6.1.1Global Challenges

The Distribution and Abundance of Organisms — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

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📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 8Distribution and abundance

Distribution describes where a species is found, while abundance is how many individuals of the species there are in an area.

Samples

To study these, ecologists cannot count every organism, so they take samples.

A sample is a small part of the habitat studied carefully.

It must be large enough and unbiased so that the findings can be used to represent the whole habitat.

Taking more samples increases reliability, and random sampling prevents the investigator from choosing areas that look more interesting.

Part 2 of 8Sampling with quadrats

A quadrat is a square frame used for sampling plants or slow-moving organisms. For a random sample:

Mark out a grid with two tape measures at right angles along the sides of the study area.

Use a random number generator to find coordinates, and place the quadrat there.

Count the number of individuals of each species inside, or estimate the percentage of the quadrat covered by each.

Repeat many times, at least 10, and calculate the mean per quadrat.

Estimated population size = mean number per quadrat x (total area / area of one quadrat). Percentage cover is useful where individual plants such as grass cannot be counted.

Part 3 of 8Transects

If you want to see how organisms change along a gradient, for example from the sea up a beach or from a path into a hedge, use a transect.

A tape measure is stretched along the line.

At regular intervals, say every 2 m, a quadrat is placed and species are recorded.

Belt transect

A belt transect uses a continuous strip or quadrats placed edge to edge.

The results are plotted against distance and compared with measurements of abiotic factors made at each point, such as light intensity, soil moisture, pH and temperature.

This helps to explain why a species is found in one place but not another.

Remember that correlation shows a link but does not prove that one factor causes the change.

Part 4 of 8Other methods for animals

Animals that move cannot be sampled with quadrats alone. Methods include:

Pitfall traps

Pitfall traps a container sunk into the ground to catch ground-dwelling insects, which fall in. They must be checked often and the animals released.

Pooters

Pooters and sweep nets for small insects on plants, and kick sampling for invertebrates in streams.

Capture-mark-recapture

Capture-mark-recapture catch a sample of animals, mark them harmlessly, release them, then catch a second sample later.

The population is estimated as (number in first sample x number in second sample) / number of marked animals in the second sample.
The method assumes that marked animals mix evenly with the others, that there is no migration and few births or deaths,
- and that marks do not harm the animals or make them more visible.

Part 5 of 8Factors that affect distribution

Organisms are found where conditions suit them.

Abiotic factors include light, temperature, moisture, soil pH, oxygen concentration (in water), wind and salinity.

Biotic factors include food availability, competition, predation and disease.

For example, daisies grow well in the open but not in the deep shade where light is limiting; mayfly larvae live only in oxygen-rich water, so their presence indicates clean water.

Indicator species

Such species are called indicator species and give a quick check on pollution.

Other organisms, including lichens, which are sensitive to sulfur dioxide, can show air quality: more lichens mean cleaner air.

Part 6 of 8Evaluating a sampling investigation

Exam questions often ask you to judge the quality of a field study.

A sound design uses random positions, a quadrat size suited to the plants, and enough quadrats for the mean to settle; a useful check is to see whether the running mean stops changing as more quadrats are added.

Control other variables when comparing two areas: sample at the same time of year and day, and use the same quadrat size.

A small number of samples, bias in placing quadrats, trampling of the area, and counting plants that are partly inside the frame inconsistently are all sources of error, and a rule such as counting only plants touching two chosen sides helps.

Plants that grow in clumps or species that are hard to tell apart make counts less accurate, in which case percentage cover is a better measure.

When results from two sites differ, compare the means and the spread of the data before concluding that the difference is real.

Part 7 of 8Zonation on a shore

A rocky shore is a good example of how distribution follows abiotic gradients.

Moving from the low tide mark up the shore, the time spent out of water increases, so desiccation, temperature range and salinity changes become more extreme.

Different seaweeds and animals live in distinct bands, or zones, because each species tolerates a different level of exposure.

Species lower down are often limited by competition and predation, while those high on the shore are limited by the harsh abiotic conditions.

A belt transect from the low to the high water mark, with quadrats at fixed intervals, records this pattern.

Part 8 of 8Exam-style questions

A student uses 0.25 m² quadrats in a field of 2000 m². The mean number of dandelions per quadrat is 3. Estimate the total number of dandelions. [3]3 marks
In a mark-recapture study, 40 snails are marked and released. A second sample has 30 snails, of which 6 are marked. Estimate the population size. [2]2 marks
Describe how you would investigate the effect of light intensity on the distribution of a plant from a hedge into an open field. [4]4 marks

Key terms

Term	Meaning
Distribution	Where an organism is found.
Abundance	How many individuals of a species are present.
Quadrat	A square frame used for sampling.
Transect	A line along which samples are taken to show changes across a habitat.
Random sampling	Choosing sample positions by chance to avoid bias.
Indicator species	A species whose presence or absence reflects environmental conditions.
Percentage cover	The proportion of the quadrat area covered by a species.
Mark-recapture	A method of estimating animal populations by marking and re-catching.

Avoid these mistakes

Placing quadrats in areas that look best. This introduces bias; use random coordinates.

Using too few samples, which makes the mean unreliable.

Forgetting to scale up the quadrat mean to the whole area.

Assuming that a correlation along a transect proves that one abiotic factor is the cause.

In mark-recapture, using marks that could harm the animals or make them easier for predators to spot.

Last updated: 20 October 2026