Organisation of an ecosystem

7. Ecology

Every food chain starts with a producer that builds biomass using energy from light. This sub-point also carries the fieldwork: quadrats, transects and the maths for turning a handful of counts into an estimate for a whole habitat.

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172 questions4 subtopicsFoundation and Higher2 required practicals

What organisation of an ecosystem covers

Every subtopic below has its own questions, mark schemes and notes.

4.7.2.1Food chains, predators and sampling

4.7.2.2Cycling materials — water and carbon

There is no new supply of materials on Earth, so everything living things are built from has to be released and used again. Two cycles are examinable — carbon and water — and microorganisms drive both by breaking down what is left behind.

4.7.2.3Decomposition, compost and biogas

Biology-only content. Decay is enzyme-controlled, so anything that affects the decomposers' enzymes affects the rate: temperature, water and oxygen. Required practical 10 measures that rate using lipase, milk and a colour change.

4.7.2.4Impact of environmental change

Biology-only and Higher Tier only. Three environmental changes shift where species can live — temperature, water availability and the composition of atmospheric gases — and the questions usually ask you to weigh up evidence rather than recall a fact.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Producer — An organism that makes its own food molecules using energy transferred from light by photosynthesis.

Biomass — The mass of living material in an organism.

Consumer — An organism that gets its food by eating other organisms.

Predator — A consumer that hunts and kills other animals for food.

Prey — An animal that is hunted and eaten by a predator.

Distribution — Where an organism is found within a habitat.

Abundance — How many individuals of a species there are in an area.

Quadrat — A square frame enclosing a known area, used for sampling.

Transect — A line across a habitat along which organisms are recorded, used to study how distribution changes.

Percentage cover — The percentage of a quadrat's area that is covered by a particular species.

Abiotic — The non-living parts of an ecosystem, such as air, water and soil.

Biotic — The living parts of an ecosystem.

Decay — The breakdown of dead material and waste by microorganisms.

Detritus feeder — An animal that feeds on dead remains and waste material.

Mineral ions — Ions released into the soil by decay and absorbed by plant roots for growth.

Combustion — Burning — it releases carbon dioxide from wood and fossil fuels.

Transpiration — The loss of water vapour from a plant.

Condensation — Water vapour cooling and turning back into liquid water.

Precipitation — Water falling from clouds as rain, snow or hail.

Decomposer — A microorganism, such as a bacterium or fungus, that breaks down dead material and waste.

Denatured — An enzyme whose active site has changed shape, so it no longer works.

Optimum temperature — The temperature at which the rate of a process is fastest.

Compost — Decomposed organic matter used as a natural fertiliser.

Anaerobic decay — Decay by microorganisms in the absence of oxygen; it produces methane.

Biogas — A fuel gas, mainly methane, produced by the anaerobic decay of waste material.

Lipase — The enzyme used in the required practical; it breaks fats down into fatty acids.

Phenolphthalein — An indicator that is pink in alkaline conditions and colourless below about pH 8.3.

Seasonal change — An environmental change that repeats with the time of year.

Geographic change — A change caused by the physical features of an area, such as altitude or soil.

Indicator species — A species whose presence or absence shows something about environmental conditions, such as lichen and air pollution.

Global warming — The rise in average global temperature, caused by human activity increasing greenhouse gases.

Where marks get lost

The mistakes that come up again and again in this topic.

Draw food chain arrows from the eaten to the eater — they follow the food, not the hunter.
Count consumers, not boxes: in a four-organism chain the last organism is the tertiary consumer, not the fourth consumer of the chain including the plant.
On a predator-prey graph, identify the predator by which curve peaks second, not by which peaks are smaller.
When the quadrat is not 1 m², convert the mean to a number per m² before multiplying by the area — and remember a smaller quadrat means a bigger number per square metre.
A transect is deliberately not random: random sampling is for estimating abundance across a whole area, while a transect is for showing change along a line.
Decay does not release carbon dioxide by itself — it is the respiration of the decomposers that does. Say so.
Plants respire all the time, not only in the dark, so they release carbon dioxide as well as absorbing it.
Carbon enters an animal in its food, not through its lungs; breathing in supplies oxygen.
The water cycle provides fresh water — the word fresh is often the mark.
The nitrogen cycle is not on this specification, so do not spend revision time on it.
Enzymes are denatured, not killed — organisms are what get killed. Using the wrong word loses the mark.
"Warmer is always faster" is only true up to the optimum; past it the rate falls.
Rate = 1000 ÷ time, so a longer time means a slower rate; check your answer is the right way round.
Give the unit s⁻¹ when calculating rate, and watch for questions asking for three significant figures.
Naming a factor is only half of a "describe and explain" answer — say how the factor affects the decomposers or their enzymes.
This sub-point is Higher Tier only, and biology only — it is not on the combined science course.
"Evaluate" means give both sides and then a judgement; answers that only agree, or only criticise, rarely reach full marks.
Not every change is caused by humans — seasonal and geographic causes are named in the specification too.
Say which of the three changes you are using (temperature, water, atmospheric gases) rather than writing vaguely about "the environment".

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