Sampling Populations

  1. It is often difficult or impossible to perform a complete count (true census) of a population.
  2. Sampling techniques are used to estimate a population based upon counting a subset of the population.
  3. The sampling method varies with survey goals, species, environment, and surveying resources such as personnel.

Quadrat

  1. An sample plot of area, a, is specified in a larger area, A.
  2. The location of the sample plot within the larger area, A, is randomly determined!
  3. The organisms in area, a, are counted.
  4. More than one sample plot is typically counted.
  5. The relationship of the area surveyed to the total area allows the population to be estimated.

Transect

  1. A line of length, L, is set in the area to be sampled.
  2. The organisms along that line are counted.
  3. A width restriction for counting may also be specified (strip transect).
  4. The relationship of the area surveyed to the total area allows the population to be estimated.
  5. For animals, the count is assumed to be incomplete. Adjustments are made using the distance and angle of line of sight when the animal was sighted.

Mark-recapture

  1. Periodic surveys (captures) of animals are made.
  2. On the first survey a number of the animals are marked.
  3. On subsequent surveys, counts of the marked and unmarked animals are made.
  4. The population is estimated with this formula:
    • N = n2 * n1 / m
    • N = estimated total population.
    • n1 = number of animals marked in first survey.
    • n2 = number of animals in the subsequent survey (both marked and unmarked).
    • m = number of marked animals in the subsequent survey.
  5. It is assumed the population is closed.
  6. This technique is known as the unmodified Lincoln-Petersen estimator.

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