5 - Energy and Ecosystems

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5.1 - Food Chains and Webs (key definitions)

Producers: Photosynthetic organisms that maufacture organic substances from light energy, water and carbon dioxide. 

Primary Consumers: Obtein their energy from feeding on producers (other organisms)

Secondary Consumers: Obtein their energy from consuming primary consumers

Tertiary Consumers: Obtein their energy from consuming secondary consumers 

Secondary and tertiary consumers tend to be predators or parasites. 

Food Chains: Producer - Primary consumer - secondary consumer - tertiary consumer

                      Each term is refered to as a trophic level

Food Webs: systems of interlocking and interdependent food chains.

(Many animals dont rely on just one food sources, and the diets of different animals may overlap if they compete for the same food)

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5.2 - Energy Transfer Between Trophic Levels

Energy Loss: 


  • sunlight reflected back into space
  • not all wavelengths can be absorbed in photosynthesis
  • light may not fall on chlorophyll molecule
  • limiting factors


  • some of organism is not eaten
  • some parts cannot be digested, lost as waste
  • energy lost in excretion
  • heat from respiration
  • heat to environment
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Calculating Efficiency of energy transfers


                                     ENERGY AVAILABLE AFTER TRANSFER

                                -------------------------------------------------------------------------B  X 100

                                    ENERGY AVAILABLE BEFORE TRANSFER

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5.3 - Ecological Pyramids

Pyramids of numbers:   

  • measured in numbers
  • no account taken of size
  • number can be too large to represent

Pyramids of biomass:

  • Biomass is the total mass of the plants or animals in a particular place
  • measured in grams per square meter
  • seasonal differences are not apparent (significant in marine ecosystems)

Pyramids of energy:

  • Most accurate
  • difficult to collect data
  • results are more reliable than pyramids of biomass
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5.4 - Agricultural Ecosystems


  • used to produce foor for mankind
  • minimise energy loss between trophic levels
  • as much energy from food web to humans as possible


  • Rate at which something is produced
  • Net productivity= gross productivity - respiratory losses

Affected by: 


  • Efficiency of photosynthesising crop (can be improved by removing limiting factors)
  • Area of ground covered by crop
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Comparison of natural and agricultural ecosystems


  • Solar energy only, no aditional input
  • lower productivity
  • more species diversity
  • more genetic diversity in species'
  • nutrients naturally recycled in ecosystem
  • populations controlled by natural means
  • natural climax community


  • solar energy plus fossil fuels ect
  • higher productivity
  • less species diversity
  • less genetic diversity in species
  • artificial fertilisers used as nutrient source
  • population controlled by natural and use of pesticides and cultivation
  • prevented from reaching natural climax?
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5.5 - Pest control

Pesticides: poisonous chemicals that kill pests

Pesticides should be:

  • specific to direct organism
  • biodegradable - breaks down into harmless chemicals
  • cost effective
  • not accumulate - does not build up in organisms or down food chain

Biological agents:

  • very specific
  • once introduced, the organism reproduces itself
  • pests do not become resistant

Chemical pesticides:

  • Effects on non- target species
  • must be reapplied
  • pests develop genetic resistance
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Agricultural pests

Integrated pest control:

  • choosing well suited crops/ animals that are pest resistant
  • provide suitable habitats for natural predators
  • monitor for signs of pests
  • removing pests mechanically
  • biological agents
  • pesticides as last resource

How pests affect productivity:

  • Reduce productivity
  • Compete for resources
  • lacking resources may become limiting factor for photosynthesis or growth
  • damaging to crops
  • disease causing to animals
  • consuming crops, competition to humans
  • monocultures enable rapid spread of pests or fungi
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5.6 - Intensive rearing of domestic livestock

Effect of intensive rearing on energy conversion:

  • movement restricted - less energy lost in muscle contraction 
  • warm environment - no heat loss to surroundings
  • controlled feeding - optimum diet, minimum waste
  • Predators excluded - no loss to other organisms in food web
  • selective breeding - animals that are most efficient at converting food to body mass                                 are bred with eachother to encourage this trait. 
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Features of intensive rearing

  • efficient energy conversion
  • low cost
  • quality of food
  • use of space
  • safety
  • disease
  • use of drugs
  • animal welfare
  • pollution
  • reduced genetic diversity
  • use of fossil fuels

Environmental issues

  • animal welfare
  • destruction of habitats
  • reducing genetic variation
  • over grazing land
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