Investigating populations

• abundance and distribution:

  • investigating populations of organisms involves looking at the abundance and distribution of species in a particular area
• abundance:

  • abundance is the number of individuals of one species in a particular area (i.e. population size) abundance can be estimated by simply counting the number of individuals in samples taken.
  • there are other measures of abundance that can be used to:
  • frequency - the number of samples a species is recorded in e.g. 70%
  • percentage cover - non-motile or slow-moving species only - how much of the area you're investigating is covered by a species
• distribution: is where a particular species is within the area you're investigating
• random sampling:

  • most of the time would be too time consuming to measure the abundance (population size) and the distribution of each species present in the entire area investigating
  • so take samples
• • choose an area to sample - small area within the area being investigated
  • samples should be random to avoid bias - can use a random number generator to ensure sample are random
  • use an appropriate technique to take a sample of the population
  • repeat the process taking as many samples as possible - will reduce the likelihood that results are down to chance
  • the no. of individuals in the whole area can be estimated by taking the mean of the data collected in each sample and multiplying it by the size of the whole area
  • % cover of the whole area can be estimated by taking the mean of all the sample
• random number generators:

  • if investigating populations in a field could pick random sample sites by dividing the field into a grid and using a random no. generator and a random letter generator to select coordinates
  • will give coordinates at random
  • then take samples from these coordinates
  • using tables of random numbers is another way of generating random numbers
• running means:

  • it is important you take enough samples to give a good estimate
  • one way of doing this is to take a running mean - where you work out the mean of all the data each time you collect a new sample
  • once the mean no longer changes by a large amount - should have data that gives a realistic estimate for the whole area
• the point at which the running mean stabilises will be different in each area you study
• methods for investigating populations:

  • there are many methods - need to choose the most suitable one to use - depends on the type of organism and its habitat
  • quadrats and transects can be used for studying non-motile organisms e.g. plants and corals, or slow-moving organisms like limpets
  • on the other hand - if studying more motile organisms, like insects - nets and traps are more appropriate
• quadrats:

  • a quadrat is a square frame which is usually divided into a grid of 100 smaller squares by strings attached across the frame
  • quadrats are placed on the ground at different points within the area you are investigating
  • the species frequency (how often a species is found) or the number of individuals of each species is recorded in each quadrat
• • the % cover of a species can also be measured by counting how much of the quadrat is covered by the species - count a square if it is more than half covered
  • % cover is a quick way to estimate populations and don't have to count all the individual organisms
• quadrats are useful for quickly investigating areas with plant species that fit within a small quadrat - areas with larger plants and trees need large quadrats
• non-motile organisms don't move around - they're fixed to a surface

  can also be called sessile organisms

  motile organisms are able to move around freely but they might move really slowly
• transects:

  • you can use lines called transects to help find out how organisms are distributed across an area e.g. how plant species change from a hedge towards the middle of a field
  • 2 kinds
• • belt transects:
  • quadrats are placed next to each other long the transect
  • to work out species frequency and percentage cover along the transect
• • interrupted belt transects:
  • instead of investigating the whole transect you can take measurements using a quadrat placed at regular intervals
  • e.g. every 2 meters
  • can make it easier to cover a large distance
• interrupted belt transects are quicker to carry out - but give you less information
• transects can be used in any ecosystem, not just fields. Fir example, along a beach
• capturing motile organisms:

  • if investigating motile organisms, you might need to use equipment to capture them
  • the best method of capturing organisms will depends on what you are studying
  • for flying insects - you'd use a sweep net (net on a pole)
  • for aquatic animals - use a net
  • for ground insects - use a pitfall trap (steep-sided container that is sunk into the ground) (flowerpot or similar container, raised lid, walking insects fall in and are trapped - stone to raise lid)
• when using equipment to take a sample of organisms in an area - important to use the same method each time e.g. with a sweep net you could sweep three times at shoulder height in each area you sample
• mark-release-recapture:

  • a method used to measure the abundance of more mobile species
• 1. capture a sample of a species using an appropriate technique and count them
  2. mark them in a harmless way e.g. by putting a spot of paint on them or by using an identification tag
  3. release them back into their habitat
  4. wait a week and then take a second sample from the same population
  5. count how many of the second sample are marked
  6. use the equation to estimate the total population size
• total population size = number caught in 1st sample X no. caught 2nd

  number marked in 2nd sample
• the accuracy of the mark-release-recapture method depends on a few assumption:

  • the marked sample has had enough time and opportunity to mix back in with the population
  • the marking hasn't affected the individual's chances of survival e.g. by making them more visible to predators and the marking itself is still visible e.g. it hasn't rubbed off
  • there are no changes in population size due to births, deaths and migration during the period of study
• • some people think that capturing animals for study is unethical - might cause them unnecessary stress
  • also if animals are put under too much stress during capture - could reduce their chances of survival after release/ influence them to avoid the trap in future
  • these would interfere with the accuracy of any estimates of population size made using the mark-release-recapture method
  • to minimise stress investigations should be planned so that organisms are treated carefully and are kept and handled as little as possible