Required Practical: Ecosystems

Random Sampling (Estimating Population Size)

• Step 1: Lay out two tape measures at right angles to create a coordinate system (e.g., $10 m \times 10 m$) over the study area.
• Step 2: Use a random number generator to produce pairs of coordinates to determine where to place the quadrat.
• Step 3: Place the quadrat at the generated coordinates and count the number of individuals of the target species within the frame.
• Step 4: Repeat the process at least 10 times to obtain a representative sample and calculate the mean number of organisms per quadrat.

Systematic Sampling (Investigating Distribution)

• Step 1: Lay a transect line (tape measure) across the area where an environmental change is observed (e.g., from a tree trunk into an open field).
• Step 2: Place quadrats at regular intervals along the tape measure (e.g., every 2 meters).
• Step 3: Record the abundance of the species and measure the abiotic factor (e.g., light intensity using a light meter) at each interval.
• Step 4: Repeat the transect several times parallel to the first to ensure the trend is consistent across the area.

• Formula Precision: Always remember the population estimation formula: $\text{Total Population} = \frac{\text{Total Area}}{\text{Area Sampled}} \times \text{Total Counted}$. Ensure all units (e.g., $m^2$) are consistent before calculating.

• Sample Size: If an exam question asks how to improve the investigation, the most common answer is to 'increase the number of quadrats' to make the mean more representative and reduce the effect of chance.

• Abiotic Measurements: When using a transect, ensure that abiotic factors (like light or temperature) are measured at the same time of day to avoid temporal bias, as these factors fluctuate over hours.

• Consistency: State that the same size quadrat and the same counting method (e.g., only counting plants if their roots are inside the frame) must be used for all samples to ensure a fair comparison.

• Miscounting at Edges: Students often struggle with whether to count organisms that lie on the boundary of the quadrat. A consistent rule (e.g., 'count if on the top or right edge, ignore if on the bottom or left') must be applied to prevent overestimation.

• Confusing Frequency and Cover: Frequency is the percentage of quadrats a species appears in, while percentage cover is the area of the quadrat occupied by the species. Use percentage cover for species that are difficult to count individually, like moss or grass.

• Non-Random 'Randomness': Simply throwing a quadrat over your shoulder is not truly random and can be influenced by the thrower's subconscious. Always use coordinate-based random sampling for scientific validity.