Sampling Organisms (Edexcel GCSE Biology)

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.

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

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