Preliminary experiment: -
Before I decided to do my investigation I had carried out a preliminary experiment of limpet density at two different heights across one shore. One height was 4 meters from chart datum and the other height was 6 meters from chart datum. There were transect lines placed across the shore at both these heights. I then picked random coordinates and placed a 50x50 cm² quadrat at these coordinates and counted the amount of limpets within the given area at each coordinate. The main point of this preliminary experiment was to see if there is a difference in limpet density across two different heights on the same shore.
Null hypothesis: -
There will be no significant difference in the limpet density at two different heights across a vertical range.
Rank 4m = 213 ΣR1 = 213
Rank 6m = 87 ΣR2 = 87
U1 = (n1 * n2) + 0.5 * n2 (n2 + 1) - ΣR2
= (12 * 12) + (0.5*12) (12 + 1) – 87
= 144 + 78 – 87
= 135
To check if this calculation is correct, n1 * n2 should equal U1 + U2.
12 * 12 = 144
135 + 9 = 144
I have used the mann-whitney U test to see if the medians of the two unmatched samples are significantly different. The U value that I got was 9 because it was the lower of the two U values. Using the significance table I can determine that the critical value is 37. The U value is lower than the critical value so I can reject the null hypothesis and can accept the alternate hypothesis, which is, there is a significant difference in the density of limpets at two different heights across a shore.
From conducting this preliminary experiment, I have decided that the best height to conduct my investigation from will be 3.5meters from chart datum. I also found out that this is the optimum niche range for limpets, also conducting the investigation at this height will enable me to get all the necessary results within the given time. If the height was any lower than 3.5 meters there is a risk of the tide coming in, which would affect the results.
Independent variable: -
The independent variables in this investigation are the two shores, the sheltered shore and the exposed shore. The sheltered shore is a lot rockier, where organisms are more protected from wave action and can survive the harsher abiotic conditions. Organisms in the exposed rocky shore are more affected by the biotic conditions rather than the abiotic.
Dependent Variable: -
The dependent variable in this investigation is the limpets. The method that I will be using to count the amount of limpets within the given area of the quadrat will be by hand. Considering that there is not a lot of time to do this investigation, measuring the limpet density by using the naked eye will be the most effective and accurate way to do it.
Controlled Variables: -
There will have to be a lot of factors that will have to be controlled to make this investigation a fair test. Timing will be an important factor. Obviously it’s not possible to conduct the investigation on both shores at the same time. As soon as I finish the experiment on one shore I will go to the other shore as quick as possible, as long as the tide does not come in to much the results will not be affected that much.
Another factor that will need to be controlled will be temperature. This factor I cannot really control because I am going to be outside. Again timing will play an important role. As long as the experiment is conducting on both shores within the time limit, the temperature will not change that much to have an affect on the results.
The transect line will be 3.5 meters from the tide and will go 30 meters across the shore. So I have to make sure that the transect line is 3.5 meters away from the tide all the way across. It will be quite difficult to get this accurate because both shores are rocky and the transect line cannot be placed in straight line across the shore. I will use the apparatus that is provided to place the transect line as accurately as possible across both shores.
Another factor that will have to be controlled is the readings taken from the quadrat. Every time the quadrat is placed down, only the limpets within the quadrat should be counted. Any limpets that are counted and added to the results that are not in the quadrat, will make this experiment an unfair test.
The amount of readings that are taken from both shores will have to be the same. If I take ten readings from the exposed shore and twenty from the sheltered shore this would make my investigation unfair and I would probably be unable to reach a conclusion.
The accuracy that this investigation is done at also plays a part in keeping this experiment fair. There are a lot of other organisms that will be on both shores, and there are organisms that look similar to limpets. So when counting the limpets I will have to make sure that I do not count an organism that looks like a limpet because this will make it an unfair test. There is no definite way that I can prevent this from happening, so I have to make sure that when I am counting the limpets I do it properly and accurately as possible.
Null hypothesis: -
The null hypothesis is that there will be no significant difference in the abundance of patella vulgata across a sheltered rocky shore and an exposed shore.
Apparatus: -
- 50cm by 50cm quadrat.
- 1 meter ruler
- Clipboard
- Spirit leveller
- Calculator
- 30 meter measuring tape
The reason that I am using a 50cm by 50cm quadrat is because it is quite small. Limpets are very small animals so using a large quadrat will be time consuming and I will not be able to get as many samples as I would like.
Method: -
- Before you set out to the shores to conduct your experiment, you have to decide on how many readings you want to take and decide on how you are going to get your random coordinates. The method to use here is to go to the shore and take as many readings as possible within one hour and then do the same on the next shore. To calculate your random digits you can use your calculator, by pressing SHIFT and RAN#*30. This will generate a random number.
