18.2.4—Methods to assess distribution and abundance

Choose the field method that matches the evidence needed: a transect shows how distribution changes across a gradient, while quadrats record what is present within a standard area.

1. Define the area, species and physical gradient or comparison being investigated.
2. For distribution along a gradient, lay a line transect across the area and record organisms touching the line at fixed intervals; this gives qualitative presence data.
3. For abundance within areas, place the same-sized quadrat at random or at regular transect intervals, then record each species in every quadrat.
4. Use frequency for how often a species occurs in the quadrats; use density for individuals per unit area, or percentage cover when individuals are difficult to count.
5. Repeat across multiple points, keep quadrat size, interval, observer rule and recording time consistent, and present the data against position or sample area.

A belt transect combines a line across the gradient with quadrats at regular intervals, so it can show how abundance changes quantitatively. A line transect alone records which organisms meet the line and does not provide the same abundance measure.

Randomly choosing quadrat locations reduces selection bias when estimating an area-wide value. A planned interval is useful for a clear environmental gradient, but the starting point and interval should be stated. Use the same area and sampling rules so differences are not caused by inconsistent effort.

Method choice and recording units come before statistical interpretation. Do not treat a single transect or quadrat as the whole ecosystem, and do not move this card into Pearson’s or Spearman’s correlation calculations or Simpson’s index.