18.14 Sampling

Purpose

• Sampling helps estimate species abundance and population size in ecosystems.
• It is impractical to count every organism, especially in large or complex areas, so samples are taken and used to make estimates.

Sampling Methods

1. Random Sampling:

• Definition: Positions of samples are chosen by chance to eliminate bias.
• When to Use:
  • The area is uniform, or there is no visible pattern in species distribution.
• Process:
  • Use measuring tapes to mark out the area.
  • Use a random number generator to determine sampling points (coordinates) for placing quadrats.

1. Systematic Sampling:

• Definition: Samples are taken at regular intervals or positions determined by the researcher.
• When to Use:
  • To study patterns in species distribution (e.g., along a transect line from a beach to a forest).
• Process:
  • Sampling points are evenly spaced, e.g., every 2 meters along a transect.

Using Quadrats for Sampling

Quadrats

• Definition: Square frames used to define a specific area for sampling populations of organisms.
• Common Sizes:
  • 0.25 m² for small organisms or dense populations.
  • 1.0 m² for larger or less dense populations.
• Purpose:
  • Identify species within the quadrat.
  • Measure abundance using methods like frequency, density, or percentage cover.

Abundance Measures

1. Species Frequency:

• Definition: The proportion of quadrats where a species is present.

1. Species Density:

• Definition: The number of individuals per unit area.

1. Percentage Cover:

• Definition: The proportion of the quadrat area occupied by a species.
• Method:
  • Divide a quadrat into smaller squares (e.g., 100 squares in a 1 m² quadrat).
  • Estimate the percentage of squares covered by each species.
• Advantages:
  • Useful for species that form a continuous covering (e.g., grass, moss).
• Disadvantages:
  • Subjective estimates can reduce accuracy.
  • May not capture overlapping species, leading to totals >100%.

1. Abundance Scales (e.g., Braun–Blanquet Scale):

• Assign values based on plant count or cover (Table 18.4).
• Example:
  • Cover <1% = “+”, cover 26–50% = “3”, etc.
• Disadvantages:
  • Estimates can be subjective.
  • Values may not accurately reflect actual abundance.

Worked Examples

Example 1: Red Sea Anemone Survey

• Data from Table 18.5:
• Quadrat area = 0.25 m².
• Total number of anemones = 12.

• Calculations:

Example 2: Benghal Dayflower Survey

• Data from Table 18.6:
• Quadrat area = 1 m².
• Two ecosystems: Lawn and Field.

• Lawn:

Key Terms

• Random Sampling: Unbiased sampling method using random coordinates.
• Systematic Sampling: Sampling at regular intervals or positions.
• Species Frequency: Percentage of quadrats where a species is found.
• Species Density: Number of individuals per unit area.
• Percentage Cover: Proportion of a quadrat covered by a species.

Summary

• Sampling methods like random quadrats allow for unbiased estimates of species abundance and distribution.
• Use species frequency for general presence/absence and species density for detailed population estimates.
• Tools like percentage cover and abundance scales are useful for plants and continuous-cover species but can introduce subjectivity.
• Random placement of quadrats ensures results are representative of the whole ecosystem.