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Limiting factors and primary productivity questions
Light intensity, carbon dioxide concentration and temperature as limiting factors and how to read them off a graph, the commercial use of the principle in glasshouses, the compensation point, gross and net primary productivity with their units, and calculating percentage energy transfer between trophic levels.
6 original questions · 21 marks · the limiting factors and primary productivity notes · Photosynthesis and primary productivity
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A graph shows the rate of photosynthesis against light intensity, with three curves obtained at three different carbon dioxide concentrations. The curves lie on top of one another at low light intensity and level off at different heights. Explain how to identify the limiting factor in each region of this graph.
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- B1 on the steep part at low light intensity the rate rises as light intensity rises, so light is the limiting factor there
- B1 the three curves coincide in that region, which shows the extra carbon dioxide is doing nothing, because there is not enough light energy to use what is already present
- B1 where a curve levels off, more light no longer raises the rate, so light has stopped being limiting and something else has taken over
- B1 the curves plateau at different heights, and since carbon dioxide concentration is what differs between them, carbon dioxide is the factor limiting the rate at the plateau; a plateau marks the point where the factor on the horizontal axis stopped mattering, not an absolute maximum
In a grassland the gross primary productivity is 18 000 kJ m⁻² year⁻¹ and the producers lose 6 000 kJ m⁻² year⁻¹ in respiration. The primary consumers accumulate 1 080 kJ m⁻² year⁻¹. Calculate the net primary productivity and the percentage of it transferred to the primary consumers.
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- M1 net primary productivity = gross primary productivity minus the energy lost in respiration by the producers
- A1 18 000 − 6 000 = 12 000 kJ m⁻² year⁻¹, with both parts of the unit kept
- M1 percentage transfer = energy in the consumers ÷ net primary productivity × 100 = 1 080 ÷ 12 000 × 100
- A1 9.0 per cent; dividing by the gross figure would be wrong, because the energy the producers had already respired away was never available to a herbivore
Explain why only about ten per cent of the energy in one trophic level reaches the level above it.
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- B1 not all of the material in the level below is eaten: roots, wood and dead litter are left behind
- B1 not all of what is eaten is digested, and the indigestible part leaves the body as faeces
- B1 some of what is absorbed is lost in excretion, as urea in mammals
- B1 a large share of the rest is respired to power movement and, in birds and mammals, to maintain a constant body temperature, so only what is left builds new tissue; transfer to a bird or mammal is often only 5 to 10 per cent, while transfer to an insect or a fish can be higher
A grower runs carbon dioxide enrichment continuously in a glasshouse through a dull week in December and finds no increase in yield at all. Suggest why the enrichment failed to work, and suggest how the system should be run instead.
Explain what is happening to a plant at its compensation point, and explain why a shade plant on a woodland floor can survive there while a sun plant of open ground would not.
State what is meant by a limiting factor, and state the units in which primary productivity is measured.
Practise limiting factors and primary productivity one question at a time
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