AQA GCSE Combined Science (8464), Higher tier · Biology › Ecology › Organisation of an ecosystem
Practise Levels of organisation. 20 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Food chains, the names of the feeding levels (producer, primary, secondary and tertiary consumer), predator–prey cycles, and how ecologists use quadrats and transects to measure the abundance and distribution of species. This includes a required practical, so expect planning, calculation and graph questions as well as recall.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name producers and consumers in a food chainThe producer comes first, then the primary, secondary and tertiary consumers.
4
State why food chains start with producersProducers (green plants and algae) make glucose by photosynthesis, so they produce the biomass for the rest of the chain.
4
Calculate a mean, median and modeMean = total ÷ number of values; median = middle value when in order; mode = most common value.
5
Describe using quadrats to estimate population sizePlace quadrats randomly, count the organisms, find the mean per quadrat, then scale up to the whole area.
6
Estimate population size from quadrat dataMean number per m² × total area in m².
6
Use a transect to investigate distributionPlace quadrats at regular intervals along a line and measure the factor at each point.
7
Explain predator–prey cycles from a graphPrey rise first, giving predators more food; predators rise and eat more prey, so prey fall, then predators fall.
Notes
Food chains
Photosynthetic organisms are the producers of biomass for life on Earth.
Every food chain starts with a producer, usually a green plant or alga, which makes glucose by photosynthesis.
Producers are eaten by primary consumers, which may be eaten by secondary consumers, and then by tertiary consumers.
The arrows point from the organism that is eaten to the organism that eats it.
Consumers that kill and eat other animals are predators; the animals they eat are prey.
Predator–prey cycles
In a stable community, the numbers of predators and prey rise and fall in cycles.
More prey → more food for predators → more predators survive and breed.
More predators → more prey eaten → prey numbers fall.
Fewer prey → less food → predator numbers fall → prey numbers can rise again.
On a graph, each predator peak comes after a prey peak, and there are usually fewer predators than prey.
Quadrats: how many?
A quadrat is a square frame, e.g. 50 cm × 50 cm (0.25 m²).
Place quadrats randomly (e.g. at coordinates from a random number generator on a grid) to avoid bias.
Count the organisms in each quadrat and calculate the mean number per quadrat.
Estimated population = (mean number per quadrat ÷ area of one quadrat) × total area of the habitat.
The more quadrats you use, the more representative the estimate.
Transects: where?
A transect is a line (e.g. a tape measure) across a habitat where a factor changes, such as from a hedge out into a field.
Place a quadrat at regular intervals along the line. Count the organisms (or estimate percentage cover) and measure the abiotic factor at each point.
Plot the results to see how the distribution of the species changes with the factor.
Producers make glucose by photosynthesis (usually green plants or algae)
Arrows point towards the organism doing the eating
Predator–prey graph: predator peaks come after prey peaks
Mean = sum of values ÷ number of values
Median = middle value when in order; mode = most common value
Population estimate = mean number per m² × total area in m²
Random quadrats → population size; transect → distribution along a gradient
How to answer each type of question
Name the feeding levels in a food chain
1 mark each3
The first organism is the producer.
Count along the arrows: next is the primary consumer, then the secondary, then the tertiary.
A herbivore eats producers, so it is a primary consumer.
Example. Here is a food chain from a rocky shore: seaweed → limpet → crab → herring gull (a) Name the producer. (b) Name the secondary consumer. (c) Name the herbivore.
Show the model answer
(a) Seaweed (1) (b) Crab (1) (c) Limpet (1)
Calculate a population estimate from quadrat data
2 to 3 marks6
Calculate the mean number per quadrat (total ÷ number of quadrats).
Divide by the area of one quadrat to get the number per m².
Multiply by the total area of the habitat.
Show every step; each one can earn a mark.
Example. A student placed 10 quadrats randomly in a field. Each quadrat had an area of 0.25 m². The numbers of daisies were: 4, 7, 2, 0, 5, 3, 6, 1, 4, 8 The field has an area of 1500 m². Estimate the number of daisies in the field. (3 marks)
Show the model answer
Mean = 40 ÷ 10 = 4 daisies per quadrat (1) Number per m² = 4 ÷ 0.25 = 16 (1) Estimate = 16 × 1500 = 24 000 daisies (1)
Describe how to investigate the effect of a factor on distribution (required practical)
4 to 6 marks6
Lay a tape measure (transect) from one area to another where the factor changes.
Place a quadrat at regular intervals, e.g. every 2 m.
Count the organisms or estimate percentage cover in each quadrat.
Measure the factor at each quadrat, e.g. light intensity with a light meter.
Repeat along more transects and calculate means.
Example. Plan an investigation to find out how the number of daisies changes with distance from a tall hedge at the edge of a school field. (4 marks)
Show the model answer
Lay a tape measure from the hedge out into the field, at right angles to the hedge (1). Place a quadrat at regular intervals along the tape, e.g. every 2 m (1). Count the daisies in each quadrat (1). Repeat along several transects at different points along the hedge and calculate a mean for each distance (1). Also credit: measure the light intensity at each quadrat with a light meter.
Explain a predator–prey graph
3 to 4 marks7
Describe the pattern: both populations rise and fall in cycles, with the predator peak after the prey peak.
