 1000 - 900 = 100 g

at stage 2 the mass loss is 100 - 90 = 10 g

at stage 3 the mass loss is 10 - 9 = 1 g

At the end of stage 3 the total mass loss is 900 + 90 + 9 = 999 g, only 1 g becomes part of the bird of prey (kestrel) at the top of the food chain!

100 x 999/1000 = 99.9% of the mass has been lost in the process!

The reasons for inefficient transfer of biomass and its loss are discussed in section (e).

You can also work out the efficiency of biomass transfer at each stage in the food chain

Using the following formula to calculate the efficiency of biomass transfer:

  biomass available/transferred to next level  
Efficiency = ------------------------------------------------------------------------ x 100
  biomass available from the previous level  

For stage 1: Efficiency = 100 x 100 / 1000 = 10%

For stage 2: Efficiency = 100 x 10 / 100 = 10 %

For stage 3: Efficiency = 100 x 1 / 10 = 10%

 

You can also express the efficiency of energy transfer for each stage in the food chain

Using a similar formula to calculate the efficiency of energy transfer:

  energy available/transferred to next level  
Efficiency = ------------------------------------------------------------------------ x 100
  energy available from the previous level  

Incidentally, you can draw pyramids of energy, in exactly the same way as for biomass.

Do diagrams of food chain showing energy losses ↓ in a food chain →

 

In the food chain: plants ==> rabbits ==> foxes, all these fields of plants of large areas of grass support a relatively smaller population of rabbits, which in turn support a very small number of foxes - you only get a relatively small numbers of a top predator!

This is the reason why you rarely get food chains of more than five stages (feeding/trophic levels) because there is so little mass/energy resource left at the end of the food chain.

Once the energy is lost, it can't be used by the animal in the next stage of the food chain i.e. the next trophic level.

 

It can be difficult sometimes to construct an accurate pyramid of biomass because some organisms may feed at more than one trophic level - examples have already been mentioned in the context of food webs.

 

Examples of biomass and efficiency calculations

Ex. 1. Food chain: Grass ==> rabbits ==> foxes

Suppose 500 kg of grass supports 25 kg of rabbit, which in turn supports 2 kg of fox.

(a) Calculate the biomass efficiency at each stage.

  biomass available/transferred to the next level  
Efficiency = ------------------------------------------------------------------------ x 100
  biomass available from the previous level  

For grass ==> rabbit: Efficiency of biomass transfer = (25/500) x 100 = 5.0%

For rabbit ==> fox: Efficiency of biomass transfer = (2/25) x 100 = 8.0%

(b) Calculate the overall biomass efficiency from grass to fox.

For grass ==> fox: Efficiency of biomass transfer = (2/500) x 100 = 0.4%

(c) Calculate the biomass lost between the 2nd and 3rd trophic levels.

mass of rabbits - mass of foxes = 25 - 2 = 23 kg

What is this as a percentage of biomass lost?

(23/25) x 100 = 92%

 

Ex. 2. A typical food chain (summarised) in a more complex ecosystem of food webs,

but starting with a given initial input quantity of sunlight energy of 1 MJ:

sunlight (1 000 000 J) ==> plants 8 000 J ==> small animal herbivores (600 J) ==> top predator carnivores (50 J)

After the 'sun' input, the figures in () represent the chemical potential energy store of the organisms.

  energy available/transferred to the next level  
Efficiency = ------------------------------------------------------------------------ x 100
  energy that was available to the previous level  

The formula for calculating the efficiency of energy transfer up a food chain.

(a) Calculate the efficiency of energy transfers involved.

sunlight ==> plants: efficiency = (8 000/1 000 000) x 100 = 0.8%

plants ==> herbivores: efficiency = (600/8000) x 100 = 7.5%

herbivores ==> carnivores: efficiency = (50/600) x 100 = 8.3%

(b) What is the overall efficiency from sunlight to top predator carnivores?

Efficiency = (final energy store of carnivores/initial sunlight energy) x 100

Overall efficiency of energy transfer = (50/1 000 000) x 100 = 0.005%

(c) What % of energy is lost from the 1st to the 3rd trophic level?

1st trophic level are the plants (8000 J)

3rd trophic level are the top carnivores (50 J)

Energy lost = 8000 - 50 = 7950 J

% energy wasted = (7950/8000) x 100 = 99.4%

Note: The efficiency of energy transfer = (50/8000) x 100 = 0.63%

 

The reasons for inefficient transfer of biomass and its loss are discussed in section (e).


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(g) More examples of food chains expressed as pyramids of biomass

 

Food chain and biomass pyramid example 1

Butterflies/caterpillars feed off cabbage ==> butterflies eaten by blue/great tit birds ==> bird of prey eg kestrel, catches the smaller birds like blue/great tits

plants producers (100%) ==> primary consumers (caterpillars/butterflies, 15%) ==> secondary consumers (small birds 2%) ==> bird of prey (0.15%)

The percentages in () represent the biomass remaining based on the original 100% of cabbage plants.

It takes plenty of vegetables to feed the local population of cabbage white butterflies, as gardeners know to their cost!, and look how little is left to produce the biomass of the kestrel.

 

Food chain and biomass pyramid example 2

Pondweed eat by tadpoles ==> eaten by water beetle ==> perch fish eat water beetle ==> otter eats perch fish

It takes al lot of pondweed to feed a batch of tadpoles.

 

The reasons for inefficient transfer of biomass and its loss are discussed in section (e).


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(h) Pyramids of numbers of individuals - how to construct them - examples explained

Construction of pyramids of numbers (NOT biomass).

Pyramids of numbers are similar to pyramids of biomass but each bar represents the relative total number of individuals in a population for one of the organisms in a food chain, NOT their total mass.

Although a pyramid of numbers can be the same shape as a regular pyramid shape of biomass, it is quite often a very different shape and not really a pyramid.

For example a bird feeds off caterpillars on a large cabbage plant. One small bird might feed on 25 caterpillars on one large cabbage plant. The 'non-pyramid' of numbers looks like the left-hand diagram below.

 

This shape is rather different for the pyramid of biomass for the same food chain (shown right-above).

 

However, in the case of another food chain e.g. in a freshwater lake/stream/river system:

Pondweed eat by tadpoles ==> eat by water beetles ==> perch fishes eat water beetles ==> otter eats perch

The pyramid of numbers is the same shape as the pyramid of biomass

Pyramid of numbers

Pyramid of biomass

 


See also

Carbon cycle, nitrogen cycle, water cycle and decomposition  gcse biology revision notes

Ecosystems - biotic & abiotic factors - interactions between organisms - interdependency


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