Sampling Organisms

Determining the Abundance & Distribution of an Organism

• Ecology is the branch of biology that studies the distribution and abundance of species, the interactions between species, and the interactions between species and their abiotic environment
  • Ecologists are biologists that study these interactions by investigating ecosystems
  • The distribution of a species describes how it is spread throughout the ecosystem
  • The abundance of a species is the number of individuals of that species
• The distribution and abundance of a species in an area can be assessed using different practical methods, including:
  • Quadrats
  • Belt Transects

Quadrats

• Some ecosystems are very complex with large numbers of different species of different population sizes
  • Measuring all the different levels of biodiversity within an ecosystem could be very time consuming
  • Finding out which species live in an ecosystem and the size of the populations requires the identification and cataloguing of all organisms present to build a species list
  • This is possible for areas that are very small or where the species are very large like trees
  • However, for larger and more complex ecosystems like rainforests, it is simply impossible to find, identify and count every organism that exists there
• To simplify things, sampling is often used to estimate the distribution and abundance of species (different samples of the area can be taken and used to make an estimate for the total species numbers in the area)
• When carrying out sampling, square frames called quadrats can be used to mark off the area being sampled
• Quadrats are square frames made of wood or wire
• They can be a variety of sizes eg. 0.25m² or 1m²
• They are placed on the ground and the organisms within them are recorded
• Plants species are commonly studied using quadrats to estimate their abundance

Using a quadrat to investigate population size or distribution

• Quadrats must be laid randomly in the area to avoid sampling bias
  • This random sampling can be done by converting the sampling area into a grid format and labelling each square on the grid with a number
  • Then a random number generator is used to pick the sample points
• Once the quadrat has been laid on the chosen sample point the abundance of the different species present can be recorded
  • The abundance of a species is the total number of individuals of a species within a given area

Belt transects

• Throughout some areas, there can be changes in the physical conditions
  • For example, there may be changes in altitude, soil pH or light intensity
• When investigating the species distribution in these kinds of areas sampling is appropriate
• Methods using transects can help show how species distribution changes with the different physical conditions in the area
  • A transect is a line (usually formed by a measuring tape), along which samples are taken
• One form of transect is a belt transect. For a belt transect:
  • Lay out a measuring tape in a straight line across the sample area
  • Place quadrats at regular intervals along the tape and record the abundance of each species within each quadrat
• Another simpler form of transect is a line transect. 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
  • This method only measures which species are present in the area and should not be used to measure the abundance (number of individuals) of a particular species

A line transect and belt transect is carried out in the field

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.

Exam Tip

Take care with your spelling of the word ‘quadrat‘ – it is commonly written as ‘quadrant’ by students in examinations.

Practical: Determining the Abundance & Distribution of an Organism

• It is possible to investigate the relationship between organisms and their environment using field-work techniques, including quadrats and belt transects
• There are two practical scenarios you need to be aware of:
  • Investigating the population size (of a particular species) in two different areas using quadrats
  • Investigating the distribution (of a particular species) across an environmental gradient using a belt transect

Practical scenario 1: Investigating population size in two different areas using quadrats

Apparatus

• 2 tape measures
  • These are used to lay out a survey area
• Quadrat
  • This is used to sample your chosen species
• Random number generator
  • This is used to generate random sets of coordinates (locations within the survey area where you will place the quadrat)
• Species identification guide
  • This is used to help accurately identify and count individuals of your chosen species

Method

How to estimate the population size of a plant species in a survey area. You must repeat steps 1-5 in the second study area.

Results

• Once the results have been collected and the averages calculated, we can compare the abundance of the study species in each survey area
• Species abundance is likely to be influenced by biotic factors such as:
  • Competition
  • Predator-prey relationships
  • Interactions with other organisms within the food chain or food web
• The abundance will also be influenced by abiotic factors such as:
  • Light intensity
  • Mineral availability
  • Water availability
  • pH
  • Temperature
  • Salinity

Limitations

• It can be easy to miss individual organisms when counting in a quadrat, especially if they are covered by a different species
  • Solution: Use a pencil or stick to carefully move leaves out of the way to check if there is anything else underneath
• Identifying species may be tricky
  • Solution: Use a species identification guide to identify the species

Applying CORMS to practical work

• When working with practical investigations, remember to consider your CORMS evaluation

CORMS evaluation

• In this investigation, your evaluation should look something like this:
  • C – We are changing the study area where we are collecting the data
  • O – We will count the same species of organism in each quadrat
  • R – We will repeat the investigation several times to ensure reliability
  • M1 – We will count the number of the designated study species found across all quadrats
  • M2 –  this isn’t really relevant in this scenario
  • S – We will control the size of the quadrat, the random way that quadrats are placed on the ground and the day that the results were collected

Practical scenario 2: Investigating the effect of a factor on the distribution of a species

How to investigate the effect of a factor on the number of plants across a survey area

Exam Tip

When completing an ecological study, we must ensure that the data collected is random and representative of the survey area. We do this by selecting the position of the quadrat randomly and taking a minimum of 10 readings in each survey area.