Explain the rise in predators: more prey means more food, so more predators survive and breed.
Explain the fall in prey: more predators eat more prey.
Explain the fall in predators: less food.
Example. A graph shows that the numbers of owls and of voles (their main prey) in a forest rise and fall in cycles. Each owl peak comes about a year after a vole peak. Explain this pattern. (4 marks)
Show the model answer
When there are many voles there is plenty of food for the owls (1), so more owls survive and breed and their numbers rise (1). More owls eat more voles, so the number of voles falls (1). With fewer voles there is less food, so the owl numbers then fall; this is why the owl peaks come after the vole peaks (1).
Shortcuts and memory tricks
Predators lag: on a cycle graph the predator line always peaks a little after the prey line.
Quadrat check: dividing by 0.25 m² is the same as multiplying by 4. If a 0.25 m² quadrat holds 4 daisies, 1 m² holds 16.
Random quadrats count 'how many'; a transect shows 'where'.
Median: put the values in order first, then find the middle one.
Where marks are lost
Drawing food chain arrows the wrong way. They point towards the organism that does the eating.
Forgetting to divide by the quadrat area when it is not 1 m².
Placing quadrats where the organisms are, instead of randomly. This biases the estimate.
Saying that predator and prey numbers peak at the same time.
Using a transect to estimate the total population size. Transects show distribution; random quadrats estimate abundance.
Exam technique
In practical questions, give the details that make a method repeatable: quadrat size, how positions were chosen, the interval along the transect and the number of repeats.
Show every step of a population calculation.
When describing a graph, give values (e.g. 'the prey peaked at 800 in year 3') before explaining.
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A food chain in a woodland is shown. oak leaves → caterpillar → blue tit → sparrowhawk Which organism in the food chain is the producer?
Oak leaves (oak tree)
A student counted the snails in five quadrats. The numbers were: 5, 2, 8, 2, 6 Give the mode and the median of these numbers.
Mode 2; median 5
In the Southern Ocean:
phytoplankton are single-celled algae
krill are small animals that eat phytoplankton
Adélie penguins eat krill
leopard seals eat Adélie penguins.
Use the information to write a food chain.
phytoplankton → krill → Adélie penguin → leopard seal
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
A food chain in a woodland is shown. oak leaves → caterpillar → blue tit → sparrowhawk
(a) Which organism in the food chain is the producer?[1]
(b) Producers make glucose. Name the process they use.[1]
(c) Which organism is a secondary consumer? Tick (✓) one box.[1]
Oak leaves
Caterpillar
Blue tit
Sparrowhawk
(d) The sparrowhawk is a predator. What is a predator?[1]
(e) Explain why all food chains begin with a producer.[1]
Show the answer and mark scheme
(a)Answer: Oak leaves (oak tree)
oak leaves / oak tree
(b)Answer: Photosynthesis
photosynthesis
(c)Answer: Blue tit
(d)
an animal / consumer that kills and eats other animals
(e)
producers make their own food / synthesise molecules (by photosynthesis), so they are the source of biomass for all other organisms
Question 2Medium7 marks
A student wanted to compare the number of dandelions in two areas of a park. One area is mown every week. The other area is mown only once a year.
(a) Describe how the student could use quadrats to take samples at random positions in each area.[3]
(b) The student used 10 quadrats in each area. Explain why using more quadrats would improve the investigation.[1]
(c) The quadrats were 0.5 m × 0.5 m. The mean number of dandelions in the area mown every week was 2.4 per quadrat. Calculate the mean number of dandelions per m2 in this area.[2]
(d) There were more dandelions in the area mown every week. Suggest one reason why.[1]
Show the answer and mark scheme
(a)
lay out two tape measures at right angles along the edges of the area / divide the area into a grid
use random numbers (e.g. from a random number generator) as coordinates
place the quadrat at each pair of coordinates and count the dandelions inside it
(b)
the mean would be more representative / closer to the true mean / the effect of unusual quadrats would be reduced
(c)Answer: 9.6 per m2
area of quadrat = 0.25 m2
2.4 ÷ 0.25 = 9.6
(d)
dandelion leaves grow flat against the ground, so they are not cut by the mower (but taller plants are)
less competition for light from taller grasses / plants
more light reaches the dandelions
Question 3Hard7 marks
Limpets are small animals that live on rocks on the seashore. A student noticed that there seemed to be more limpets near the low tide line than near the high tide line.
(a) Plan an investigation to find out whether the distance from the low tide line affects the population density of limpets on a rocky shore.[6]
(b) Suggest one reason why there may be more limpets near the low tide line.[1]
Show the answer and mark scheme
(a)
at low tide, lay a tape measure (transect) from the low tide line up the shore to the high tide line
place a quadrat at regular intervals along the tape, e.g. every 2 m
count the number of limpets in each quadrat
use the same size of quadrat each time, e.g. 0.5 m × 0.5 m
repeat with several transects at different places along the shore and calculate a mean for each distance
calculate population density = mean number per quadrat ÷ area of quadrat
keep other variables the same where possible, e.g. same type of rock, same day
safety: check tide times; take care on slippery rocks
Marked with levels of response: the full level descriptors are in the app.
(b)
they are covered by water for longer, so are less likely to dry out