Measuring Changes in Ecosystems (DP IB Environmental Systems & Societies (ESS))

Measuring Changes in Ecosystems

Sampling

• Sampling is a method of investigating the abundance and distribution of species and populations

• There are two different types of sampling

  • Random

  • Systematic

• In random sampling, the positions of the sampling points are selected at random

  • This method avoids bias by the person that is carrying out the sampling

  • Bias can affect the results - for example, a student might choose to carry out samples in a particular location because it looks interesting, and this might give the impression that the habitat contains more species than it really does

• In systematic sampling, the positions of the sampling points are located at fixed intervals throughout the sampling site

  • This avoids accidentally missing out sections of habitat due to chance

  • Systematic sampling allows researchers to investigate the effect of the presence of certain environmental features on species distribution e.g. by taking samples along a line that extends away from an environmental feature such as a river

  • A line of this type is known as a transect

Random sampling involves selecting sample sites at random while systematic sampling involves placing sample sites at regular intervals.

Transects

• Systematic sampling is used when there is a clear change in the physical conditions across the area being studied

  • For example, there may be changes in altitude, soil pH or light intensity

  • Methods using transects can help show how species distribution changes with the different physical conditions in the area

  • A transect is a line represented by a measuring tape, along which sample are taken

• For a line transect:

  • Lay out a measuring tape in a straight line across the sample area

  • At equal distances along the tape, record the identity of the organisms that touch the line (e.g. every 2 m)

• For a belt transect:

  • Place quadrats at regular intervals along the tape and record the abundance or percentage cover of each species within each quadrat
    

    

A line transect and belt transect is carried out in a habitat

An example of a belt transect on a hillside. A quadrat is placed at regular intervals (every 10m of altitude gained) and the number of individuals (of the species being investigated e.g. buttercups) in each quadrat is recorded

Representing Results

• The results of an investigation into the distribution and abundance of organisms can be represented visually using a type of graph known as a kite diagram

• Kite diagrams can show both distribution and abundance

  • The distribution of a species along a transect can be shown by its position along a central horizontal line in each section of a kite diagram

  • Each section represents a different species  

  • The distance along the transect is given on the x-axis, to which the horizontal line is parallel

  • The abundance of a species can be shown by the width of the 'kite' around the central horizontal line

  • The shape is referred to as a kite because it extends an equal distance on each side of the central horizontal line

• Additional sections can be added to a kite diagram to show the changes in abiotic factors at different points along a transect e.g. the height above sea level or the pH of soil

Kite diagrams can be used to provide a visual representation of both abundance and distribution of species, as well as changes to abiotic factors such as elevation