3.6 Estimating Population Size
Unit B
Module 3 ~ Lesson 3
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
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
Read "Sampling Populations in Ecosystems" and "Estimating Numbers Using Transects or Quadrats" on pages 105 to 106 of your textbook.
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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Other Counting Technologies