Investigating Species Distribution (AQA A Level Biology)

• Understanding how to investigate the effects of named environmental factors on the distribution of a given species is an important skill
• The distribution of a species depends on a range of different factors, including:
  • Abiotic factors, such as light intensity, nutrient concentration and temperature
  • Biotic factors, such as intraspecific or interspecific competition for resources, or predators and pathogens

Investigating Abiotic Factors Affecting the Distribution of a Plant Species

• The distribution of plant species in an area can be assessed using two different practical methods:
  • Frame Quadrats
  • Belt Transects

• Throughout some areas, there may be obvious changes in the physical conditions (i.e. abiotic factors)
  • For example, if surveying an area of increasing altitude, temperature will clearly decrease, or if surveying from a shaded woodland into an open field, the light intensity will clearly increase

• When investigating the species distribution in these kinds of areas, systematic sampling using belt transects is more appropriate
• If there are no clear and obvious changes in the physical conditions within the area being sampled, random sampling using frame quadrats is more appropriate

Apparatus

• Quadrat
  • To calculate percentage cover of the plant species at each sampling site

• Two tape measures (at least 5m in length)
  • To set up a 5m by 5m grid (in which to randomly place quadrats) or to set up a 10m transect (along which to systematically place quadrats)

• Appropriate apparatus to measure the abiotic factor being investigated, for example:
  • Digital thermometer to measure soil temperature
  • Digital pH meter to measure soil pH
  • Photometer to measure light intensity

• Random number generator (e.g. an app, website or simply rolling dice)
  • This is used in random sampling to randomly select the coordinates at which to place the quadrats

Method for Random Sampling Using Frame Quadrats

• In the habitat you are investigating (e.g. grass field or woodland), set up the two tape measures like x and y-axes to form a large 5m by 5m grid
• Use a random number generator to generate 10 sets of random coordinates
• At each coordinate:
  • Place the quadrat with its bottom left corner on the coordinate
  • Record the percentage cover for the plant species being investigated (As the quadrat is divided into 100 smaller squares, the number of squares the species is found in is equivalent to its percentage cover in that quadrat)
  • Take a measurement of the abiotic factor being investigated

Method for Systematic Sampling Using Belt Transects

• Create a transect by placing the tape measure (or multiple tape measures if a longer transect is required)
  • The transect should allow you to see how the changing abiotic factor you are investigating affects the distribution of your plant species of interest. For example, if investigating the effect of light intensity, make sure your transect goes from an area of low light intensity (e.g. under the tree canopy) to an area of high light intensity (e.g. outside of the wooded area)

• At regular intervals along the tape measure transect (e.g. every 1m):
  • Place the quadrat
  • Record the percentage cover for the plant species being investigated
  • Take a measurement of the abiotic factor being investigated

Results

• A graph can now be plotted of the percentage cover of your plant species (the dependent variable) against the abiotic factor you are investigating (the independent variable)
  • A correlation from the graph will indicate the effect of the chosen abiotic factor on the distribution of the species

• Statistical tests, such as Spearman's Rank Correlation test, can be carried out on the data to assess how strong the correlation is (i.e. whether or not the distribution of your plant species is correlated with the abiotic factor in a statistically significant way)

Limitations

• Remember that correlation does not always mean causation
  • Even if the distribution of your plant species is correlated with the abiotic factor in a statistically significant way, this may be due to chance, or there may be other abiotic or biotic factors that are influencing the results
  • For example, your plant species may become increasingly abundant and you move from woodland into an open field but this may not be due to light intensity alone and could also be caused by other factors such as a change in soil pH or due to a change in the presence of a pollinator specific to that species

• Further investigation may be required to explore the effects of other abiotic and biotic factors on your plant species before conclusions can be confidently drawn