Sampling and Field Investigations

It would be impossible to count every organism in a large area. Sampling allows scientists to:

• Estimate the total population of a species
• Study the distribution of organisms (where they are found)
• Investigate how environmental factors affect organisms
• Compare different habitats or the same habitat over time

A quadrat is a square frame (usually $0.5 \text{ m} \times 0.5 \text{ m} = 0.25 \text{ m}^2$) placed on the ground to sample an area.

Using Quadrats

1. Choose the area to study
2. Place the quadrat randomly in the area:
   • Use a random number generator or random number table to determine coordinates
   • This prevents bias (choosing locations that look interesting)
3. Count or estimate the number/coverage of each species within the quadrat
4. Repeat in multiple locations (at least 10 quadrats) to make results reliable
5. Calculate the mean number of organisms per quadrat

Measuring with Quadrats

For individual organisms (e.g., daisies):

• Count the number of organisms in each quadrat
• Calculate the mean per quadrat

For organisms that cover the ground (e.g., moss, grass):

• Estimate the percentage cover — the percentage of the quadrat covered by the species
• Some quadrats have a grid of smaller squares to help estimate percentage

Random Sampling

To ensure random sampling and avoid bias:

1. Lay out two measuring tapes at right angles to form a coordinate grid
2. Generate random coordinates using a random number table or calculator
3. Place the quadrat at each set of random coordinates
4. This ensures every part of the area has an equal chance of being sampled

Once you have quadrat data, you can estimate the total population in the area:

$\text{Estimated population} = \text{Mean number per quadrat} \times \frac{\text{Total area}}{\text{Area of one quadrat}}$

Worked Example

A student placed 20 quadrats ($0.5 \text{ m} \times 0.5 \text{ m}$) randomly in a field measuring 50 m × 40 m. The mean number of buttercups per quadrat was 6.

Step 1: Area of one quadrat $= 0.5 \times 0.5 = 0.25 \text{ m}^2$

Step 2: Total area of field $= 50 \times 40 = 2000 \text{ m}^2$

Step 3: Number of quadrats that would fit in the field $= \frac{2000}{0.25} = 8000$

Step 4: Estimated population $= 6 \times 8000 = 48{,}000$ buttercups

A transect is a line placed across an area to study how the distribution of organisms changes along an environmental gradient.

Types of Transects

Line transect:

• A tape measure or string is stretched between two points
• At regular intervals (e.g., every 1 m or 5 m), record which species are touching the line
• Provides a simple picture of species distribution

Belt transect:

• A tape measure is stretched as above
• At regular intervals, a quadrat is placed next to the line
• The organisms within each quadrat are counted
• More detailed data than a line transect

When to Use Transects

• When investigating how organisms are distributed along a gradient (change in conditions)
• Examples:
  • From a path into woodland (light gradient)
  • Across a rocky shore (from sea to land — moisture gradient)
  • From a pond edge outward (water availability gradient)

For mobile animals (e.g., woodlice, snails, fish), quadrats don't work well. The capture-recapture (Lincoln Index) method is used:

Method

1. Capture a sample of animals from the habitat
2. Count them — this is the first sample ($M$)
3. Mark each animal (e.g., small dot of non-toxic paint, nail polish, tag)
4. Release them back into the habitat
5. Wait a period of time (allow them to mix with the population)
6. Recapture a second sample
7. Count the total in the second sample ($n$) and the number of marked individuals in the second sample ($m$)

The Formula

$\text{Estimated population} (N) = \frac{M \times n}{m}$

Where:

• $N$ = estimated total population
• $M$ = number caught and marked in the first sample
• $n$ = total number caught in the second sample
• $m$ = number of marked individuals in the second sample

Worked Example

A student captures 40 woodlice and marks them. After releasing them, a second sample of 50 is captured, of which 10 are marked.

$N = \frac{M \times n}{m} = \frac{40 \times 50}{10} = \frac{2000}{10} = 200$

Estimated population = 200 woodlice

Assumptions (Important!)

• The population has not changed between samples (no births, deaths, immigration, or emigration)
• Marking does not affect the animal's survival or behaviour
• Marked animals mix completely with the unmarked population
• The marking does not wear off or become invisible
• Each individual has an equal chance of being captured

You may be examined on using quadrats and transects to investigate the effect of a factor on organism distribution.

Example Investigation

Question: How does distance from a tree affect the number of daisies?

Method:

1. Place a tape measure from the base of the tree outward (transect line)
2. At regular intervals (e.g., every 2 m), place a quadrat
3. Count the number of daisies in each quadrat
4. Repeat the transect in a different direction from the tree
5. Record results and calculate means

Expected results: Closer to the tree, there may be fewer daisies (less light due to shade from the tree canopy). Further from the tree, more daisies (more light for photosynthesis).

Variables:

• Independent: Distance from the tree
• Dependent: Number of daisies per quadrat
• Control: Same size quadrat, same time of year, same tree species

• Always mention random placement when describing quadrat use — this is essential.
• Show full working for population estimation calculations.
• For capture-recapture, learn the formula $N = \frac{Mn}{m}$ and the assumptions.
• Transects are used when there's a gradient (change in conditions). Quadrats alone are for general population estimates.
• In practical questions, always identify the independent, dependent, and control variables.

• Quadrats are used to sample organisms in a fixed area; they should be placed randomly to avoid bias.
• Population estimate = mean per quadrat × (total area ÷ quadrat area).
• Transects investigate how organisms are distributed along an environmental gradient.
• The capture-recapture method estimates mobile animal populations using $N = \frac{Mn}{m}$.
• Key assumptions: no population change, complete mixing of marked individuals, marking doesn't affect survival.