AQA GCSE Biology Foundation (8461), Foundation tier · Ecology › Organisation of an ecosystem
Practise Levels of organisation. 12 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.
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
Notes
Food chains
diagram
Organisms that photosynthesise produce the biomass that all other life on Earth depends on.
Every food chain begins with a producer: usually a green plant or an alga that makes glucose by photosynthesis.
Next come the primary consumers (which eat producers), then the secondary consumers (which eat primary consumers), then the tertiary consumers.
The arrows point from the organism that is eaten to the organism that eats it.
A food chain always starts with a producer. Arrows show what is eaten by what.
A predator is a consumer that kills and eats other animals, which are its prey.
Predator–prey cycles
diagram
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.
Predator–prey cycle: the predator peak lags behind the prey peak.
Quadrats: how many?
diagram
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.
Random sampling: two tapes make a grid and random numbers give each quadrat's coordinates.
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?
diagram
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.
Transect: count the plants and measure the factor (e.g. light) in a quadrat at each point along the line.
Plot the results to see how the distribution of the species changes with the factor.
How to answer each type of question
Name the feeding levels in a food chain
1 mark eachGrade 3
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 answerHide the model answer
(a) Seaweed (1) (b) Crab (1) (c) Limpet (1)
Don’t lose marks
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.
More tips
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.
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.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name producers and consumers in a food chainThe producer comes first, then the primary, secondary and tertiary consumers.
Grade 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.
Grade 4
Calculate a mean, median and modeMean = total ÷ number of values; median = middle value when in order; mode = most common value.
Grade 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.
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