Investigating Species Distribution & Abundance (WJEC GCSE Biology: Combined Science)

• It is possible to investigate the factors that affect the abundance and distribution of organisms in a habitat; such factors might include:
  • Light intensity
  • Competition from other species
  • The presence of a feature such as a road or stream
  • Disturbance, e.g. from grazing or mowing

The effect of mowing on the abundance of daisies on a school field

Apparatus

• 2 long tape measures, e.g. 10 m
• A random number generator, e.g. a calculator, a phone or a set of dice
• 1 m² quadrat
• A mowed and an unmowed area of grass within the school grounds
• An identification guide for common field plants

Method

1. Lay out the two tape measures at right-angles to each other on a mowed section of the school field, creating a set of grid axes
2. Use the random number generator to generate two random numbers to act as grid coordinates
3. Lay the quadrat at the point at which the grid coordinates meet
   • Using this system to place quadrats ensures that sample sites are chosen at random; this avoids bias and increases the likelihood that the sample results will accurately represent the area being sampled
4. Count the number of daisy plants within the quadrat
   • Include plants that fall more-than-half within the quadrat
   • You may choose to count plants that are not in flower; a field guide can be used to identify the shape of the leaves in this situation
5. Repeat steps 2-4 until you have values for at least 10 quadrats
   • The larger the number of samples collected, the more representative the samples will be and the more likely it is that any conclusions will be valid
   • The number of samples that are possible here may be limited by the time available
6. Repeat steps 1-5 on an unmowed section of field
7. Use the equation below to calculate an estimated population size for each field

Estimating abundance using a quadrat diagram

Quadrats can be placed at random grid coordinates to collect sample data for plant abundance on a school field

Analysis of results

• The sample data can be used to produce an estimate of the daisy population size for the mowed and unmowed fields as follows:

Population estimate = number of daisies in sample x (total area of field ÷ sample area)

Variables

• The independent variable here is whether or not the field has been mowed
• The dependent variable is the estimated daisy population for each field
• Control variables here might include:
  • • The time of year at which the samples are collected
    • Soil type
    • Light intensity
    • Level of disturbance, e.g. from students using the field
  • Note that some of these variables may be outside the control of a student, so you would need to be aware of the impact of the possible impact of these factors when drawing conclusions about the effect of mowing

Risk assessment

• A hazard in an experiment is something that could potentially harm you
• A risk is the harm that could occur as a result of the risk
• A control measure is the action taken to reduce the chance of the hazard causing a problem

Risk assessment for investigating species abundance practical table