RESULTS
Individual group results
Total data obtained.
STATISTICAL ANALYSIS
Lincoln - Peterson Method
In theory, the ratio of the total population (N) to the number of marked individuals (M) is equal to the ratio of the number captured (n) at a later date to the number that were recaptured (R) on the later date. This equation can be rearranged to the following equation.
In essence, by knowing the number of individuals captured and marked on the first day (M), the number captured on the second day (n), and the number recaptured on the second day (R), you can estimate the number of individuals in the population.
To estimate the total population of the whole 60m, each groups’ results are collated below.
In estimating the size of a population, there is always going to be a level of uncertainty due to examining samples rather than the population as a whole, which in most cases is impossible. To measure this level of uncertainty, Standard Error is calculated by putting the recorded data in the formula below (using the totals):
This formula simply measures the amount of error in the estimate. By multiplying the SE times 1.96 you can calculate the approximate confidence (CI) interval for the mark-recapture estimate, which is then added and subtracted from the estimate (N).
The results show
We could then say that there is a 95% chance that the actual population size is between the upper and lower limit of the confidence interval. The confidence interval is a measure of the estimate precision, not the estimate accuracy. Therefore, the precision of the mark-recapture method is inversely dependent on the number of marked animals.
When carrying out statistical analysis on Microsoft Excel, the standard deviation for the population of snails is 28.52 suggesting that the data is unreliable. The confidence level is 22.82, this suggests that the data is reliable, as it is smaller than the calculated mean 63.
χ²
To determine whether or not the difference between the expected snail population and the observed snail population is significant at each area site. The chi square test is used. Where ‘O’ is the observed count and ‘E’ is the expected count.
The null hypothesis is stated that:
"There is no difference with the observed count and the expected count."
The results from the chi-square suggests that probability (P) < 5% then there is a significant difference from each other, therefore the null hypothesis is rejected.
This reinforces what the original table shows, the difference in vegetation affects the number of snails caught, i.e. more snails were caught amongst the dense nettle area.
EVALUATION
When analysing the data and the result obtained, it can be said that the results are reliable to an extent. It shows that there is a significant difference between each site and the number of snail caught and recaptured, however there is a level of error in the total population count.
There are a couple of factor to be considered which may have affected the results. For instance the weather conditions varied on the days the mark and recapture took place. On day one of the original capture took place in damp conditions early morning, and the day of recapture took place on a dry afternoon. The sites varied in vegetation and cover, i.e some areas where largely dense with nettles whilst others where sparse. Snails prefer moist damp conditions, and are more likely to be found in crevices, and within the nettles. The reason behind this is that the nettles and crevices in walls, aids as protection against predators like birds, aswell as keeping them protected from the sun. As the second day was dry not as many snails were recaptured, also on day one many on their habitats amongst the nettles were damaged.
These factors could be taken into account for future reference by making sure that the mark and recapture take place around the same time of day, with similar weather conditions. This would also make it a fairer test as it limits some variables. There is also a need to try and avoid altering the habitat during sampling
There are many limitations of the mark and recapture technique, which are:
- It must be carried out under comparable conditions
- It is confined to species living in discrete populations inhabiting a definable area.
- The Lincoln index assumes that there are no births or deaths whilst sampling therefore does not change the total population.
- Estimations of the population tend to be high, especially if there are a low number of individuals recaptured.
By using a technique such as Bailey Triple Catch method, which involves multiple capture and recapture techniques could improve and give a more accurate set of results.
