The Distribution & Abundance of Organisms (OCR Gateway GCSE Biology: Combined Science)

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Biology Project Lead

Ecological Field Investigations

Field work

  • Trends in ecology can be spotted by measuring the numbers and species of organisms in an ecosystem and how these change over time
    • Knowledge of these trends can inform humans about how to take the best care of the land, lakes, rivers and oceans
  • However, a typical ecosystem is a big place and we cannot count every single organism in it
  • In order to have an idea of how many organisms occupy a particular ecosystem, we have to make sensible estimates, based on thorough sampling methods
  • The idea is that a well-selected sample can be scaled up to give an accurate estimate of the whole population
  • Field work involves scientists getting outside and carrying out work to sample and measure various aspects of ecosystems
  • Abundance is the number of organisms of a particular species in a habitat
  • Distribution is the geographical spread of a species

A field investigation into the distribution and abundance of organisms in a habitat

  • Aim: To measure the population size of a common plant species in a habitat and use sampling techniques to investigate the effect of a factor on the distribution of this species
  • Methods:
    • Use a quadrat to estimate the population size of a plant species in a survey area
    • Use a transect line and a quadrat to investigate the effect of a factor on the number of plants in a survey area
  • Scientists can't put a quadrat on every single 50 cm × 50 cm patch of ground
    • That would be very time-consuming and tedious work
    • So a reliable sampling technique is required
    • Essentially, parts of the population are counted, so sampling has to be done in a completely random manner
    • The number of organisms in the sample is multiplied up to give a population size estimate
    • Data can also be gathered on the distribution of species within the sampling area
      • This may be due to abiotic or biotic factors which can also be investigated

Random sampling

  • Scientists must go to great lengths to ensure that sampling is random and free from bias
    • For example, in a quadrat study looking at thistle plant distribution in a field, scientists must not just focus on the areas where thistles are seen to be growing
    • There may be valuable data about why thistles are not growing in a certain part of the field e.g. soil conditions are unfavourable
  • Quadrat studies must use random number generators to identify co-ordinates within a marked-out area for quadrats to be analysed

Investigating biodiversity with quadrats 1_investigating-biodiversity-with-quadrats-2

Estimating Population Size Method

  • Stratified sampling is also useful
    • Divide a habitat into zones which appear to have different communities and take samples from each zone
      • For example, if vegetation cover in an area of moorland is 70% heather and 30% grass, take 70% of the samples from within the heather and 30% of the samples from within the grass
  • Systematic sampling is used e.g. with transects
  • Systematic sampling is used where the study area includes an environmental gradient (change of conditions across the study area)
    • For example, samples taken, every 10 meters along a line running from the sea shore, inland across a sand dune system
  • Sampling mobile organisms (most animals) presents risks of missing certain individuals or double-counting others
    • A technique called capture-recapture is employed in these cases
  • Sampling the abiotic environment is also required to be random
    • e.g. taking water samples from the right place in a stream/river system, rather than from the place that might be easiest to reach (the water)

Pooters, Pitfalls, Nets & Kick Sampling

Capturing live animals can present its challenges

  • Scientists should adopt ethical practices at all time, employing a 'do no harm' approach to field work
  • This requires animals species to be captured alive wherever possible and released back into the same habitat directly after being assessed
  • The most common types of animal to be captured are invertebrates because they present a low risk to the scientist and are relatively abundant
  • Larger, more dangerous animals require professional assistance for sampling
    • e.g. gathering data on lion populations in a game reserve may require a ranger trained in the use of tranquiliser darts
  • The following techniques can be used to capture invertebrates:


  • A small, hand-held suction device that can capture small invertebrates like flies, spiders etc
  • The scientists sucks on one tube and 'vacuums' up the creature with the other tube
  • The suction pulls the creature into the pooter chamber
  • A piece of gauze on the suction pipe prevents the operator sucking the animal into their mouth
Invertebrate sampling techniques (2)

A pooter

Pitfall traps

  • These are dug in-situ within a habitat and left for 1-2 days to fill with invertebrates
  • A canopy is placed over them to prevent flooding by rainwater
  • Leaf litter is placed inside to provide some protection to smaller animals who may become prey to larger ones inside the trap
  • Organisms fall in but cannot climb out
    • So can be identified, counted and released quickly
  • The pitfall trap is refilled with soil after use and the canopy removed

Invertebrate sampling techniques (1)

A pitfall trap


  • Sweep nets are large, hand-held nets that can be swept across a tree's leaves to capture the invertebrates in that tree
  • These must be used free of bias and in a systematic and safe way 
    • e.g. Sweep the tree for 10 seconds at 2 metre height and 10 seconds at 4 metre height etc.
  • The invertebrates are collected, counted then returned to the approximate same location

