19.5 - Investigating Populations

• quadrats

  square frame usually divided into a grid of smaller squares placed on the ground at different points in the area being investigated
• frame quadrat

  square frame divided by string/wire into equally sized subdivisions
• how is a frame quadrat used?

  it's placed in different locations in the area being studied and the abundance of each species in the quadrat is recorded
• point quadrat

  horizontal bar supported by two legs and at set intervals along it there are 10 holes where a pin is dropped

  each species the pin touches is recorded
• what type of organisms are quadrats used to investigate?

  non-motile or slow moving organisms
• factors to consider when using quadrats

  size of the quadrat: large species = larger quadrat; population not evenly distributed = smaller quadrats

  number of sample quadrats to record within a study area: more sample quadrats = more accurate

  position of each quadrat within the study area
• random sampling - method

  divide sample area into a grid and assign coordinates

  use a random number generator to obtain coordinates and count number of organisms in that square

  repeat process to gain a large sample and calculate the mean

  use the mean to estimate the number of organisms by multiplying by the total area
• systematic sampling - method

  mark out a line between two points using a tape measure

  place quadrats at regularly spaced intervals along the belt transect

  count number of organisms in the quadrats

  record data on abiotic factors in each quadrat

  repeat process multiple times to calculate a mean value for each quadrat

  plot graph to see if there's a correlation between number of organisms and changing abiotic factor
• abundance

  number of individuals of a species within a given area
• how can abundance be measured?

  frequency

  percentage cover
• frequency

  the likelihood of a particular species occurring in a particular quadrat
• benefits of using frequency to measure abundance

  quick

  shows general distribution

  good where a species is hard to count
• limitations of using frequency to measure abundance

  doesn't provide information on density and detailed distribution of a species
• percentage cover

  estimate of the area within a quadrat a particular plant species covers
• benefits of using percentage cover to measure abundance

  data is collected quickly

  individual plants don't need to be counted
• limitations of using percentage cover to measure abundance

  not useful when organisms occur in overlapping layers
• how can reliable results be obtained when measuring abundance?

  use a larger sample size - many quadrats are used and mean of all samples is obtained
• what kind of organisms is mark release and recapture used to investigate?

  motile organisms
• mark release and recapture method

  known number of organisms collected and marked then released back into their community

  after some time another sample is collected and total number of organisms and marked organisms is recorded

  estimated population size is calculated
• estimated population size equation

  (number of organisms in first sample × number of organisms in second sample) / number of individuals recaptured
• assumptions in mark release and recapture

  proportion of marked and unmarked individuals is the same in both samples

  marked individuals distribute evenly among population after release

  population has definite boundary - no immigration or emigration

  very low birth and death rates

  marking method isn't toxic or make individual more prone to predation

  mark isn't lost