Desiccation is the process by which salt in the surroundings of an animal cause it to dehydrate. The limpets avoid this very well. They have adapted by using a waterproof shell with water held in the body cavity meaning that they do not have to look outside for much water reducing the risks of desiccation. It can also be avoided by the limpet when it returns to its home scar sealing the shell margin when necessary.
Wave action is also another coastal process which a limpet must be protected from. It occurs when the shear physical force of the wave pounding and battering would remove the limpets from the rocks. It tolerates the effects of wave action with its hard shell being streamlined in shape to dissipate the energy of the wave around its shell. The tremendous suction power of the foot also plays a key part to the survival against wave action.
The amounts of nutrients that are available in the environment also affect the density of limpets. Although the basic nutrients are absorbed by plants the diversity of these plants is dependent upon the amount of nutrients available to the system. The more diverse the producers are the more animals will be able to be supported, the same way that more limpets will be able to be supported by the excess in growth of micro-algae (the producers) with the availability of more nutrients.
Both shores are susceptible to the conditions described above but with varying degrees. In the exposed rocky shore, the environment is much harsher. The limpets experience accelerated wind speed and the effects of wave action would be more prominent. This is due to the length of fetch, which is large and the aspect, usually towards the prevailing winds. These harsh conditions reduce the amount of diversity in the environment and so smaller populations of species would be present here.
On the sheltered rocky shore, conditions are more favourable for limpet existence. The limpets experienced reduce wind speeds and the shelter can provide protection from wave action. Typically, the sheltered rocky shore has a small fetch with the aspect away from the prevailing winds.
The extent to which all the limiting factors above varies being both the environments being investigated, and it is this difference in density of limpets from both environments I wish to prove.
I decided to investigate how two different types of abiotic and biotic conditions affected the density of limpets. I then carried out a preliminary investigation to see where I should take my samples from. The aim was to determine if there is a difference in limpet density at two different points in their vertical range at Castle Beach Bay.
At heights of 4m and 6m above chart datum we used random coordinates and at these points we used a 50 cm quadrat in which we counted the density of limpets for that area.
To test if there was a significant difference I decided to use the Mann – Whitney U test.
H0 = No significant difference in limpet density at 2 different vertical heights
ΣR1 = 213 Note → U1 + U2 = n1 * n2
ΣR2 = 87
U1 = (n1 * n2) + 0.5n2 (n2 + 1) - ΣR2
= (12 * 12) + (0.5*12) (12 + 1) – 87
= 144 + 78 – 87
= 135
U2 = (n1 * n2) + 0.5n1 (n1 + 1) - ΣR1
= (12 * 12) + (0.5*12) (12 + 1) – 213
= 144 + 78 – 213
= 9
Therefore 135 + 9 = 12 * 12
144 = 144
So the equation is correct
Critical value found by looking at the table was 37. The lowest U value we got was 9 so from the calculation we can reject the null hypothesis and accept the alternative hypothesis that there is a significant difference of limpet density at two different vertical heights.
From the results above I thought that the best height to conduct the experiment would be at 3.5 m. I chose a slightly lower value than the one of the preliminary experiment because I found out that the closer to shore you got the higher the density of limpets, and also allowed myself enough time to get the readings before the tide came in.
When deciding on the number of samples to take I considered the implications of taking to little or too many. Too little wouldn’t allow me to make accurate and concise conclusions about my results and too many would be too time consuming. Knowing that my quadrat was half a metre square in size, I decided that I would take as many samples as possible in an hour, this way I could keep my timings as well as having a sufficient amount of results to base my conclusions upon.
Safety was taken into account by filling out two working information cards for both sites that samples will be taken from, Frenchman’s Steps and Castle beach bay. These contain information about nearest telephones, escape routes etc.
Looking at the size of the beach it was decided that a 30m transect would be the most appropriate length of transect. This would allow a big enough area to survey and give a good set of results for that site as the sampling wouldn’t be localised to one area but spread across much of the beach.
Random sampling would be the best sampling technique for this investigation as I will be able to generate random coordinates across the 30m tape. The random coordinates are generated using the RAN# key on a calculator and multiplying the number by 30.
Locations
Castle Beach Bay –
Grid Ref. – SM 819050
Aspect – S.S.E. (towards prevailing winds)
Fetch – Large (the Atlantic Ocean)
Frenchman’s Steps –
Grid Ref. – SM 819064
Aspect – N.N.E (away from prevailing winds)
Fetch - Small
Variables
Independent variables – these are the variable that one must change during the investigation which includes:
- Sheltered Rocky shore
- Exposed Rocky shore
Controlled variables – these are the variables which must be kept constant during the investigation which include:
- Height above chart datum (3.5 m)
- Temperature – although short changes in temperature do not affect the density of limpets they are seasonal animals so they move with seasons.
