Sampling Organisms (Edexcel GCSE Biology: Combined Science)

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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

Quadrat in use, downloadable IGCSE & GCSE Biology revision notes

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

Line and Belt Transects, downloadable AS & A Level Biology revision notes

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

A hillside transect, downloadable IGCSE & GCSE Biology revision notes

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.

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

MethodRP Ecosystems_ Estimating Population Size Method (1)_1, downloadable IGCSE & GCSE Biology revision notes

RP Ecosystems_ Estimating Population Size Method, downloadable IGCSE & GCSE Biology revision notes

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

RP Ecosystems_ Investigating the Effect of a Factor on the Distribution of a Species Method (1)_1, downloadable IGCSE & GCSE Biology revision notes

RP Ecosystems_ Investigating the Effect of a Factor on the Distribution of a Species Method (2)_1, downloadable IGCSE & GCSE Biology revision notes

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

Examiner 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.

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.