1. Ecosystem – a group of populations of organisms interacting together in a specific environment
e.g. a lake.
2. Producer - an organism that makes its own organic nutrients from inorganic raw materials using
an energy source, usually the sun.
3. Consumer – an organism that gains energy and organic nutrients from eating other organisms.
4. Herbivore – an organism which gets its energy from eating producers (plants)
5. Carnivore – an organism which gains its energy by eating other animals
6. Decomposer – an organism that gains its energy from dead or waste organic matter.
7. Food chain - a chart showing the flow of energy from one organism to the next, beginning with a
producer and ending with a carnivore
Rose → Aphid Blackbird Sparrow Hawk
8. Food web - a network of interconnecting food chains showing the energy flow through an
Fox grass snake blackbird
rabbit squirrel grasshopper
9. Trophic level – the position of an organism in a food chain, web or pyramid.
vegetation 1st trophic level (producer)
1st order consumer snail 2nd trophic level (herbivores only)
2nd order consumer thrush 3rd trophic level (omnivores, carnivores)
3rd order consumer cat 4th tropic level (usually carnivores)
10. Population – a group of organisms of one species living in the same area at the same time.
Energy flow through an ecosystem. - Energy cannot be recycled. It enters through the producers as
light energy and they transform it to chemical energy which passes up the food chain. An ecosystem
therefore needs a constant supply of energy.
Only about 10% of the energy available at one trophic level is available to the next. This is because
o Animals and plants use energy for respiration.
o Not all the food that is eaten is digested.
o Some substances are excreted
S This means that food chains rarely exceed 5 links and populations of supercarnivores are small.
S The human population is more easily sustained by eating at the 2nd
trophic level as there is more
energy available. At this level than the amount available from eating meat (about 18kg of grass is
needed to produce 1 kg of meat)
Pyramid of numbers. A pyramid, drawn to scale, shows the population present in a community or food chain
at each trophic level. Can be inverted if the producers are large e.g. forest (trees)
Pyramid of biomass A pyramid, drawn to scale, shows the mass of the population (population x average mass
of individual) present in a community or food chain at each trophic level. Can be inverted if the producers
reproduce sufficiently quickly.
15,476 Aphids x 0.4g
= 6190g = 6.19Kg
1. Use the total numbers at each trophic level and draw a pyramid of numbers on graph paper. Use a
scale of 1cm representing 100 organisms in the population.
2. Given the following masses of the organisms present, complete the table and draw to scale a
pyramid of biomass for the same organisms using a suitable scale.
Organism Average mass of
one organism g.
Population Biomass Size to scale on
Sparrow Hawk 248.0
3. What happens to the size of a step in the pyramid at each trophic level?
4. If the pyramids were drawn for a forest ecosystem would they appear different?
2 sparrow hawks
11 thrushes and 9
620 snails and 560
The raw materials needed by an ecosystem are recycled, for example, CO2 released by animal
respiration is utilised in plant photosynthesis, converted to glucose and then could return to a herbivore
when it eats the plants. The same applies to all raw materials including minerals, oxygen and water. The
flow of these nutrients in ecosystems is known as a nutrient cycle.
CO2 enters the atmosphere through respiration and is absorbed by plants for photosynthesis. Animals
then eat the plant carbohydrates, oils, proteins and organic carbon passes up the food chain. Combustion
of fossil fuels and wood also adds CO2 to the atmosphere
Labels for arrows:
Decomposition, combustion, fossilisation, photosynthesis, respiration, excretion/death, digestion
and assimilation. (some arrows can be used twice)
1. Most CO2 is found in atmosphere – around 0.04%, rising to 0.045% at night and dropping to
0.038% in daytime (depending on the season) due to changes in photosynthesis rates.
2. As seasons and day/night are in apposition, the overall atmospheric level is balanced
3. Combustion of fossil fuels and deforestation has added approximately 0.005% more CO2 to the
atmosphere in the last 50 years and the rise is increasing, contributing to global warming.
