How is the distribution of organisms affected by their environment, and how is it measured?
The effect of abiotic and biotic factors on the distribution of organisms, the use of indicator species to judge environmental conditions, and sampling techniques such as quadrats and pitfall traps with their sources of error.
An SQA National 5 Biology answer on the distribution of organisms, covering the effect of abiotic and biotic factors on distribution, the use of indicator species to judge environmental conditions, and sampling techniques such as quadrats and pitfall traps with their sources of error.
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What this dot point is asking
The SQA wants you to explain that the distribution of organisms is affected by abiotic and biotic factors, describe how indicator species reveal environmental conditions, and describe sampling techniques (quadrats and pitfall traps), including how to use them well and the sources of error to avoid.
What affects distribution
For example, a plant that needs plenty of light will be found in open ground and not in deep shade, an abiotic effect. A plant heavily grazed by rabbits may be absent where rabbits are common, a biotic effect.
Measuring abiotic factors
Abiotic factors are measured with instruments:
- Light intensity with a light meter.
- Temperature with a thermometer.
- pH of the soil with a pH meter.
- Moisture of the soil with a moisture meter.
Each has a source of error to control. For instance, when using a light meter, shading the sensor with your body lowers the reading; when using a moisture or pH meter, the probe must be cleaned between readings so soil from one place does not affect the next.
Indicator species
For example, some lichens grow only in clean air, so finding them indicates low air pollution; certain river invertebrates survive only in clean water, so their presence indicates good water quality.
Sampling techniques
You cannot count every organism, so you take a sample and scale up.
A pitfall trap needs a cover (so rain does not flood it and birds cannot eat the catch), small drainage holes, and regular checking so trapped animals do not die or eat one another.
Examples in context
Example 1. Lichens and air pollution. Surveys of lichens on tree bark show fewer species in city centres than in the countryside, because many lichens cannot tolerate polluted air. Using lichens as indicator species lets scientists map air quality without expensive equipment.
Example 2. Daisies in shade and light. A quadrat survey across a field often finds more daisies in the open, sunlit areas than under trees. This links the distribution to an abiotic factor, light intensity, measured with a light meter, showing how distribution and abiotic factors connect.
Try this
Q1. Name the apparatus used to estimate the number of a plant species in a field. [1 mark]
- Cue. A quadrat.
Q2. State why quadrats should be placed randomly. [1 mark]
- Cue. To avoid bias and get a representative sample.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksDescribe how you would use quadrats to estimate the number of daisies in a field, and explain why the quadrats must be placed randomly.Show worked answer →
A 4-mark method answer needs the technique, the calculation idea and the reason for random sampling.
Place a quadrat on the ground and count the number of daisies inside it. Repeat this many times across the field.
Work out the average number of daisies per quadrat, then scale up by the area of the field to estimate the total.
The quadrats must be placed randomly (for example using random number coordinates) so the sample is representative and not biased towards one patch.
Markers reward (1) counting within quadrats, (2) repeating and averaging, (3) scaling up to the whole area, and (4) random placement to avoid bias.
SQA N5 style2 marksExplain what an indicator species is and give an example of what it can indicate.Show worked answer →
Two ideas: the definition and an example.
An indicator species is an organism whose presence, absence or abundance gives information about the environmental conditions, such as the level of pollution.
For example, certain lichens grow only in clean air, so their presence indicates low air pollution, while their absence can indicate high pollution.
Markers reward (1) the definition linking the species to environmental conditions and (2) a valid example such as lichens for air quality or certain invertebrates for water quality.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)