18.16 Systematic Sampling

Definition

• Systematic Sampling: Used to study how species distribution and abundance change across a gradient, such as altitude, soil pH, moisture, light intensity, or tidal zones.

Using Transects for Systematic Sampling

1. Transect Definition:

• A transect is a straight line across a habitat, usually marked with a measuring tape. Samples are taken along this line to detect changes in species composition along environmental gradients.

1. Types of Transects:

• Line Transect:
  • Record the identity of species touching the line at set intervals.
  • Provides qualitative data (presence or absence of species).
• Belt Transect:
  • Place quadrats at regular intervals along the line.
  • Record species abundance within each quadrat.
  • Provides quantitative data, useful for detailed distribution studies.

Procedure for Systematic Sampling

1. Choose a starting point within the habitat.
2. Lay a measuring tape in a straight line across the environmental gradient.
3. Sample along the transect:

• Line Transect: Note the species touching the line at fixed intervals.
• Belt Transect: Place quadrats at regular intervals along the line to record abundance.

Worked Example: Interrupted Belt Transect on a Rocky Shore

Students used an interrupted belt transect to study seaweed and mollusc distribution along a rocky shore. Sampling was done every 20 meters from 200 meters to 0 meters relative to mean sea level. Species abundance was recorded using an abundance scale, where 5 indicates high abundance and 1 indicates low abundance.

Table: Data Collected on a Rocky Shore

Presenting Data from Belt Transects

1. Bar Charts:

• Plot the abundance of each species at each distance along the transect.

1. Kite Diagrams:

• Visual representation of species distribution and abundance:
  • Length of kite: Represents the range of species distribution.
  • Width of kite: Reflects species abundance at each interval.
• Kite diagrams allow quick visual interpretation of species distribution across an environmental gradient.

Kite Diagram Example (based on the data above):

• Distance (m): Horizontal axis.
• Species: Each species is represented by a “kite” shape, with the width corresponding to abundance at specific distances.

Example Analysis of Data

1. Seaweed Distribution:

• Bladder Wrack and Serrated Wrack are most abundant from 40 m to 100 m.
• Kelp is only present at 0 m, indicating it likely thrives in areas closer to mean sea level.

1. Mollusc Distribution:

• Rough Periwinkle is found at higher distances (180 m and 160 m).
• Limpets and Edible Periwinkles are more abundant between 20 m and 80 m.

This data reflects how species distributions vary with distance from sea level, with certain species favoring specific zones.

Advantages of Systematic Sampling

1. Detects Patterns Along Gradients:

• Useful for studying changes in species distribution along environmental gradients (e.g., tidal zones, altitude).

1. Provides Qualitative and Quantitative Data:

• Line transects give presence/absence data, while belt transects provide abundance data.

1. Effective Visualization:

• Data can be effectively visualized in kite diagrams, helping to identify zones of high and low abundance.

Limitations of Systematic Sampling

1. Potential to Miss Rare Species:

• Species that do not fall on the transect line or within quadrats may be missed.

1. Time-Intensive:

• Belt transects, especially in large areas, require significant time and resources.

1. Only Shows Patterns Along One Axis:

• Does not account for variation perpendicular to the transect line.

Key Terms

• Transect: A line across a habitat used to sample species distribution.
• Line Transect: Records species touching the line at set distances, providing qualitative data.
• Belt Transect: Places quadrats along the line to measure species abundance, providing quantitative data.
• Interrupted Belt Transect: A belt transect where quadrats are placed at intervals rather than continuously.

Summary

• Systematic Sampling with transects is suitable for studying species distribution across environmental gradients.
• Line transects are simpler and provide presence/absence data, while belt transects give detailed abundance data.
• Data Representation: Kite diagrams visually illustrate species distribution and abundance across gradients.
• Case Study: An interrupted belt transect on a rocky shore shows how seaweeds and molluscs vary with distance from mean sea level, highlighting species preferences for certain tidal zones.