The key idea
Sampling estimates abundance and distribution without counting every organism.Good samples are representative and reduce bias.
The bit that matters
Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.
Using quadrats
A quadrat is a square frame used to sample the distribution of slow moving or non moving organisms such as plants.Quadrats are placed randomly, often using random number coordinates, to avoid bias, and the number of organisms or percentage cover in each is recorded.The mean number per quadrat is calculated and multiplied up by the ratio of the total area to the quadrat area to estimate the population of the whole habitat.
Using transects
A transect is a line, often a tape measure, laid across a habitat to investigate how the distribution of organisms changes across an area, for example from a path into a field.Quadrats are placed at regular intervals along the line and the organisms counted.This is used to study how an abiotic factor, such as light or trampling, affects the distribution of species.
Biodiversity
Biodiversity is the variety of all the different species of organisms on Earth or within an ecosystem.A greater biodiversity makes an ecosystem more stable because species depend on each other for resources.Human activities such as pollution, deforestation and increasing greenhouse gases are reducing biodiversity, which can have serious consequences for the future of life on Earth.
Maintaining biodiversity
Programmes have been put in place to try to reduce the negative effects of humans on ecosystems and maintain biodiversity.These include breeding programmes for endangered species, protection and regeneration of rare habitats, reintroduction of field margins and hedgerows on farms, reducing deforestation and recycling resources rather than dumping waste in landfill.There is often a conflict between these aims and the needs of farming and development.
Definitions to learn
Quadrat
A square frame used to sample the abundance of organisms in an area.
Transect
A line along which organisms are sampled to study changes in distribution.
Random sampling
Sampling at random positions to avoid bias and give a fair estimate.
Biodiversity
The variety of different species in an ecosystem or on Earth.
Mean
The average value found by adding the values and dividing by the number of values.
Abundance
The number of individuals of a species in a given area.
A student finds 18 daisies in five 1 m2 quadrats. Estimate the number in a 120 m2 field.
Find the mean per quadrat: 18 / 5 = 3.6.
Multiply by field area: 3.6 × 120 = 432.
Estimated population = 432 daisies.
Use random coordinates to place quadrats, not convenient patches.State the mean number per quadrat before scaling to area.Justify sampling size — more quadrats gives a more reliable estimate.
Use random coordinates for abundance estimates to reduce sampling bias.
How to score full marks
- 1State that quadrats are placed randomly to avoid bias and make the sample representative.
- 2When estimating population size, show the calculation: mean per quadrat multiplied by the total area divided by the quadrat area.
- 3Use a transect, not random quadrats, when the question asks how distribution changes across a gradient or with distance.
Try these yourself
Open each answer only after you have explained the full biological process.
1When is a transect useful?
- 1.Think about change across an area.
2Why should a student use many quadrats?
- 1.Link sample size to reliability.
3What is biodiversity?
- 1.Mention variety of living organisms.
4What is biodiversity?[1 mark]
- 1.State it is the variety of species.
5Why are quadrats placed at random positions in a field?[1 mark]
- 1.Think about bias and a fair sample.
6Describe how you would use a transect to investigate how the distribution of a plant changes from a footpath into a field.[3 marks]
- 1.Lay a line, place quadrats at intervals, record counts.
7Give two reasons why maintaining biodiversity is important and one human activity that reduces it.[3 marks]
- 1.Give benefits and a threat.
8A student uses a 0.5 metre by 0.5 metre quadrat and finds a mean of 6 daisies per quadrat in a field measuring 20 metres by 20 metres. Estimate the total number of daisies in the field, showing your working.[4 marks]
- 1.Find the quadrat area and the field area.
- 2.Scale up the mean count.
9Explain why increasing the number of quadrats used in a sampling investigation increases the reliability of the population estimate.[3 marks]
- 1.Link sample size to reducing the effect of anomalies.
10A student counts 20 woodlice under rocks along a transect and 3 woodlice on open ground. Explain this distribution in terms of abiotic factors.[4 marks]
- 1.Identify relevant abiotic factors.
- 2.Link them to woodlice biology.
11A conservation charity wants to assess whether a management programme has increased the species diversity of a meadow. Describe how they could compare biodiversity before and after the programme.[4 marks]
- 1.Describe the sampling method.
- 2.State what data to collect and how to compare.
12Explain how deforestation reduces biodiversity, including effects on both species in the forest and on the wider ecosystem.[5 marks]
- 1.Describe direct habitat loss.
- 2.Include knock-on effects on food webs and climate.