IB Environmental systems and societies (ESS) - Old 2.1F Population Abundance Estimation Notes

Abundance Estimation: Match the Sampling Method to the Species

1. Estimating population abundance tells us how the distribution and health of a species changes across an ecosystem.
2. You cannot count every organism, so you sample a small part of the area and scale up.
3. The three main methods are random sampling, systematic sampling, and transect sampling.
4. Your choice of method depends on four things.
   1. The mobility and distribution of the organisms.
   2. The size and accessibility of the study area.
   3. The precision the investigation needs.
   4. The time, resources, and equipment available.

Random Sampling: Unbiased Points in Uniform Habitats

1. In random sampling, every point in the study area has an equal chance of being selected.
2. This avoids researcher bias and works best when the population is evenly distributed.

How Random Sampling Works

1. Mark the study area with two tape measures to form a grid, for example 10 m by 10 m.
2. Use a random number generator to pick coordinates for the sampling points.
3. Place a quadrat, a square frame usually 0.25 m² to 1 m², at each coordinate.
4. Count all individuals inside the quadrat or estimate their percentage cover.

Advantages of Random Sampling

1. It reduces human bias and gives representative data.
2. It suits homogeneous habitats like grasslands or meadows.

Limitations of Random Sampling

1. It is inefficient for clumped or patchy populations.
2. It can miss rare species where the distribution is uneven.

Systematic Sampling: Regular Intervals Along a Gradient

1. Systematic sampling takes samples at regular intervals along a grid or line.
2. It fits areas with a clear environmental gradient, such as changing soil moisture, light intensity, or altitude.

How Systematic Sampling Works

1. Set up a baseline and place sampling points at equal distances, for example every 5 m.
2. Collect data at each point using quadrats or other methods.

Advantages of Systematic Sampling

1. It covers the whole area evenly.
2. It detects spatial patterns and population gradients clearly.

Limitations of Systematic Sampling

1. It can introduce bias if the species pattern lines up with the sampling interval.
2. It is not truly random, which can reduce statistical accuracy.

Transect Sampling: Tracking Change Across a Gradient

What Transect Sampling Is

1. Transect sampling lays a line across the study area and takes samples at regular intervals along it.
2. It is especially useful for studying change across environmental gradients.