Biology: Ecosystems

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(a) define the term ecosystem

All living organisms and all the non-living components in a specific habitat, and their interactions

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(b) state that

ecosystems are dynamic systems

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define the terms biotic factor and abiotic factor, using named examples

Biotic factor

How living organisms affect each other

Food supply, predation, disease

Abiotic factor

The effect of non-living components of the ecosystem

pH, temperature, soil type

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(d) define the terms


An organism that converts simple inorganic compounds into complex organic compounds


An organism that gains energy from complx organic matter


An organism that feeds on waste from other organisms, or dead organisms

trophic level

Each feeding level in a food chain

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describe how energy is transferred though ecosystems

Energy is transferred by organisms consuming each other. This is shown in a food web, with the arrows representing the flow of energy between organisms

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outline how energy transfers between trophic levels can be measured

The energy content of samples of organisms from each trophic level is measured

Each sample is dried in an oven

The samples are weighed

The samples are burned in a bomb calorimeter

The energy produced passes to a known mass of water and the temperature rise of the water is measured

How much energy is released per gram is calculated

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discuss the efficiency of energy transfers between trophic levels

Energy is lost between trophic levels because animals:

  • Never eat all of the available food
  • Cannot digest all of the food they eat
  • Use energy to respire
  • Lose heat energy to the surroundings
  • Lose energy in urine and faeces
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explain how human activities can manipulate the flow of energy through ecosystems

Replacing natural vegetation and fauna with crops and livestock

Deflecting natural succession to maintain grassland

Increasing productivity of producers through:

  • Soil improvement
  • Irrigation
  • Fertilisers
  • Removal of
    • Competing weeds
    • Damaging pathogens and pests
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Increasing productivity of producers and consumers through selective breeding or genetic engineering

Sheltering organisms from damaging environmental factors

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describe one example of primary succession resulting in a climax community

A sand dune

Pioneer plants such as sea rocket colonies the sand just above the high water mark. These can tolerate salt water spray, lack of fresh water and unstable sand.

Wind-blown sand builds up around the base of these plants, forming a ‘mini’ sand dune. As the plants die and decay, nutrients accumulate in this mini dune. As they dune gets bigger, plants like sea couch grass colonise it, which has underground stems to help stabilise the sand.

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With more stability and accumulation of more nutrients, plants like marram grass start to grow. Marram grass shoots trap windblown sand and, as the sand accumulated, the shoots grow taller to stay above the growing dune, thus trapping more sand. As the sand dune and nutrients build up, other plants colonise the sand. Many are members of the bean family, which have nodules in their roots which contain bacteria which convert nitrogen to nitrates. With nitrates available, more species colonise the dunes, stabilising them further


Algae and lichens begin to living on the bare rock

Erosion of the rock and build up of dead and rotting organisms produces enough soil for larger plants, such as mosses and fern to grow

Larger plants succeed the small plants until the climax community is reached

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describe how the distribution and abundance of organisms can be measured, using

line transects,

On a line across the habitat and record every species touching the line, and their position.

belt transects,

Quadrats places sequentially along a line transect


A square frame is placed at random in the habitat. Each species present is identified, and the abundance of each is estimated.

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point quadrats,

Frames with long pins. Lowered vertically at random. Each species which touches a pin is recorded, along with the number of times it is touched.

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describe the role of decomposers in the decomposition of organic material

Decomposers feed on waste from other organisms. They recycle materials such as Carbon and Nitrogen. If they did not break down dead organisms, energy and valuable nutrients would remain in the dead organism

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describe how microorganisms recycle nitrogen within ecosystems. (Only Nitrosomonas, Nitrobacter and Rhizobium need to be identified by name).

N2 --> NH4+

Nitrogen is fixed by bacteria such as Rhizobium that live in root nodules.

They have a mutalistic relationship with the plant- they fix the nitrogen for the plant and the plant provides the bacteria with carbon compounds (glucose)

There are proteins which absorb oxygen, keeping conditions anaerobic, so that nitrogen reductase can reduce nitrogen gas to ammonium ions

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NH4+ --> NO2-

Ammonium ions are released by bacteria in the putrefaction of proteins found in dead or waste organic matter

Nitrosomonas bacteria obtain their energy by oxidising ammonium ions to nitrites under aerobic conditions

NO2- --> NO3-

Nitrobacteria obtain their energy by oxidising nitrites to nitrates under aerobic conditions

NO3- --> N2 + N2O

Either plants absorb the nitrates from the soil to make nucleotide bases.

Or denitrifying bacteria under anaerobic conditions use nitrates as a source of oxygen for respiration, producing nitrogen gas (N2) and nitrous oxide (N2O)

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