Practical: Ecological Sampling (SQA National 5 Biology): Revision Note

Investigating population size

• Sampling techniques can be used to investigate organisms in their environment, e.g. it is possible to measure:

  • the abundance of plants or invertebrates in an area

  • the effect of light / moisture on the abundance of plants in an area

Investigating the abundance of plants in an area

• A simple quadrat investigation can be used to estimate the abundance of a plant species at a particular sample site, e.g. on a school field

Apparatus

• Long measuring tape x 2

• Calculator

• Quadrat

• Clipboard with paper and pencil

• Plant ID guide (optional)

Method

1. Position your quadrats at random sample sites

   • Lay out two tape measures at the edges of a habitat; this creates a grid

   • Use a random number generator to choose two numbers that will function as grid co-ordinates

   • Place the quadrat at the intersect of the co-ordinates

2. Take a measure of abundance, e.g.:

   • count the number of individuals of a particular species within the quadrat

   • determine the percentage cover of a particular species by counting the number of small squares in which the species occurs

3. Repeat, while keeping a running mean; this ensures that your sample is large enough to be representative:

   • Start with 5 quadrats and calculate the mean number of individuals per quadrat (number of individuals ÷ 5)

   • Sample another quadrat and recalculate the mean (number of individuals ÷ 6)

   • Repeat until the mean value stabilises

4. Estimate abundance for the entire sample site, e.g.:

   • calculate the final mean number of individuals per quadrat

   • calculate the area of a single quadrat

   • calculate the number of quadrats that will cover the whole sample site

   • multiple the mean by the number of quadrats

Investigating the effect of water availability on the abundance of plants in an area

• The availability of water in the soil affects plant growth and survival; areas with more moisture often support a greater abundance of plants, while drier areas may have fewer individuals

• It is possible to investigate how water availability influences plant abundance in a habitat

Apparatus

• Long measuring tape x 2

• Calculator

• Quadrat

• Soil moisture meter

• Clipboard with paper and pencil,

• Plant ID guide (optional)

Method

1. Choose two areas: one wetter and one drier

   • E.g. a wetter area might be closer to a water source or at a lower elevation

   • Aim to keep other abiotic factors in the habitat as similar as possible, e.g. similar light, soil type and climate conditions

2. Random sampling:

   • Lay out two tape measures at the edges of a habitat; this creates a grid

   • Use a random number generator to choose two numbers that will function as grid co-ordinates

   • Place the quadrat at the intersect of the co-ordinates

3. In each quadrat record:

   • abundance of one species: either count individuals or estimate % cover

     • The method used will depend on the species being measured

   • soil moisture: take 3 quick readings with the meter and record the average

4. Repeat steps 2-3 for both areas so that you have 10 quadrats per area

5. Analysis:

   • Calculate summary values for quick comparison:

     • the mean plant abundance for each area and

     • the mean soil moisture.

   • Plot a bar chart with area on the x-axis (e.g. wetter and drier) and mean plant abundance on the y-axis.

     • Compare the heights of the bars to describe any difference in abundance between the two areas

Expected results

• In general, areas with greater water availability are expected to have a higher overall abundance of plants, as water is needed for photosynthesis, transport and cell support

• However, the results for a single species may not match this pattern:

  • Species that are adapted to dry conditions may be more common in drier soil

  • Species that need more water will be more abundant in wetter areas

Limitations

Investigating distribution of organisms

• The distribution of a species can vary across a habitat depending on abiotic factors such as light intensity, soil moisture, temperature and pH

• A line transect can be used to investigate how the distribution of a species changes in response to variation in an abiotic factor across a habitat

Apparatus

• Tape measure (30–50 m)

• Pegs or markers

• Quadrat

• Data logger with compatible probes: light, temperature, soil moisture, pH

  • If no data logger then use a separate light meter, thermometer, moisture meter and pH meter/test kit, then record manually

Method

1. Lay the transect: stretch the tape in a straight line across the study area.

   • Mark sampling points at regular intervals, e.g. every 2 m

2. At each point:

   • Place the quadrat with one edge touching the tape

   • Record abundance of your chosen species; count individuals or estimate % cover

3. Measure abiotic factors and record/log immediately, either manually or via the data logger

   • Light intensity: hold the light probe at plant height

   • Temperature: log air and ground surface temperature

   • Soil moisture: insert the probe to the same depth each time

   • Soil pH: use a pH probe with the logger or a chemical pH test

4. Repeat at every point along the transect

5. Replicate: run two more parallel transects

• Data from a line transect can be presented visually as a kite diagram as follows:

  • draw a horizontal line representing the transect distance in metres

  • for each sampling point, plot a point above and below the line to show the abundance recorded; the distance from the line represents the abundance of the species at that point

  • join the points with smooth lines to form a continuous, kite-shaped outline

  • plot the abiotic factors on separate axes that align with the distance along the transect

Expected results

• Abundance should peak where conditions meet the needs of the species and fall where they do not, e.g.:

  • higher abundance in brighter or drier spots suggests a light-demanding or drought-tolerant species

  • higher abundance in shade or wetter areas points to shade-tolerance or moisture preference

• If abundance hardly changes, the species likely has a broad tolerance, or other factors, e.g. competition, are more important

Limitations