Kick sampling of streams

  • A scientist can stand in a stream and use their feet to agitate the stream bed
    • This must be done gently to release invertebrates whilst avoiding damage to them
    • A net is placed immediately downstream to catch the organisms that are disturbed
    • The net's contents are transferred to a water-filled tray for identification/counting of the organisms
    • All the tray's contents are tipped straight back into the stream as soon as possible after data has been gathered
Invertebrate sampling techniques (4)

Kick sampling a stream bed

Exam Tip

Remember the two main considerations for scientists doing this kind of work:
  • That the work is done ethically
  • That sampling is done randomly and free from bias

Quadrats, Transects & Capture-Recapture

Quadrats & transects

  • Quadrats are square frames made of wood or wire
  • They can be a variety of sizes e.g. 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 the abundance

Quadrat in use, downloadable IGCSE & GCSE Biology revision notes

Using a quadrat to investigate population size or distribution

Quadrats can be used to measure abundance by recording

  • The number of an individual species: the total number of individuals of a single species (eg. buttercups) is recorded
  • Species richness: the total number of different species (but not the number of individuals of each species) is recorded
  • Percentage cover: the approximate percentage of the quadrat area in which an individual species is found is recorded (this method is used when it is difficult to count individuals of the plant species being recorded e.g. grass or moss

Estimating percentage cover of one or more species, downloadable IGCSE & GCSE Biology revision notes

How to estimate percentage cover of one or more species using a quadrat

A transect is a row of quadrats or points placed in a line at pre-set intervals

  • Transects are useful for measuring the change in distribution of organisms across a area known to differ in abiotic factors
    • e.g. down a hillside where altitude changes
    • Across a beach and sand dune system and into mature woodland

Line and Belt Transects
Example of a belt transect setup

Capture - recapture

  • This technique is used for estimating the population size of mobile organisms
    • To avoid the risk of double-counting because organisms move about
  • Capture/collect a sample (of named species), mark them and release
  • Method of marking must not harm the organism OR make the organisms more visible to predators
  • (Release and) leave sufficient time for the (marked) organisms to (randomly) distribute around their habitat before collecting a second sample from the same area

P o p u l a t i o n space equals space fraction numerator f i r s t space s a m p l e space s i z e space cross times space s e c o n d space s a m p l e space s i z e over denominator n o. space o f space m a r k e d space o r g a n i s m s space i n space t h e space s e c o n d space s a m p l e end fraction

Worked example

Estimating the population size using capture-recapture

  • In a capture-recapture study, scientists were attempting to estimate the population of banded snails in a meadow habitat
  • On Day 1, 54 snails were collected in a 30-minute period and marked with a small band of non-toxic white paint on their shells
  • They were then released back into their habitat
  • The following day, 48 snails were captured, 9 of which were ones that had been marked the day before

Estimate the population of the yellow banded snails in the meadow habitat. You can assume that no snails entered or left the meadow, and no snails were born or died during the study. 

P o p u l a t i o n space o f space b a n d e d space s n a i l s space equals space fraction numerator 54 space cross times space 48 over denominator 9 end fraction space equals space 288
Population estimate = 288 snails

Exam Tip

Remember that all these population counting techniques only provide estimates of the population. Estimates like these are generally acceptable if the sampling techniques used have been sufficiently random.

Identification Keys

Identification of species

  • When sampling a habitat, a scientist is unlikely to know exactly which species is which
  • To help with the field work, identification keys are used
  • These ask a series of Yes/No questions which lead the scientist through a flowchart towards a positive identification
  • A good identification key for use in the field will contain detailed images/photos of various species
  • Because all the questions are Yes/No or only have only two possible answers, they are sometimes called dichotomous identification keys
  • Questions can include these examples
    • Are the leaves oval-shaped? (Yes/No)
    • Does the leaf have a sharp tip (Yes/No)
    • Does the animal have 6 or 8 legs?
    • Is the skin covered in scales or a shell?

An Example of an Identification Key


Measurement of Abiotic Factors

  • An important part of field work is to measure abiotic factors in a habitat
  • Because abiotic factors can easily change, they may be causing the populations of species to change
  • The types of abiotic factors routinely measured in field work include
    • Light intensity
      • Using a light meter/light sensor
    • Temperature (and range)
      • Using a thermometer or temperature probe
    • Soil moisture
      • This can be measured in-situ with a probe or a sample can be taken and analysed in the lab
    • Soil or water pH
      • With a pH probe or indicator solutions
    • Turbidity (cloudiness) of water
      • By measuring the maximum depth of water through which a black 'X' shape can be seen
    • Dissolved oxygen in water
      • Using an oxygen meter
    • Wind speed/direction 
      • Using an anemometer
    • Pollutant levels
      • Using various analytical methods, depending on the pollutant

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Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.