3.2Unit 3 — Life on Earth
Distribution of Organisms — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 2 parts Lesson 2 of 6
Section 1 of 2Sampling techniques and minimizing error
| Sampling Technique | Target Organisms | Sources of Error | How to Minimise Error / Ensure Reliability |
|---|---|---|---|
| Quadrats | Stationary or slow-moving organisms (plants, ground cover) | Non-random placement (biased results); miscounting overlapping plants | Throw/place quadrats randomly (using coordinates or random number generators); establish a consistent rule for plants crossing borders; repeat multiple times to calculate a reliable average. |
| Pitfall Traps | Small ground-dwelling invertebrates (beetles, spiders, woodlice) | Trap opening not level with soil; organisms drown in rain; predators eat trapped prey; organisms escape | Set trap opening flush with ground surface; camouflage with a raised lid/stones (keeps rain and birds out); punch drainage holes in bottom; check and empty trap frequently. |
Section 2 of 2Paired-statement keys and indicator species
1Paired-statement keys2 points
:
- Identification guides consisting of numbered pairs of alternative contrasting statements based on observable physical characteristics.
- Following the instructions at the end of each chosen statement leads sequentially to the correct identification of the organism.
2Indicator species8 points
:
- Species whose presence, abundance, or absence provides measurable information about environmental quality and pollution levels in an area.
- Air pollution indicators (Sulfur dioxide, SO2)
- Crustose lichens: tolerant of high pollution (found in urban/industrial zones).
- Foliose / Fruticose (shrubby) lichens: highly sensitive to SO2; only survive in exceptionally clean air.
- Freshwater pollution indicators (Dissolved oxygen / organic waste):
- Stonefly nymphs and Mayfly larvae: require high dissolved oxygen levels (clean, unpolluted water).
- Freshwater shrimp: moderately clean water.
- Bloodworms and Sludgeworms: survive in severely polluted, low-oxygen waters.
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