The Water Cycle
Addition of water to the atmosphere by evaporation from seas, lakes, rivers and transpiration from plants leads to the
formation of clouds by condensation.
The clouds return water to the ground by precipitation (rain) and the water is absorbed by plant roots or enters soil or
river, lake sea water.
Loss of forests lowers transpiration and therefore local rainfall (as well as leading to soil erosion and contributing to
atmospheric CO2 increase).
Carbon dioxide in air and
dissolved in water
carbon e.g. sugar,
carbon e.g. sugar,
Fossil fuels e.g. coal, oil,
S The Nitrogen Cycle
Plants absorb soil nitrate and convert it to protein. Their protein can either be ingested by herbivores
and pass up the food chain or, on death, leaf fall, broken down by saprotrophs to decay products in the
soil. Animal excreta, egesta and dead remains are also broken down by saprotrophs to decay products in
the soil. Nitrifying bacteria then convert these decay products back to nitrate to be re-used by the plants.
Denitrifying bacteria use up nitrate to gain oxygen, nitrifying bacteria make nitrate from atmospheric
1. Lightning can cause N2 (air) to react with O2 + H2O to form Ammonia, which dissolves in clouds and comes to soil in rain.
2. Nitrification. Animals excrete urea, ammonia (NH4
) is formed by saprotrophs and both are useless to plants. They are
oxidised by nitrifying bacteria to NO3
(Nitrate) which can be absorbed by plant roots and converted to protein. The
plants are then eaten by animals which manufacture animal protein.
3. Farmers add nitrate and ammonia fertilisers to soil (the amount of NO3
in soil determines how much protein plants can
make and therefore how fast they can grow).
4. Some nitrogen-fixing bacteria can fix atmospheric nitrogen, converting it directly into nitrate. Some live freely in soil,
some live in root nodules of plants of the Pea family (hence growing peas and beans and clover in a crop rotation which
is a natural way to increase soil nitrogen). The pea plant symbioses with the bacteria -- the plant supplies sugars and the
bacteria supplies nitrates, both benefit by the association.
5. Leaching -- both ammonia and nitrates are highly soluble and so can be washed out of the soil by rain, the use of nitrate
fertilisers increases the population of denitrifying bacteria
6. Denitrification - denitrifying bacteria change NO3
to N2 which leaves soil for the atmosphere.
Place all the words in bold on the correct arrows in the cycle above.
N as products of
decay in the soil
N as protein in
N as protein in
N as NO3
The main factors affecting the size of a population are
Food supply – Resources are plentiful in a small population with little competition and/or at a favourable
time of year. They may be scarce in winter.
Predation – as the supply of prey increases, the predator population will increase and probably reduce the
prey population. The predator prevents the prey population from overexploiting the ecosystem.
Disease – can be fatal in both animals and plants. It is more common in crowded conditions (as transfer
from one organism to another is easier) and more fatal in populations weakened by lack of food
When there is a natural balance within a community in terms of the physical and living components, a stable
population is maintained.
For a theoretical population, the Sigmoid growth curve normally applies when there are limited resources and
therefore competition for raw materials. It has three phases as the population reaches maturity:
S The phases of the growth curve are
S Explain the three phases of the curve for the following populations:
Population Explanation of Phase
Lag Exponential Stationary
Bacteria introduced into
Rabbit population over a
Spring and Summer
Replanted forest x
Sigmoid Growth Curve of a Population
Lag – establishment, finding resources, preparation for reproduction before the population grows.
Exponential – resources are abundant, disease and predation low allowing a very fast increase in
Stationary– resources match population and the checks provided by predation, disease,
emigration, immigration give a stable, mature population where birth and death rates are equal.
(Death – if the population overexploits the resources.)
Human population growth
is in the exponential phase and therefore the population requires ever-increasing resources. This leads to:
1. Habitat destruction for building and farming
2. Increased carbon dioxide emissions and
3. Global warming
http://www.prb.org/ is an interesting site on world human population.