Investigate the effect of different sampling methods on estimates of the size of a population7/1/2016 When calculating the abundance or distribution of an organism, there are numerous different methods that are used. These methods include quadrats, both square and point, transects and mark release and recapture involving the Lincoln index. All of these methods have their advantages and disadvantages and affect the accuracy of the population size estimate.
Quadrats This involves a frame, usually square, acting as the area in which the sample of the population will be taken. The quadrat is laid on the ground at a random point. This random point may be chosen by a random number generator and the quadrat placed on a coordinate grid. Once the quadrat is on the ground, a value for the estimated population can be obtained through either individual counts or percentage cover. Both of these are examples of quantitive sampling techniques as the results are quantifiable. A number of quadrats are laid down at different points and a mean of the population size is calculated. This number is then multiplied by total area to gain an estimation for the whole area. Quadrats are advantageous because they are simple and easy to use and because of the random nature of their position, they provide an objective result. There are however limitations including the problems with the sample area that can be analysed and deciding whether or not to include those organisms on the edges of the quadrat. Another major issue is that they cannot be used to examine fast-moving animals as they are not present in the quadrat for long enough to be taken into account. A final issue would be with the randomness of the sample site as there is often bias as to where the quadrat is placed, especially if they are thrown on the ground to determine the area to measure. Another type of quadrat is the point quadrat which is essentially a horizontal bar consisting of pins which stick in the ground. The organisms that these pins come into contact with are measured to achieve a population estimate. These are quick and efficient and don’t squash any vegetation meaning they are more ethical to use. Additionally, they create a clearer picture as the organisms are easier to see. The drawbacks to this method is that it is limited to habitats with soil as the pins are unable to stay upright in any other material. As well as this, they are unsuitable for animals as the pins cannot touch them. The final limitation would be that it is easy to miss out the rarer, smaller species using pins. ACFOR scale This scale is used for measuring the abundance of species and is very subjective as it involves the scientist using the ranking scale to determine a value for the population size. The scale ranges from A meaning abundant to R meaning rare and either is calculated by the percentage cover or the number of organisms to work out how common the organism is in that particular area. The major issue with this is that it is subjective, meaning the same result may not be achieved upon repeats. As well as this it is hard to say exactly the point at which the value goes from abundant to common or occasional to rare. This ties in with the limitation of the scale being subjective. To add to this, different species may be more easily recognised which has a downside as it causes bias and may result in the scientist identifying that particular species as more abundant due to it being more clear and easier to see. Transects Transects are used to systematically sample a site meaning it is not a random method. This method involves stretching a tape between 2 points and recording very individual organism, whether that is a plant or an animal, that touches the tape. Transects are positioned so that as you move along the tape, there is environmental change, meaning the measurements taken are not replicates but in fact completely different readings. An alternative method would be the belt transect involving 2 tapes aligned and the ground between them is surveyed. Quadrats can also be used along these tape measures to achieve systematic samples of the population at different stages along the tape. This is great for measuring the distribution of organisms dependent on changes in abiotic factors because these factors will change as you move along the tape. To look at these correlations between population size and abiotic factors such as soil pH, samples must be taken at regular intervals along the tape measure. Mark, release and recapture This method is used to estimate the population size of a certain species in a particular area. To do this you collect a sample of the population by using, for example, netting or a pitfall trap, and recording the number of individual organisms caught. Before releasing these organisms, they must be marked. After a suitable time period, repeat this process in the same area and capture some more organisms. Record those organisms which are marked and count those which are unmarked. The Lincoln index is then used to estimate the population size. The total population size equals the number of animals captured first multiplied by the number of animals recaptured the second time all divided by the number of marked organisms recaptured. This method is beneficial for calculating population sizes when you cannot see the organisms in their habitat, for example crabs. However there are numerous limitations with the method because it assumes there have been no births, deaths or migrations over the sampling period. As well as this, there is a possibility that marking the organism reduces the individual’s chance of survival, for example by inhibiting their visibility, or decreasing their ability to move, both of which can affect the estimate for the population size. Another assumption made is that the individual organisms redistribute themselves evenly, whereas in fact they may be more concentrated in the place of release which can affect results. A final limitation is that the method assumes all organisms are equally likely to get caught, when in fact some organisms are trap-happy and some are trap-shy. This affects the population estimate using the Lincoln index because those organisms that were caught in the first place and then marked, are either the ones likely to get caught again (trap-happy) or have learnt from their mistakes and will avoid being caught again (trap-shy). Overall each method has its advantages which makes them suitable for estimating populations of certain types of organisms, for example, the mark, release and recapture method involves moving organisms whereas quadrats are for organisms that do not move. We can see that, however, each method has its own limitations whether that can affect the accuracy of the population size value. Moreover, the style of sampling, whether it is random, systematic or stratified can affect the reliability of the results as it may lead to bias and the results might not be an accurate representation of the population size.
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Ciara Branagan
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October 2016
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