The most common limpet found in northwest Europe is the Patella spp. Patella vulgata is found wherever there is substratum firm enough for its attachment on rocks, stones and in rock pools. This species can be found on a rocky shore from the high shore to the sublittoral fringe. It is abundant on all rocky shores of all degrees of wave exposure. The abundance of limpets on wave-swept shores makes it ideal for examining the effects of wave exposure on them.
Project Hypotheses:
Alternative Hypothesis: There will be a statistically significant change in the volumes of limpets found on an exposed shore compared with a sheltered shore.
Null Hypothesis: There will be no statistically significant change in the volumes of limpets found on an exposed shore compared with a sheltered shore.
Method:
The location of this investigation was at a wave exposed rocky shore and a wave sheltered rocky shore at West Angle Bay SM 852 032 on the Atlantic coast of Pembrokeshire in Wales. The exposed shore has steep, sloping rock faces and facing south, the shore has a large fetch. The sheltered shore faces north and the geology of both sites is Carboniferous limestone.
In order to carry out this investigation it was necessary to choose a suitable location on both shores. The middle shore was chosen on both shores because preliminary work carried out on rocky shores showed that a greater number of limpets were found on the middle shore and therefore enough data could be collected to provide as accurate and reliable results as possible. The middle shore was chosen on both shores to make sure the investigation was as fair as possible and so the middle shore was not compared with the upper or lower shore. After choosing the location of the investigation, a 0.25m2 quadrat was placed on a rock stratum on the middle shore. Each limpet inside the quadrat was measured using callipers. Callipers were chosen to measure the limpets instead of a ruler because they are capable of measuring lengths to the nearest tenth of a millimetre, which is far more accurate and therefore reliable. The diameter of the base of each limpet was measured along with the maximum vertical height. These two measurements were needed to calculate the volume of each limpet using the formula ‘⅓πr2h’ which is the formula to work out the volume of a cone. After the volume of each limpet in the quadrat was worked out, the quadrat was flipped horizontally in order that the investigation took place along a continuous horizontal belt transect at a fixed vertical height. The quadrat also ran along the same rock stratum. This was done because different species of limpet could be found on different types of rock and these species could be bigger or smaller than the Patella spp. This would affect any trends and patterns found in the volumes of the limpets. The volume of each limpet was worked out in this quadrat and the quadrat was continuously flipped until a running mean was established. The running mean was calculated after every volume was worked out until half of the results obtained were 2.5% either side of the mean. The same apparatus and the same method were used on both shores. It was important to use the same quadrat size on both shores because a bigger quadrat would mean the horizontal belt transect would be larger and therefore a larger section of the shore would be investigated on one shore compared with the other. This could provide a greater variation in the limpets and also different species may be included in the results. This investigation was carried out on two consecutive days at the same time in the day, with similar weather conditions observed on both days. This was important as limpets feed at different times, with some feeding out of the water and some in the water. The Patella spp. is especially variable in its behaviour. If the investigation was carried out at different times or even different seasons then the limpets could have moved far away from where they originally were and therefore the results would be unreliable. A key was also used in this investigation to ensure that the correct specie of limpet was measured and that it was in fact a limpet that was being measured and not a periwinkle or topshell. The method used here had no further modifications than that stated in the plan for this investigation.
This is a full list of the apparatus used in this investigation:
Callipers
0.25m2 Quadrat
Key
Pen, Paper, Calculator
Results and Analysis:
The raw data obtained at the wave-exposed site can be seen in Appendix 1 and the data obtained at the sheltered site can be seen in Appendix 2.
This is a summary table of the results obtained on both the exposed shore site and the sheltered shore site.
The graph on the following page shows the running means of both sets of data in order to establish when enough data had been collected. The 2.5% boundary lines show that half the data collected at each site was within 5% of the mean. A similar number of samples needed to be taken at both sites in order for this to happen. The graph clearly shows a difference in the volumes of limpets found at the exposed site compared with the sheltered site. The mean volume of a limpet found on the sheltered shore is just over 3 times greater than the mean volume found at the exposed site. The standard deviations of both sites show that the volumes found at the exposed site are more closely clustered around the mean. This means that there is a smaller range of volumes and less variance at the exposed site. The tables in appendices 1 and 2 show that there are some anomalies in the data collected. The sheltered site shows one extremely large limpet that is 2500mm3 bigger than any other limpet recorded (sample number 47). On the other hand, limpets with a much smaller volume than the mean were found at the exposed site (sample numbers 4, 29). The results obtained here seem to be very conclusive, however a statistical test must be carried out in order to make sure.
The Student’s T-Test
It is important to carry out a statistical test in this investigation in order to see if the data obtained is significant and whether the hypotheses can be rejected or accepted. The t-test is the most appropriate test to use in this case as this test allows you to see whether the means of two samples differ significantly. In this investigation, it is the mean volume of limpet at the exposed site compared with that of the sheltered site. This is the formula used to calculate the t-value:
T =
Where xa and xb stand for the means of each set of data,
sa and sb stand for the standard deviations of each set of data and
na and nb stand for the number of measurements of each set of data.
Using this formula and the data obtained in the investigation the t value was 14.848. In order to convert this figure into something meaningful, it is necessary to know the degrees of freedom for this set of data. For the t-test, this is simply two less than the total number of measurements in the two samples. In this case the degrees of freedom is 125.
In order to see if the results obtained are statistically significant, the value of t must be compared with the table of t values (Appendix 3). The p values at the bottom of this table show the probability that chance alone could produce the difference between the two sets of data. The p-value used here is 0.05. The degrees of freedom for this investigation lies between 120 and ∞, which give the critical values of 1.980 and 1.960 respectively. The t value obtained was 14.8 and is clearly bigger than the critical value for the degrees of freedom. This means that I can be 95% confident that the difference between the means is significant and therefore the results are statistically significant. This strong probability shows that the null hypothesis has been falsified by the data. Therefore the null hypothesis can be rejected and the alternative hypothesis accepted.
The statistical test has shown that this hypothesis is correct:
There will be a statistically significant change in the volumes of limpets found on an exposed shore compared with a sheltered shore.
This hypothesis can be rejected:
There will be no statistically significant change in the volumes of limpets found on an exposed shore compared with a sheltered shore.
References:
BRANCH, G. M. 1986. Limpets: their role in littoral and sublittoral community dynamics. Pp. 97-113. In: P. G. Moore & R. Seed (eds.), The Ecology of Rocky Coasts: Essays Presented to J. R. Lewis. Columbia Press: New York.
Todgham A, Boulding E, Megill WM 1997. Wave exposure effects on the density, habitat selection and size distribution of intertidal limpets species
