Food chains, food webs, trophic levels, pyramids of biomass and energy transfer efficiency,
food chains, pyramids of numbers
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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)
Definitions, 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
and
Biodiversity, land management, waste management,
maintaining ecosystems - conservation
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(a)
Energy and biomass in food chains
Plants and algae make food via
sunlight energy and photosynthesis and this increases their biomass -
which is an important source of food and energy for animals.
Almost all life on Earth depends
on plants and algae at the base of a food chain.
This also means that most chemical potential energy for
life is indirectly derived from the Sun, which is used to make a range
of organic molecules from complex carbohydrates, sugars, proteins and
lipids (fats and oils).
The carbohydrates store chemical potential energy which
is passed down a food chain from plants/algae to animals when animas eat
these photosynthesising organisms.
Only plants, algae like seaweed or phytoplankton and
some bacteria can carry out photosynthesis.
Many land based plants depend on insects to
pollinate them and without the intervention of bees, moths and
butterflies, many species of plants would struggle to reproduce.
Carbon dioxide is slightly soluble in water (as is
oxygen) so aquatic organisms in water can carry out photosynthesis
near the surface.
This has implications for the food chains from which
we derive our own food.
So, if pollination is disrupted, so is our food
supply, so the more bees, moths and butterflies we have, the better
for our ecosystems including our food supply.
Plants use carbon dioxide and produce oxygen in
daylight, but use oxygen and produce carbon dioxide in respiration at
night.
Animals use oxygen all the time and produce carbon
dioxide all the time from respiration.
Fortunately for animals, plants keep the oxygen
level at around 20% in the Earth's atmosphere and respiration by
animals keeps the carbon dioxide level at around 0.04%, which is
sufficient for plants to thrive.
For more details see
Carbon cycle,
nitrogen cycle, water cycle, decomposition - decay etc.
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)
Definitions,
food chains, trophic levels
and decomposers
- examples explained
Reminders of
some important
definitions (in alphabetical order, explained in more detail
where appropriate)
Carnivore - an animal that only eats other
animals, not plants.
Consumer - not plants or algae, all animals
that eat plants or other animals.
Decomposer - an organism
that breaks down dead plant and animal material,
so nutrients can be recycled
back to the soil or water.
Ecosystem -
all the living things in a given area and their non-living
environmental conditions.
Environment -
The surrounding air, water, rocks and soil where an organism lives.
Food chain - a linear part
of a food web, starting with a producer, ending with a top predator.
Food web - shows how food
chains in an ecosystem are linked - shows interdependence.
Herbivore - an animal that only eats plants -
e.g. aphids, caterpillars, rabbits, tadpoles.
Interdependence - organisms in food chains
and food webs are dependent on each other,
change in one organism population results in
changes in other organism populations.
Omnivore - an animal that eats both plants or
animals.
Population -
The number of a group of the same species living in an area.
Primary consumer - an animal that eats
producers e.g. eats plants or algae.
Producer - plants and algae - initial
producers of food from sunlight energy using photosynthesis
Secondary consumer - an animal that eats
primary consumers i.e. another animal
Tertiary consumer - an animal that eats
secondary consumers i.e. another animal
Top carnivore - an animal not eaten by any
other animal.
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 on this page.
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 (top
predators).
(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, 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.
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.
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).
TOP OF PAGE and
sub-index
(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).
TOP OF PAGE and
sub-index
(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
More examples of pyramids of numbers
u = producer,
v =
primary consumer, w
= secondary consumer,
x =
tertiary consumer numbers
Pyramid of numbers 1 food chain examples
mosses => beetles => birds => sparrowhawk
algae => crabs => seals => eskimos
Pyramid of numbers 2 food chain examples
grass => rabbits => foxes => fleas
grasses => zebra => lion => fleas
Pyramid of numbers 3 food chain examples
grass => cattle => ticks => birds
grass => antelope => mites => birds
Pyramid of numbers 4 food chain examples
rose bush => greenfly => ladybirds => swallows
tree => beetles => mice => owls
Pyramid of numbers 5 food chain examples
fruit bush => caterpillars => birds => fleas
tree => monkeys => tigers => mosquitos
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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