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F215
5.3.1 Ecosystems
& Energy Flow
By Ms Cullen
Terminology
Try and define the
following terms used when
studying the environment.
Biosphere
Ecosystem
Biotic factors:
living organisms
these involve other
Abiotic factors:
Habitat
Producer (autotroph)
Can you remember the balanced equation for
photosynthesis?
6CO2 + 6H20
C6H12O6 + 6O2
Consumer (heterotroph)
Decomposer (saprotrophs)
Trophic Level
Food Chain and Food Web
Food chains and food webs describe the
feeding relationships between species in an
ecological community. They graphically
represent the transfer of material and energy
from one species to another within an
ecosystem.
 Organisms are connected to the organisms
they consume by arrows representing the
direction of biomass and energy transfer.

Pyramids of Numbers
Oak trees are very large,
but caterpillars are very
small. One oak tree can
feed many caterpillars, but
many caterpillars are
needed to feed a blue tit,
and many blue tits are
needed to feed a sparrow
hawk.
Draw a pyramid of
numbers for this food
chain.
It should look like this:
Pyramids of Biomass



The dry mass of all organisms
per unit area of ground (or water)
is called biomass. The total
biomass at a particular step in a
food chain is always less than
the total biomass at the step
before it.
Note the large biomass of the
one oak tree, compared with the
small biomass of lots of
caterpillars, indicated by the
shape of the pyramid.
The wider the bar, the more
biomass there is. Pyramids of
biomass always have this shape
(but pyramids of numbers can be
any shape).
The circle represents the producer. All of
the stored energy in the body of the
producer organism is eaten by the primary
consumer.
This circle represents the primary consumer.
Only the stored energy is eaten by the
secondary consumer.
This circle represents the secondary
consumer. Only a very small fraction
(shown in green) of the producer's
original energy is stored by the
secondary consumer. This energy is
taken into the body of the tertiary
consumer.
Energy Flow in Food Chains


As you can see from the previous diagrams, only
about 10% of the energy which the plant used for
growth is taken into the body of the carnivore. The
second consumer uses some of this energy for its
own body activities and some of the energy will be
wasted. Therefore, the amount of energy available for
the tertiary consumer is only 1% of the energy which
the primary consumer gained from the plant.
As the energy is passed along the food chain much
of it is either used or lost. Therefore there is a limit to
the number of organisms in a food chain. The top
carnivore is usually the third or fourth consumer.
Look at the diagram from Feltham
book that shows what happens to
solar radiation in an ecosystem.
Pyramids of Energy
Pyramids of energy show us the
amount of energy transfer between
each trophic level.
 It is measured in kJ per m2 per year, as
there are seasonal variations.
 Pyramids of energy include the initial
transfer of energy from the sun.

NB these figures are in Joules – convert to kJ
Productivity
The Primary Productivity of an ecosystem
is the rate at which biomass is produced per
unit area by the producers. It can be divided
into 2:
 Gross Primary Production (GPP) is the
amount of energy stored by the producers.
 Net Primary Production (NPP) is the
amount of energy in excess of what the
producer requires for its own respiration (R)
and is therefore available for the next trophic
level.
NPP= GPP – R
 Units are kJm-2 yr-1

Ecosystem
Extreme desert
Mean NPP (kJm-2
yr-1)
260
Open ocean
4700
Temperate
grasslands
Temperate
deciduous forest
Intensive
agriculture
Tropical rainforest
15000
26000
30000
40000
Mean values for NPP in a range of ecosystems
Look at graphs and information on
productivity and how it can be
calculated per year.
Questions

Answer exam Q’s on sheet.

Complete worksheet ‘Year 13 Ecology’.

Complete Qs 2, 3 & 6 on P.404-5
‘Advanced Biology For You’.
OCR A2 textbook P.196-7

Write notes/ summary on how we as
humans manipulate energy transfer.
Changing Ecosystems
Using your fieldwork notes and OCR A2 books
P.198-201 write notes on:
 How succession occurs (including terms
pioneer and climax communities).
 The stages of succession on sand dunes.
 Fieldwork methods for sampling eg quadrats
and transects.
Recycling of
Nutrients
The Carbon Cycle
The Carbon Cycle involves:





Photosynthesis which converts atmospheric
CO2 into carbon compounds
Respiration from plants and animals
Combustion of fossil fuels
Decomposition micro-organisms feed on
carbon compounds in dead plant and animal
material and release it as CO2 during
respiration
Slow release of CO2 from oceans where
carbon is locked up in carbonates, in shells
and rocks.
The Carbon Cycle
Carbon sinks
The concept of carbon sinks is based
on the natural ability of trees, other
plants and the soil to soak up carbon
dioxide and temporarily store the carbon
in wood, roots, leaves and the soil.
The Nitrogen
Cycle
Role of bacteria in the
nitrogen cycle
Decomposers (saprophytic bacteria)
These bacteria convert organic
nitrogen based compounds in dead
tissue, faeces and urine into
ammonium ions (NH4+).
Role of bacteria in the
nitrogen cycle
Nitrifying Bacteria
Convert ammonium into nitrates. This
involves 2 types of bacteria.
 Nitrosomonas, found in soil, that oxidise
ammonium compounds into nitrites
 Nitrobacter, also found in soil, oxidise
nitrites into nitrate ions.
NH4+
NO2NO3nitrosomonas
ammonium
nitrobacter
nitrite
nitrate
Role of bacteria in the
nitrogen cycle
Nitrogen Fixing Organisms (diazotrophs)
 These reduce nitrogen gas to ammonia.
 The process is catalysed by an enzyme called
nitrogenase.
N2 + 3H2
2NH3
nitrogen

hydrogen
ammonia
Only certain bacteria and cynobacteria can do
this some such as Azotobacter live freely in
soil. Others, such as Rhizobium, form root
nodules on leguminous plants.
Role of bacteria in the
nitrogen cycle
Denitrifying bacteria
 Convert nitrate ions to nitrites and then
to nitrogen gas which escapes into the
atmosphere.
 This usually occurs in anaerobic
conditions, such as waterlogged soil.
 Denitrifying bacteria include
Pseudomonas and Thiobacillus.