Ecosystems and sampling
Levels of organisation
individual → population → community → ecosystem → biome → biosphere.
- Population: organisms of one species in an area at one time.
- Community: all populations interacting in the same area.
- Ecosystem: community + abiotic environment, interacting as a system.
Biotic vs abiotic factors
| Biotic (living) | Abiotic (non-living) |
|---|---|
| Predation | Light intensity |
| Competition for food, mates, space | Temperature |
| Disease | Soil pH and moisture |
| New pathogens or invasive species | O₂ / CO₂ levels |
Both can be density-dependent (disease, food supply) or density-independent (a flood, a frost).
Quadrat sampling (PAG B4)
To estimate the size of a slow-moving / non-moving population:
- Generate random coordinates (random numbers — avoids bias).
- Place a 0.25 m² quadrat at each.
- Count the target species inside.
- Take ≥ 10 quadrats; calculate the mean per quadrat.
- Estimate total = (mean per quadrat) × (total area ÷ quadrat area).
For a gradient (e.g. shore to dune), use a belt transect: lay a tape and place quadrats at fixed intervals.
Capture–mark–recapture (mobile species)
Lincoln index: N = (n₁ × n₂) / m
Where n₁ is initial number captured/marked, n₂ is total captured second time, m is recaptured marked. Assumes: marked individuals mix randomly, no births/deaths/migration between samples, mark does not affect survival.
✦Worked example
Beetles: 30 marked, released. Next day 25 caught, 5 marked. N = (30 × 25) / 5 = 150 beetles.
OCR exam tip
The mark scheme always rewards the word random for placement. If you only say "10 quadrats", you can lose M1 even if everything else is right.
AI-generated · claude-opus-4-7 · v3-ocr-combined-science-leaves