Food chains, food webs and biomass transfer

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 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?



Energy and biomass in food chains

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.


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!

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. This adds to the biomass of the plant.

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

The energy from photosynthesis produces sugars and other carbohydrates, which in turn a source of energy to make fats and amino acids and proteins.

The carbohydrates and fats in the cells of plants and algae form part of the cell structure.

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

 


Food chains and trophic levels

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

Food chains must start with a producer, usually a plant.

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

Producers make their own food usually sunlight energy.

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.

Consumers are organisms that eat other organisms in a food chain.

The primary consumer is the first to eat the producer.

Primary consumers are then eaten by a secondary consumer.

Secondary consumers are the second consumers in the food chain.

Secondary consumers are eaten by tertiary consumers etc.

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

NOTE that all these organisms die and get eaten by decomposers.

 

Trophic levels

Each stage of a food chain is called a trophic level.

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

producer ==> primary consumer ==> secondary consumer ==> tertiary consumer

The arrows indicate the direction of biomass transfer, that is also the direction of energy transfer.

 

Three examples of food chains

(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 producer.

Tadpoles are the primary consumer. The water beetle is the secondary consumer.

The perch is the tertiary consumer. etc.

 

(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 consumer.

The birds are the secondary consumer. The bird of prey is the tertiary consumer - the top of this food chain.

 

(c) grass ===> rabbits ==> foxes

This food chain involves three trophic levels.

The photosynthesising grass is the producer. The rabbits are the primary consumers.

The foxes are the secondary 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

 


Food webs

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

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

All the species in a food web are interdependent. 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'.

 

Examples of food webs

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.

 

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). 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.

 

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. Although the greenfly increase in population, there are less ladybirds, so the blue tit population might not be affected - their numbers might remain the same.

(ii)

 

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, their population would begin to decline.

 

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.

 


Pyramids of biomass

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

The mass of living material (the biomass) at each stage in a food chain is less than it was at the previous stage.

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

Food chains and biomasses

Up the food chain: producer ==> primary consumer ==> secondary consumer ==> tertiary consumer etc.

The producer is usually a photosynthesising plant or algae.

How to construct a biomass pyramid:

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

In a biomass pyramid, each horizontal bar (drawn to scale) is proportional to the 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.

Up the food chain and 'up the pyramid' the biomass gets less because of loss of organic material, waste energy and even the energy from respiration, required to sustain life, eventually becomes waste energy too eg heat energy to the surroundings.

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

(i) some materials and energy are always lost in the organismsí waste materials by eg excretion (urine, droppings), fallen leaves from trees etc.

The word egestion means getting rid of undigested waste food in an animals faeces.

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

(ii) 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.

the overall simplistic equation for respiration is the opposite of photosynthesis

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

This energy is needed for all life processes, energy to do things like movement of any organism, heat to keep mammals warm,

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

plant producers (100%) ==> primary consumers (caterpillars, 40%) ==> secondary consumers (small birds 5%) ==> birds of prey (0.5%)

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

plant producers (1000 g) == stage 1 ==> primary consumers (caterpillars, 400 g) == stage 2 => secondary consumers (small birds 50 g) == stage 3 ==> bird of prey (5 g)

You can work out how much biomass is lost by subtracting the mass left at one stage from the previous stage e.g.

at stage 1 the mass loss is 1000 - 400 = 600 g

at stage 2 the mass loss is 400 - 50 = 350 g

at stage 3 the mass loss is 50 - 5 = 45 g

At the end of stage 3 the total mass loss is 600 + 350 + 47 = 995 g, only 5 g (0.5%, 1/200th) becomes the bird of prey at the top of the food chain! 99.5% of the mass and energy has been lost in the process!

 

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

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

  biomass available to next level  
Efficiency = ------------------------------------------------------------------------ x 100
  biomass that was available to the previous level  

For stage 1: Efficiency = 100 x 400 / 1000 = 40%

For stage 2: Efficiency = 100 x 50 / 400 = 12.5 % (13 % 2sf)

For stage 3: Efficiency = 100 x 5 / 50 = 10%

 

This means in this particular food chain, that of all the mass /energy you start with, only 0.5% (1/200th) eventually ends up as the owl.

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 left in the end.

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.

 

Examples of food chains expressed as a pyramid of numbers

 

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 smaller birds eg blue/great tits

plants producers (100%) ==> primary consumers (caterpillars/butterflies, 40%) ==> secondary consumers (small birds 5%) ==> bird of prey (0.5%)

It takes plenty of vegetables to feed the local population of cabbage white butterflies.

 

 

Food chain and biomass pyramid example 2

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

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

 


Pyramids of numbers

Construction of pyramids of numbers.

Pyramids of numbers are similar to pyramids of biomass but each bar represents the 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 pyramid of biomass, it is quite often a different shape.

For example a bird feeds off caterpillars on a cabbage plant. One bird might feed on 25 caterpillars on one cabbage plant. The pyramid of numbers looks like the right-hand diagram.

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

 


 

See also

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

Ecosystems - biotic & abiotic factors - interactions between organisms - interdependency  gcse biology notes

 


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