2.1F Population abundance estimation Notes

1. Estimating population abundance is essential for understanding the distribution and health of species within an ecosystem.
2. To do this, scientists use various sampling techniques depending on the characteristics of the area and species being studied.
3. These techniques include random sampling, systematic sampling, and transect sampling.

Sampling Principles

1. The choice of sampling method depends on:
   1. The mobility and distribution of organisms
   2. The size and accessibility of the study area
   3. The precision required for the investigation
   4. The available time, resources, and equipment
2. The three most commonly used methods are random sampling, systematic sampling, and transect sampling.

Random Sampling

1. In random sampling, every point or location within the study area has an equal chance of being selected.
2. This method avoids researcher bias and is most effective when the population is uniformly distributed.

Procedure

1. Mark the study area using two tape measures to form a grid (e.g., 10 m × 10 m).
2. Use a random number generator to select coordinates for sampling points.
3. Place a quadrat (a square frame, usually 0.25 m²–1 m²) at each coordinate.
4. Record all individuals within the quadrat or estimate their percentage cover.

Advantages

1. Reduces human bias and ensures representative data.
2. Useful for homogeneous habitats like grasslands or meadows.

Limitations

1. Inefficient for clumped or patchy populations.
2. May miss rare species if the distribution is uneven.

Systematic Sampling

1. Systematic sampling involves selecting samples at regular intervals along a grid or line.
2. It is appropriate when there is a clear environmental pattern or gradient, such as changes in soil moisture, light intensity, or altitude.

Procedure

1. Establish a baseline and place sampling points at equal distances (e.g., every 5 m).
2. Collect data at each point using quadrats or other methods.

Advantages

1. Ensures that samples cover the entire area evenly.
2. Detects spatial patterns and population gradients effectively.

Limitations

1. May introduce bias if the pattern of the species coincides with the sampling interval.
2. Not truly random, which may reduce statistical accuracy.

Transect Sampling

1. Transect sampling involves laying a line (transect) across the study area and taking samples at regular intervals along it.
2. This method is particularly useful for studying changes across environmental gradients.

Types of Transects

1. Line transect:
   Record species that touch the line at regular points.
2. Belt transect:
   Place quadrats continuously or at intervals along the line to record species within a given strip width.
3. Interrupted transect:
   Place quadrats at regular gaps (e.g., every 10 m) rather than continuously.

Applications

1. Examining zonation patterns (e.g., rocky shore species distribution from low tide to high tide).
2. Studying succession or the effect of human disturbance.

Advantages

1. Provides data on species change along environmental gradients.
2. Allows simultaneous recording of biotic and abiotic factors (e.g., light, soil type, salinity).

Limitations

1. Time-intensive.
2. May not account for microhabitat variations if only one transect is used.

Quadrat Sampling Technique

Materials Needed

1. Quadrat (frame of known area, e.g., 1 m²)
2. Measuring tapes and stakes
3. Identification guides
4. Recording sheets

Procedure

1. Select a study site and define boundaries using measuring tapes.
2. Choose a quadrat size suitable for the species (e.g., 1 m² for grass, smaller for moss).
3. Determine sampling points using random, systematic, or transect placement.
4. Record the species present, number of individuals, and percentage cover.
5. Repeat across multiple quadrats to increase accuracy.
6. Calculate mean abundance and population density.

Advantages of Quadrat Sampling

1. Non-invasive: Since it does not require capturing or disturbing organisms, it is especially useful for studying plants and slow-moving animals.
2. Quantitative: Provides objective and repeatable measurements of species abundance and distribution.
3. Simple to Use: Quadrat sampling is relatively easy to set up and can be applied to various ecosystems, including grasslands, forests, and aquatic habitats.

Limitations

1. Does not estimate population size directly: Quadrat sampling only gives an estimate of relative abundance, not the actual number of individuals in the entire study area.
2. Can be biased: Quadrat placement can influence the results and areas with high densities of organisms may be overrepresented.
3. Not suitable for highly mobile organisms: This method works best for sessile or slow-moving species, like plants, barnacles, or corals.

Estimating Abundance and Density

$$\text{Population Density} = \frac{\text{Total Number of Individuals in All Quadrats}}{\text{Total Area of All Quadrats}}$$

Percentage Frequency

$$\text{Percentage Frequency} = \frac{\text{Number of Quadrats Where Species Occurs}}{\text{Total Number of Quadrats}} \times 100$$

Estimating Population Size with the Lincoln Index

1. For mobile organisms, quadrat or transect sampling is unsuitable.
2. Instead, ecologists use the capture-mark-release-recapture method to estimate total population size.

The Formula

1. The Lincoln Index estimates population size based on the proportion of marked individuals recaptured in a second sample.
2. The formula for the Lincoln Index is:

$$\text{Population size estimate} = \frac{M \times N}{R}$$

Where:

1. M = Number of individuals caught and marked initially.
2. N = Total number of individuals recaptured.
3. R = Number of marked individuals recaptured.

Procedure

1. Capture a sample of individuals using ethical traps (e.g., Sherman traps, nets).
2. Mark each individual harmlessly (e.g., non-toxic dye, fur clipping, numbered tag).
3. Release the marked individuals into their habitat and allow time for mixing.
4. Recapture a second sample after a suitable time interval.
5. Record how many individuals are marked (R).
6. Apply the Lincoln Index formula to estimate total population size.

Assumptions of the Lincoln Index

1. The population is closed (no migration, births, or deaths between samples).
2. Marked individuals mix randomly with the population.
3. All individuals have an equal chance of being recaptured.
4. Marking does not affect behavior or survival.
5. Marks are not lost or overlooked.

Advantages

1. Non-destructive and repeatable.
2. Provides quantitative data on elusive or mobile species.
3. Allows tracking of population changes over time.

Limitations

1. Accuracy decreases if assumptions are violated.
2. Ethical concerns about marking methods.
3. Environmental factors (rain, temperature) may alter capture success.
4. Species with large home ranges may move outside the sampling area.