3.7.4 Populations in ecosystems

• Population


  Group of individuals of a particular species
• Abundance


  Number of individuals/how common organisms are
• Distribution


  Spread of organisms over an area/ where they are found
• Quadrat


  Square frame used for counting organisms
• Transect


  Straight line that cuts through a habitat so that standardised observations and measurements can be made
• Abiotic factor


  Non-living variable that affects where something lives
• Biotic factor


  Living variable that affects where something lives
• Percentage cover


  % of a quadrate that is covered by a species
• Species richness


  Number of species
• Standard deviation


  Spread of data around the mean
• What is used to investigate the abundance of sessile (non-motile) organisms?

  1. Point quadrats

  2. Frame quadrats
• Finding population size

  1. Lay 2 tape measures to form a "x-axis" and "y-axis" along your sample area

  2. Use a random number generator to create a set of random co-ordinates - at least 10

  3. Place a quadrat at each co-ordinate and count the number of organisms (or percentage cover) - calculate mean number of organisms

  4. (area of sample area/area of quadrat) x mean number of organisms
• Sample size MUST be large and a mean calculated for reliable and representative results.
• Investigating the abundance of motile organisms

  1. Flying organisms – sweep net

  2. Swimming organisms – net

  3. Ground insects – pitfall trap
• Mark release-recapture (MRR)

  1. Capture a sample of a species using an appropriate technique and count them

  2. Mark them in a harmless way e.g. spot of paint or a metal ring around the claw

  3. Release them back into their habitat

  4. Wait a week, then take a second sample from the same population

  5. Count how many are marked

  6. population size = (number in 1st sample x number in 2nd sample)/number in 2nd sample previously marked
• Investigating the distribution of sessile organisms

  1. Place a tape measure along an area (belt transect)

  2. Place quadrats at equal intervals along the tape measure

  3. Count the number of organisms within each
• Frequency or percentage cover can be used to investigate the abundance of sessile (non-motile) organisms.
• To calculate frequency, divide the total length of all lines by the total length of all lines that have been covered by the species being investigated.
• Percentage cover = (total length of line covered by species / total length of all lines) x 100
• Accuracy and validity depends on:

  1. Sufficient time for marked animals to reintegrate with rest of the population
  2. Ensure marking does not make the animal more vulnerable to predation or change behaviour (affecting survival)
  3. No changes in population size due to births (avoid breeding seasons), deaths or migration
  4. Large enough sample
• A community is a group of populations of different species living in the same area.
• An ecosystem is a community of organisms and their non-living (physical) components of its environment.
• The abiotic factors are physical environmental conditions such as temperature, light intensity, water availability, soil pH, humidity etc.
• A habitat is the place where an organism lives and the conditions in which it lives.
• Ecosystems are dynamic systems.
• Population size (carrying capacity) of an ecosystem can vary as a result of:

  • effect of abiotic factors
  • interactions between organisms: interspecific and intraspecific competition and predation
• Carrying capacity is the maximum population/community size that an ecosystem can support.
• A niche is the position/role of a species within a habitat.
• Intraspecific competition is competition between members of the same species.
• Interspecific competition is competition between different species.