describe and use suitable methods to assess the distribution and abundance of organisms in an area, limited to frame quadrats, line transects, belt transects and mark-release-recapture using the Lincoln index (the formula for the Lincoln index will b

Biodiversity – Assessing Distribution and Abundance

Understanding how organisms are distributed and how many individuals are present in a given area is fundamental to the study of biodiversity. The following methods are commonly used at A‑Level and are suitable for a range of habitats.

1. Frame Quadrats

A frame quadrat is a square or rectangular frame of known area that is placed on the ground to sample sessile or slow‑moving organisms (e.g., plants, lichens, intertidal invertebrates).

• Procedure

  1. Lay out a random or systematic grid across the study site.
  2. Place the quadrat at each grid point and record all organisms within the frame.
  3. Count individuals or estimate percent cover for each species.
  4. Repeat for a sufficient number of quadrats to obtain a representative sample.

• Advantages

  • Simple, inexpensive, and quick to set up.
  • Provides quantitative data on density (individuals per unit area).
  • Suitable for habitats with relatively uniform substrate.

• Limitations

  • May miss mobile species that move in and out of the frame.
  • Edge effects if the quadrat overlaps different micro‑habitats.
  • Requires a sufficient number of replicates for statistical reliability.

2. Line Transects

A line transect involves walking a straight line (or a series of connected lines) and recording organisms encountered at a fixed distance from the line. It is useful for mobile animals such as birds, mammals, and insects.

• Procedure

  1. Lay a measuring tape or rope along a straight line across the habitat.
  2. Define a detection strip of width \$w\$ on each side of the line.
  3. Move along the line at a constant speed, noting each individual seen within the strip.
  4. Record the perpendicular distance from the line for each sighting (optional for distance sampling).

• Advantages

  • Efficient for covering large areas.
  • Reduces the amount of time spent in each location.
  • Can be combined with distance sampling to estimate detection probability.

• Limitations

  • Assumes all individuals within the strip are detected – often unrealistic.
  • Bias towards more conspicuous or vocal species.
  • Requires careful training to maintain a constant speed and detection width.

3. Belt Transects

A belt transect is a hybrid of quadrats and line transects. It consists of a series of contiguous quadrats laid out along a line, forming a “belt” of known width and length.

• Procedure

  1. Mark a straight line of length \$L\$ across the study area.
  2. Place quadrats of width \$w\$ at regular intervals (e.g., every 1 m) along the line.
  3. Within each quadrat, record all organisms present.
  4. Calculate density as total individuals divided by total belt area (\$L \times w\$).

• Advantages

  • Provides a more detailed picture of spatial variation than a single line transect.
  • Suitable for habitats where organisms are patchily distributed.
  • Combines the quantitative strength of quadrats with the coverage of transects.

• Limitations

  • More time‑consuming than a simple line transect.
  • Requires careful placement to avoid overlapping quadrats.
  • Potential disturbance to the habitat if many quadrats are used.

4. Mark‑Release‑Recapture (MRR) – Lincoln Index

The Lincoln index (also known as the Petersen estimator) provides an estimate of total population size (\$N\$) for mobile animals that can be captured, marked, released, and later recaptured.

Mathematical requirement:

\$ N = \frac{M \times C}{R} \$

where

• \$M\$ = number of individuals marked and released in the first capture.
• \$C\$ = total number of individuals captured in the second sample.
• \$R\$ = number of marked individuals recaptured in the second sample.

Procedure

1. Capture a sample of the target species and mark each individual uniquely (e.g., with a harmless tag).
2. Release all marked individuals back into the population and allow sufficient time for mixing.
3. After the mixing period, capture a second sample of size \$C\$.
4. Count how many of the second sample are already marked (\$R\$).
5. Insert the values into the Lincoln formula to estimate \$N\$.

Assumptions

• Marked individuals mix completely with the unmarked population.
• Marking does not affect survival or behaviour.
• Population size remains constant between the two sampling events (no births, deaths, immigration, or emigration).
• All individuals have an equal probability of being captured.

Advantages

• Provides a direct estimate of absolute population size.
• Useful for elusive or mobile species where quadrats are impractical.

Limitations

• Violations of the assumptions (e.g., trap shyness) can bias the estimate.
• Requires sufficient recaptures; very low \$R\$ leads to large uncertainties.
• Ethical considerations for handling and marking wildlife.

Summary Comparison of Methods