Sampling Techniques

1. Sampling Techniques http://www.sciencenetlinks.com/Esheet.cfm?DocID=86

2. What is Sampling? • Sampling is a set of ways in which scientists try to accurately estimate the total population in community based upon a limited number of individuals

3. Why? • This is done because in most real life scenarios it is not possible to capture, collect or view every organism at once or over a period of time. • Look at a gram of healthy soil. One teaspoon of topsoil contains 5 billion bacteria in addition to all other organisms. http://www.rhs.org.uk/advice/profiles0305/images/topsoil.jpg

4. Limitations • Sampling is an estimate. It gives a solid approximation but…there is still a margin of error.

5. Population Density • D= n/s(area) • Number ÷ Space = Population Density

6. REPRESENTATIVE SAMPLING • Take a number of samples from around the sampling site so as to be reasonably sure that the samples represent the site in general

7. Necessities… the samples represent the whole… • It is necessary to take enough samples so that an accurate representation is obtained • It is necessary to avoid bias when sampling

8. SAMPLING UNITS • Type determined by the organisms and the physical nature of the habitat being sampled • Area of ground surface • Volume of air, water or soil • Standard units enable comparison of results

9. Examples of TYPES OF SAMPLING • Raw • Mark Recapture • Systematic • Stratified • Random

10. Types Raw Sampling • Raw sampling assumes all individuals are evenly spaced throughout the habitat. No adjustment or allowances is made for differences in distribution. • Count the individuals in a small area, the project to the entire area

11. Types Presence-absence • Identify the presence or absence of a designated species • This is the simplest method. And the least precise. It is only used in a an initial study to determine what species are there. It does not estimate population

12. Types Mark-recapture • Mark-recapture allows for differences in distribution to be accounted for in a statistically significant method: (total number captured) X (number marked)/ total number marked recaptured http://mampam.50megs.com/bok/markedbok.jpg

13. SYSTEMATIC SAMPLING • Often used when the area being studied is varied, not very large, or when time is available • Samples are taken at fixed intervals

14. Types of transect sampling

15. How to sample systematically • Systematic samples are usually taken along a transect line marked by a tape measure • Transect- a line laid across an area

16. Types Transect Line • A measured line is laid across the area in the direction of the environmental gradient • The species touching the line can be recorded along the whole length of the line (continuous sampling) or at specific points along the line (systematic sampling)

17. Types • Transect line is more accurate then raw sampling, and more appropriate then mark recapture for plants and slow moving organisms. • It requires standard counting procedure, how far from the transect line individuals will be counted, abiotic conditions, and accurate mapping

18. http://www.uwsp.edu/cnr/cwes/forestree/images/ecolog1.jpg http://el.erdc.usace.army.mil/aqua/apis/ecology/image/img0006.gif Types Transect Line http://ne.water.usgs.gov/html/Nawqa/pubs/posters/wetland95/photo5b.jpg http://www.geobotany.uaf.edu/cryoturbation/photos2/transects/hv_t1_082201.jpeg

19. Belt transect method • Similar to line transect but widens the sampling area • Transect line is laid out

20. Belt transect method • Samples are taken by determining abundance or % cover in an area that is a defined distance from the line • Samples can be taken all the way along the line, at specific intervals or even randomly

21. Types 4. Beltline Transect • A belt line transect is similar to a transect line except that it used alternating squares along a transect line to collect data from

22. Types 4. Beltline Transect • Gives good randomization of data • Has a specific fixed area to collect data from • It is time consuming to measure correctly and set up.

23. http://www.iol.ie/~carigeen/forest%20floor.jpg

24. Belt transects vs Line transects • belt transects measure the quantities of vegetation (within a quadrate frame placed along the transect line) and a line transect used to record the vegetation at different points (useful in succession) • So basically...belt = quantities of all vegetation in a % i.e ground coverageLine = species of vegetation present

25. STRATIFIED SAMPLING • Often used when there are small areas within a larger habitat that are clearly different • Strata- major differences within communities recognized before sampling begins

26. RANDOM SAMPLING • Often used when the area being studied is fairly uniform, very large, or when there is a limited amount of time available • Random = chosen by chance rather than according to a plan; all outcomes are equally likely • Samples are taken from different positions within a habitat and those positions are chosen randomly

27. How to sample randomly • Choose individuals or Place “sampling units” haphazardly • This is rarely completely random OR… • Assign numbers to the areas or individuals to be sampled • Use a random number table to select which areas or individuals will be sampled