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

Learning Objective

Describe and apply suitable methods to assess the distribution and abundance of organisms in a defined area, focusing on:

• Frame quadrats
• Line transects
• Belt transects
• Mark‑release‑recapture using the Lincoln index

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).

1. Place the quadrat randomly or at regular intervals within the study site.
2. Identify and count all individuals of the target species inside the frame.
3. Record the data and repeat the process across multiple quadrats to obtain a representative sample.

Data derived: Density (individuals per unit area) calculated as

\$\text{Density} = \frac{\text{Total number of individuals counted}}{\text{Number of quadrats} \times \text{Area of one quadrat}}\$

2. Line Transects

A line transect consists of a straight line of known length laid across the habitat. Observers record organisms encountered within a fixed distance on either side of the line.

1. Lay a measuring tape or rope to define the transect line.
2. Walk the line at a steady pace, noting each organism that falls within the predetermined width (e.g., 2 m total width).
3. Calculate the abundance per unit area using the transect length and width.

Formula for density:

\$\text{Density} = \frac{\text{Number of individuals observed}}{\text{Transect length} \times \text{Transect width}}\$

3. Belt Transects

A belt transect is similar to a line transect but covers a larger, continuous strip (or “belt”) of habitat, allowing for more detailed sampling of both mobile and sessile organisms.

1. Define the belt dimensions (e.g., 50 m × 5 m).
2. Systematically record all organisms within the belt, often using sub‑quadrats placed at regular intervals.
3. Summarise the data to give abundance per unit area or to assess species composition across the belt.

4. Mark‑Release‑Recapture (Lincoln Index)

This method estimates the total population size of mobile animals (e.g., insects, amphibians, small mammals) by marking a sample, releasing it, and then recapturing a second sample.

Procedure

1. First capture (M): Capture a number of individuals and mark each uniquely (e.g., with a non‑toxic dye). Record the total marked, \$M\$.
2. Release: Release all marked individuals back into the population and allow sufficient time for mixing.
3. Second capture (C): Capture a new sample of size \$C\$.
4. Recaptures (R): Count how many of the \$C\$ individuals are already marked; this number is \$R\$.
5. Apply the Lincoln index to estimate the total population \$N\$.

Lincoln Index formula (Mathematical requirement):

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

Assumptions for reliable estimates:

• Marked individuals mix completely with the unmarked population.
• Marking does not affect survival or behaviour.
• Population size remains closed (no births, deaths, immigration, or emigration) between the two sampling events.

Comparison of Methods