- Humidity
- Wind speed
- Salinity
- Time – moving between shore asap
- Accuracy – one must make sure that only the
As we are in a natural environment it is extremely hard to keep these variables constant and will therefore expect some of my results as anomalous.
Method
Apparatus needed:
- A half metre square quadrat
- Clipboard, recording sheet, pencil and sharpener
- 30m measuring tape
Upon reaching the site for sampling the following instructions should be adhered to:
- Choose an appropriate area to place your transect making sure that a good spread of beach is included and that the area you choose to take samples will allow you to make accurate conclusions.
- Mark 3.5 m from chart datum by following:
- Take a metre rule and place it at chart datum
- Holding the level square to 1m mark the position your eye sees on the beach.
- From the level repeat the 1m reading to get 2m
- Then repeat the 1m reading from the position of the 2m reading from the previous step giving you 3m.
- Then at this position place the level square with 50cm and place your transect running through that point.
- Using the random coordinates on the recording sheet place your quadrat with the top left hand corner touching the coordinate on the measuring tape.
- Count the amount of limpets contained within the quadrat, making sure that if the top of the limpet is contained within the quadrat to count it and if it is outside or touching to disregard that limpet.
- Record your results
Results
Null Hypothesis (H0) – Limpet density does not vary on two different types of shores.
ΣR1 = 1360.5 Note → U1 + U2 = n1 * n2
ΣR2 = 592.5
U1 = (n1 * n2) + 0.5n2 (n2 + 1) - ΣR2
= (31 * 31) + (0.5*31) (31 + 1) – 592.5
= 961 + 496 – 592.5
= 864.5
U2 = (n1 * n2) + 0.5n1 (n1 + 1) - ΣR1
= (31 * 31) + (0.5*31) (31 + 1) – 1360.5
= 961 + 496 – 1360.5
= 96.5
Therefore 864.5 + 96.5 = 31 * 31
961 = 961
So the equation is correct
Conclusion
I can reject the null hypothesis as the smallest value of 96.5 is smaller than that of the critical value of 127 and therefore accept the alternative hypothesis that limpet density does vary on two different types of shores.
From the results we can clearly see that the majority of limpets occur at Frenchman’s Steps, the sheltered rocky shore. We see this trend because the abiotic conditions experienced by the limpets are a lot more favourable to their existence and so the numbers would increase. At Frenchman’s steps we find that wind does not affect the limpet to much as the aspect is away from the prevailing wind. As the fetch is also relatively small the strength of wave action must also have been less as this is what allowed more limpets to cling on to the rocks successfully and not fall off.
I also observed that there was a more diverse species habitation. There were many types of seaweed, Dog whelks, sea anemones, and other organisms showing that the amount of nutrients in this environment must also be high. This would therefore allow the population of limpets to be high, explaining the large population in this environment.
Limpets at Castle Beach Bay were quite sparse. The highest number of limpets in any one quadrat was 22 as opposed to 60 found at Frenchman’s steps. Again the abiotic and biotic conditions experienced by the limpets is what affect there density. Here unlike Frenchman’s steps the conditions are a lot harsher. With the fetch being much larger the wave action on this shore is much more than that on the sheltered shore meaning that less of the limpets are strong enough to withstand the waves and so stay attached to the rock.
The desiccation ratio at this beach would also be higher as the wind would be stronger giving it more capacity to carry load. This means that more salt is carried and so the result is a high salinity level which doesn’t help the limpets. The limpets that are larger often survive better than those that are smaller, again accounting for the small population at this point.
Theses harsh abiotic conditions do not favour the diversity of life as only one type of seaweed was present, the bladder wrack. Other species of animal were also hard to find. We can say that the level of nutrients at this shore must also be lower as the observed diversity of life was also lower explaining the amount of limpets to be lower than at Frenchman’s steps.
Evaluation
It is clear from the results that some anomalous results had been achieved, i.e. finding 3 limpets in one quadrat at Frenchman’s steps. These anomalous results could be explained by rock pools. These microhabitat often offer a less hostile environment, but the level’s of competition inter and intra specifically are also high/. Meaning that at these places the populations of limpets would experience less numbers accounting for the anomalous result.
If I was to repeat this experiment again, I would take picture of every quadrat sample I took, so that I could easily check if results seemed a bit wrong. I would definitely have to spend a lot more time on counting because we were working with a time limit and this could have cause inaccuracies to arise.
An element of error could have been present on when measuring the chart datum. One could never be sure exactly where it was so estimates were taken and the readings done from there. This added to the level of error of the experiment.
To overcome the problem of timings, I would have liked to do the readings at the same time, i.e. on separate days.
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