Species Distribution: Skills (HL IB Biology)

Limiting factors and range of tolerance

• The term distribution refers to

Where a species is found within an ecosystem

• The distribution of a species is influenced by limiting factors, which in this context means

Any biotic or abiotic factor that restricts the growth of organisms

• Limiting factors will determine the range of tolerance, and therefore distribution, of a species, e.g. if soil water levels become severely limiting, then a plant species will be outside its range of tolerance and will no longer be able to survive in an area

Investigating the effect of limiting factors on species distribution

• It is possible to investigate the effect of a limiting abiotic factor on the distribution of a species using transect sampling
  • A transect is used to measure the change in species abundance along an environmental gradient
    • This gives an estimate of distribution at different levels of the abiotic factor in question
    • E.g. if investigating the effect of soil water availability on the distribution of a plant species, the transect would be set up along a high to low soil water gradient, and plant abundance would be measured at different soil water levels
  • Sampling is a technique that involves taking measurements from a small area to represent the measurements that might be gained across a large area
    • Sampling saves time; it is not possible to count all of the individual organisms in a habitat
    • Samples must be representative of the whole; so a large enough sample size must be taken

Transects

• Transects are lines laid out across a site that can be used to measure abundance and distribution across a habitat
  • Transects are useful for determining how species abundance and distribution might change along an environmental gradient e.g. at increasing distances from the water's edge on a rocky shore
• To carry out a transect, a tape measure is laid out along the gradient of interest, and samples are taken along the line
• There are different ways of carrying out transect studies:
  • Continuous line transect
    • Every species touching the tape measure is recorded
  • Interrupted line transect
    • Species touching the line at regular intervals, e.g. every metre, are recorded
  • Continuous belt transect
    • Quadrats are placed end-to-end along the line 
  • Interrupted belt transect
    
    • Quadrats are placed at regular intervals, e.g. every metre, along the line

Interrupted line and belt transects can be used to measure the distribution of species at different levels of an abiotic factor across a habitat

Measuring abiotic factors

• When investigating the impact of an abiotic factor on species distribution it is important to measure the relevant abiotic factor at each sample site along the transect
  • It is only necessary to record relevant abiotic factors
    • A study may only be interested in one particular abiotic factor
    • Some abiotic factors may not be relevant in certain habitats, e.g. water turbidity (cloudiness) will not be relevant in a woodland habitat
• Abiotic factors can be measured using specialised equipment and techniques

Measuring abiotic factors table

Abiotic factor	Method of measurement
Air temperature	Thermometer
Rainfall	Rain gauge; a funnel collects water in a measuring cylinder
Humidity	Hydrometer; an electronic device that measures water vapour content of air
Dissolved oxygen	Electronic oxygen sensor
Water turbidity (cloudiness)	A turbidity meter measures light scattered by particles in the water, or a Secchi disc is lowered into the water until it is no longer visible and the depth recorded
Light intensity	Electronic light meter
Landscape relief (height of land)	Contour lines on a map or a GPS
Site aspect (direction that site faces)	Compass
Slope incline	Clinometer; a specialised protractor that allows angle to be calculated using trigonometry
Soil or water pH	Indicator solution
Soil water content	Difference in mass between a soil sample and the same soil sample after it has been dried

 

Representing results

• The results of an investigation into the distribution of organisms in response to an environmental gradient 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 diagram

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