Sampling to Determine Biodiversity (OCR A Level Biology)

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Sampling to Determine Biodiversity

  • Measuring biodiversity within an ecosystem can be challenging; in large and complex ecosystems it is simply impossible to find, identify and count every organism
  • For this reason sampling can be used to make an estimate of the biodiversity in an area
  • Sampling involves measuring biodiversity in multiple small areas within a habitat and using this information to represent the habitat as a whole; it can be used to measure:
    • distribution: where organisms live within a habitat
    • abundance: how many organisms are present
  • In order to be truly representative a sample must be large enough to eliminate the effects of chance, i.e. missing a species simply because it didn't happen to occur at the sample sites assessed
    • The more samples are collected, the more likely it is that the data will be representative of the habitat
  • Sampling methods can be:
    • random
    • non-random

Random sampling

  • In random sampling the sampling sites are selected at random; this avoids bias that might result from a researcher choosing the sites to assess
  • Random sample sites are often selected as follows:
    • a grid system is set up by laying out two long tape measures along the outer edges of a habitat
    • a random number generator is used to generate two random numbers
    • the numbers are used like graph coordinates, and the sample site is placed at the point of intersection
  • When a sampling area is reasonably uniform or has no clear pattern of species distribution then random sampling is the best choice

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A grid system can be used to select sample sites at random

Non-random sampling

  • Random sampling is not always possible, or may not yield the type of results required; in these situations non-random sampling may be more appropriate
  • There are three main types of non-random sampling:
    • opportunistic sampling
      • Sampling is carried out on the basis of opportunity, e.g. sample sites that can be reached quickly and safely are chosen
    • stratified sampling
      • The number of random samples taken within each habitat type is proportional to the area covered by each habitat type
      • E.g. if 10 % of a dense woodland contains grassy clearings then 90 % of the sample locations should have tree cover and 10 % will be in clear areas
    • systematic sampling
      • Samples are taken across a habitat with reference to the changes in habitat conditions
      • E.g. sample sites may be taken at:
        • increasing altitude
        • increasing distance from the edge of a woodland
        • increasing distance from the shore line
      • Systematic sampling involves the use of transects

Sampling methods

Quadrat sampling

  • A quadrat is a square frame that is placed within the area to be studied to provide a sample
  • Quadrats are suitable for sampling plants or slow-moving animals
  • Quadrats can be different sizes depending on the species being studied
    • A 1 m² quadrat can be used to study small organisms such as herbaceous plants in a grassland or limpets on a rocky shore
    • A 400 m² quadrat can be used to study large organisms such as trees
        • Quadrats like this will usually be marked out with string rather than a frame!
  • Quadrats can be placed in a habitat randomly or along a transect
  • Measurements within a quadrat might include:
    • the number of species present
    • the number of individuals of a species present, i.e. species abundance
    • the presence/absence of a species
    • the percentage cover of a species, e.g. for species where it is difficult to determine where one individual ends and another starts

Quadrat in use, downloadable IGCSE & GCSE Biology revision notes

Quadrats are frames that can be used to sample biodiversity

Percentage Cover in Quadrat, downloadable AS & A Level Biology revision notes

Quadrats can be used to assess percentage cover when it is impossible to count all of the individual organisms

Sweeping nets

  • Sweeping, or sweep, nets are large, strong nets made from a material with very small holes
  • The nets can be swept across vegetation, e.g. grass or tree branches, to catch flying insects and insects that live on leaves
  • After sampling the insects in the net can be counted and identified

Pitfall traps

  • Pitfall traps are cans or jars that are sunk into the ground
  • Ground-dwelling invertebrates fall into the trap as they walk along the ground, and then cannot climb out again
  • A lid ensures that the trap does not fill with rainwater

Invertebrate sampling techniques (1), downloadable AS & A Level Biology revision notes

Pitfall traps are used to sample ground-dwelling invertebrates

Pooters

  • Pooters are small plastic or glass containers with two tubes extending from the lid
  • The tubes can be used to suck up small invertebrates
    • The first tube is placed over the insect and the second tube is used by the scientist to create suction
  • Once inside the container the sampled organisms can be easily viewed and identified

Invertebrate sampling techniques (2), downloadable AS & A Level Biology revision notes

Pooters can be used to collect small invertebrates for viewing and identification

Transects

  • A transect is a line along which samples can be taken
  • They are used during systematic sampling to assess how abundance and distribution of organisms is affected by changes in abiotic factors across a habitat
  • Transect lines are laid out across a habitat along the gradient that is to be investigated, e.g.
    • at right angles to the edge of a field
    • vertically up a slope
    • at right angles to a water course
  • Types of transect include:
    • line transect:
      • Lay out a measuring tape in a straight line across the sample area
      • At regular distances along the tape, e.g. every 1 m, record the identity of the organisms that touch the line
    • belt transect:
      • Lay out a measuring tape in a straight line across the sample area
      • Place quadrats at regular intervals, or continuously, along the tape and record the abundance or percentage cover of each species within each quadrat

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

Line and belt transects can be used to assess how the abundance and distribution of species changes across a habitat in response to an abiotic factor

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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.