Core Practical 11: Quadrat & Transect Study
Line Transects: A tape measure is laid across the habitat. In a Continuous Line Transect, every organism touching the line is recorded. In an Interrupted Line Transect, records are only taken at specific intervals (e.g., every 2 meters).
Belt Transects: These provide more detailed data by placing quadrats along the transect line. A Continuous Belt Transect involves quadrats placed end-to-end, while an Interrupted Belt Transect places them at regular intervals.
Use Case: Transects are the primary tool for investigating zonation, such as how species change with increasing altitude or distance from a water source.
Dual Representation: Kite diagrams visually combine species distribution and abundance data on a single graph. The x-axis represents the distance along a transect, allowing for clear identification of where species thrive.
Kite Width: The vertical width of the 'kite' shape at any given point indicates the abundance of the species. A wide kite suggests high abundance, while a narrow one suggests rarity.
Abiotic Overlays: These diagrams often include secondary axes or sections to show abiotic factors like pH, light intensity, or temperature, helping to correlate environmental changes with species presence.
| Feature | Random Sampling | Systematic Sampling |
|---|---|---|
| Primary Goal | Estimate population in a uniform area | Study changes along a gradient |
| Selection Method | Random number coordinates | Fixed intervals (e.g., every 5m) |
| Bias Risk | Low (prevents human choice) | Low (if intervals are fixed) |
| Best Tool | Frame Quadrat | Transect (Line or Belt) |
Selection of Quadrat Size: Always justify your choice. If the quadrat is too small, you may miss rare species; if it is too large, the work becomes inefficient and less representative.
Consistency in Recording: In percentage cover, decide beforehand if you will count 'any part' of a plant or 'only the base.' Sticking to one rule prevents data skewing.
Abiotic Correlations: When asked to investigate the 'effect' of a factor, you must describe how to measure both the species abundance (dependent variable) and the abiotic factor (independent variable) at the exact same sampling points.
Bias in 'Random' Choice: Students often think throwing a quadrat over their shoulder is random. True randomness requires a grid and coordinate generation to ensure every square has an equal probability of selection.
Misinterpreting Zeroes: A species recorded as 'zero' in a quadrat doesn't always mean it is absent from the habitat; it may simply be clustered elsewhere, which is why a high number of samples is required for validity.
Ignoring Overlap: In dense habitats, percentage cover can exceed 100% because plants grow in layers. Failing to account for this can lead to an underestimation of the true biomass and diversity.