Determining the Abundance & Distribution of an Organism
- Ecology is the branch of biology that studies:
- abundance of species
- the distribution of species in their habitats
- the interactions between species
- the interactions between species and their abiotic environment
- The distribution and abundance of a species in an area can be assessed using practical methods, including:
- quadrats
- belt transects
Quadrats
- Measuring all the different levels of biodiversity within an ecosystem is 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
- In large and complex ecosystems it is impossible to find, identify and count every individual organism
- To simplify things, sampling is often used to estimate the distribution and abundance of species
- Sampling involves assessing multiple small areas within a larger habitat and using these results to represent the habitat as a whole
- 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, e.g. 0.25 m2 or 1 m2
- Quadrats are placed on the ground and the organisms within them are recorded
- Quadrats are useful for investigating species that do not move around, e.g. plants or sessile animals, such as limpets on a rocky shore
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.
Examiner Tip
Take care with your spelling of the word 'quadrat' - it is commonly written as 'quadrant' by students in examinations.