# Unit B

## Population Sampling

Estimating a population size allows scientists to observe changes in population size and health. Few populations are small enough for scientists to simply count each individual; however, there are a number of reliable methods that can be used to estimate population size.

Transects or quadrats work well when counting plants or relatively stationary animals such as snails. In the transect method, researchers use stakes and string to mark out a straight line of a certain length. The researcher then walks along the line counting any individual within five meters of that line. A quadrat is a small area, usually a square shape, within which the researcher can count all of the individuals of a certain species. Usually, several quadrats are marked out to obtain a representative sample.

Read "Sampling Populations in Ecosystems" and "Estimating Numbers Using Transects or Quadrats" on pages 105 to 106 of your textbook.

By Yohan euan o4 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons

## Capture/Recapture

Another commonly used method for estimating populations is the capture/recapture method. This method allows you to estimate the size of a population by catching a small sample, tagging them, and then releasing them. The next step is to recapture a sample of the population and count the number of tagged individuals in the new sample. You can then use this information to calculate the estimated total population size using the following formula.

«math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨12px¨»«mfrac»«mrow»«mi»number«/mi»«mo»§#160;«/mo»«mi»tagged«/mi»«mo»§#160;«/mo»«mi»in«/mi»«mo»§#160;«/mo»«mi»first«/mi»«mo»§#160;«/mo»«mi»sample«/mi»«/mrow»«mrow»«mi»estimated«/mi»«mo»§#160;«/mo»«mi»total«/mi»«mo»§#160;«/mo»«mi»population«/mi»«/mrow»«/mfrac»«mo»§#160;«/mo»«mo»=«/mo»«mo»§#160;«/mo»«mfrac»«mrow»«mi»number«/mi»«mo»§#160;«/mo»«mi»with«/mi»«mo»§#160;«/mo»«mi»tags«/mi»«mo»§#160;«/mo»«mi»in«/mi»«mo»§#160;«/mo»«mi»second«/mi»«mo»§#160;«/mo»«mi»sample«/mi»«/mrow»«mrow»«mi»total«/mi»«mo»§#160;«/mo»«mi»number«/mi»«mo»§#160;«/mo»«mi»caught«/mi»«mo»§#160;«/mo»«mi»in«/mi»«mo»§#160;«/mo»«mi»second«/mi»«mo»§#160;«/mo»«mi»sample«/mi»«/mrow»«/mfrac»«/mstyle»«/math»

Rearrange the formula to estimate the total population.

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OSLI

Other Counting Technologies

As technologies advance, new techniques for counting population sizes are being developed. The polarized light and laser technology described in this video is one of them.

OSLI 2012