Sampling

Sampling Techniques

Ecological sampling techniques allow scientists to collect data that accurately represent the organisms and environmental features within an ecosystem.

Because it is rarely possible to count every individual, ecologists use structured methods to estimate abundance, distribution, and diversity.

Techniques such as quadrats, transects, mark-recapture, and remote sensing help ensure samples are reliable and repeatable.

Choosing the appropriate method depends on the habitat, the type of organisms being studied, and the questions being asked. Effective sampling reduces bias, improves accuracy and provides the data needed to compare sites and monitor ecological change.

Quadrats

  • An area where you survey the organisms present.
  • Usually a square, but can be other shapes.
  • Can be marked out by tape or by a frame.
  • Useful where the area being sampled is uniform.
    • Provides a representation of the entire area.
  • If used, consider:
    • Size
    • Number used
    • How they are placed
  • Once positioned, we collect data by:
    • Density
    • Frequency
    • Percentage Cover
    • Scaling Measure

Transects

  • A transect is a sampling line.
    • It is used to investigate change over distance in an ecosystem.

Mark-Release-Recapture / Lincoln Index

We can calculate the population size using the Lincoln Index and the capture-recapture method (also known as mark-release-recapture).

  1. Collect a sample of individuals, mark them and then release them.
  2. After a period of time, collect more individuals from the area and count the number that have been marked.
  3. We assume that a sample, if random, will contain the same proportion of marked individuals as the population does.

\fn_jvn N = \frac{M\; x \; n}{m}

Where

N = Size of whole population

M = number of individuals caught, marked and released initially

n = number of individuals caught on second sampling

m = number of individuals recaptured that were marked.

Explain how scientists could use the mark-release-recapture technique to estimate the population size of a species of insect.

  • collect a sample of insects in each area and mark unobtrusively/in a way not harmful to insects
  • release and allow time to re-integrate with rest of population
  • collect second sample and count number marked
  • estimate number in population using the Lincoln Index Equation : (number marked caught and marked initially x number caught on second sampling / number of individuals marked)

In a study of one population of phascogales, 72 animals were trapped and marked with ear tags. They were then released. One month later, fieldworkers examined 120 phascogale and found that 14 of these had ear tags. Use these figures to calculate the size of the phascogale population.

(72 x 120 / 14) = 617