Process of Stratified Sampling
1. Purpose
a) We can estimate population means of organisms, including their density and distribution.
b) We can also consider environmental gradients and profiles, including zonation and stratification.
2. Site Selection
When choosing any site, it’s important that it is clearly marked out and the location is accurately recorded. This means that further surveys (i.e. looking at temporal changes) can be carried out.
3. Choice of Ecological Surveying Technique: Quadrats
- An area where you survey the organisms present.
- Usually a square, but can be other shapes.
- Can be marked out by tape or by a frame.
- Useful where the area being sampled is uniform.
- Provides a representation of the entire area.
- If used, consider:
- Size
- Number used
- How they are placed
- Once positioned, we collect data by:
- Density
- Frequency
- Percentage Cover
- Scaling Measure
3. Choice of Ecological Surveying Technique: Transects
- A transect is a sampling line.
- It is used to investigate change over distance in an ecosystem.
4. Minimising Bias
a) Size and number of samples: The greater the number of samples taken, the greater the probability that sample mean is representative of the community. The size of quadrat should also be appropriate.
b) Random-number generators: Humans are naturally biased. We want to throw a quadrat wherever it looks most interesting. By using a random number generator, you minimize this bias.
c) Counting criteria: These are the parameters used for counting organisms within a quadrat. You need to have firm and consistent rules for the entirety of the sampling process. For example, counting every plant that is even partially in the quadrat, or the stem must be inside the quadrat, even if the foliage isn’t.
d) Calibrating equipment and noting associated precision: We use data loggers when measuring abiotic factors. However, the quality of the data collected will depend on whether the equipment has been serviced and calibrated correctly. Before any sampling occurs, they must be cleaned and checked for accuracy, then cleaned again after the excursion. Remember, if the data is not accurate, we may estimate the wrong tolerance limits for an organism.
5. Methods of data presentation
After the data has been collected, it needs to be presented. It can be presented in tables, graphs, transect line or transect profile diagrams. The information can be used to classify the ecosystem, identify the different sub-habitats within the ecosystem or identify relationships between organisms. Over time, this can provide valuable information that may inform management policies.
6. Analysis
There are a number of different things we can consider when analyzing the data. Confidence intervals and standard error of the mean serve the same purpose, to express the reliability of an estimate of the mean. When you look at scientific papers, sometimes the ’error bars’ on graphs or the ± number after means in tables represent the standard error of the mean, while in other papers they represent 95% confidence intervals.
Standard deviation is a measure of the amount of variation or dispersion of a set of values. A low SD indicates that the values tend to be close to the mean of the set, while a high SD indicates that the values are spread out over a wider range. We need this to calculate standard error.
Describe one way in which you could place quadrats randomly in a paddock.