- Once you get to the shore, you will have to measure out where you will place your transect line. You have to place the transect line 3.5 meters from chart datum and you measure this out by using the meter ruler and the spirit level.
- You will have to walk down to chart datum, place your meter ruler upwards. You will then have to bend down and get eye level with the top of the meter ruler. Then place your spirit level on top of the ruler to make sure you are looking straight down. When looking from the meter ruler you should see a point that is level with your eye and the ruler, you make a note of this point, and then do the same procedure again from that point. You will have to do this three times and the fourth time you do it, you will have to get eye level with the 50cm mark on the ruler.
-
Once you are 3.5 meters from chart datum, you will have to place your transect line across the shore. The measuring tape is 30 meters long, so you will have to place the whole measuring tape across the shore making shore it is 3.5 meters from chart datum all the way across.
- After you have placed your transect line across the shore, you will have to start placing your quadrat down at the random coordinates. You will have to place the quadrat down the same way at each coordinate. So for every reading that you take you have to place the left hand side of your quadrat just above the measuring tape.
- Once you have taken all your samples within the hour, you collect all your equipment and make your way to your second shore as quickly as possible and go through steps one to five again.
Health and Safety: -
I will make sure that the experiment is carried out safely so that nobody and no organisms are harmed. Both shores will be quite rocky and these rocks are quite jagged, so no equipment will be left lying about so no one will trip and fall. When measuring the transect line from chart datum I will have to do that carefully making sure that I don’t fall into the water.
When counting the limpets and when walking up the shore I will try to make sure that I do not knock any limpets of the rocks. If accidentally any limpets are knocked off, I will make sure they are placed back onto their home scar.
There will also be other people carrying out their own investigations on the same shore as me, so I will have to make sure that none of my work gets in the way of somebody else’s investigation.
Results: -
Rank Castle Beach Bay = 538.5 ΣR1 = 538.5
Rank Frenchman’s Step = 1291.5 ΣR2 = 1291.5
U1 = (n1 * n2) + 0.5 * n2 (n2 + 1) - ΣR2
= (30 * 30) + (0.5*30) (30 + 1) – 1291.5
= 900 + 465 – 1291.5
= 73.5
To check if this calculation is correct, n1 * n2 should equal U1 + U2.
30 * 30 = 900
73.5 + 826.5 = 900
I have used the Mann Whitney U test to see if the medians of the two unmatched samples are significantly different. The U value that I got was 73.5 because it was the lower of the two U values. Using the significance table I can determine that the critical value is 127. The U value is lower than the critical value so I can reject the null hypothesis and can accept the alternate hypothesis that is there is a significant difference in the density of patella vulgata across a sheltered rocky shore and an exposed rocky shore.
Conclusion: -
From carrying out the Mann-Whitney U test I discovered that there is a significant difference in the density of patella vulgata on a sheltered rocky shore and an exposed shore. However the Mann-Whitney U test does not tell us on which shore patella vulgata is more abundant, it just tells us if there is a difference. This doesn’t create a problem because it can be clearly seen from the samples that I took that patella vulgata is more abundant on the sheltered rocky shore.
These differences in limpet density across two shores occur due to both abiotic and biotic factors. On one shore limpets can be more affected by the abiotic factors and on the other shore they can be more affected by biotic factors. This is what causes the difference in their density because the common limpet is more adapted to living on one type of shore than the other.
An abiotic factor that will play a part in limpet density is wave action. If there is a lot of wave action on a particular shore then the limpet density would not be as high on a shore where there isn’t a lot of wave action. On the exposed shore limpets will be more affected by wave action. Waves can crash onto rocks and knock limpets of their home scar, there will be days when wave action can get intense and can have high potential to even kill the limpets. Wave action would not present such a problem on the sheltered rocky shore. This is because there are a lot of rocks and the shore is sheltered so wave action doesn’t have that big of an affect.
Patella vulgata are specialised living organisms they can survive the harsh abiotic conditions that persist on the sheltered rocky shore. On the sheltered shore limpets are likely to experience desiccation but they are adapted to survive this, they can survive up to 65% water loss. On the exposed shore patella vulgata is more likely to be immersed in water, and limpets that are more exposed to water have lower desiccation tolerance.
The sheltered rocky shore is quite limited with light and also there is high competition. There were the barnacles that competed with the Patella for space; Patella vulgata themselves were competitive towards each other for space and food. Other organisms that compete with patella vulgata for space were purple topshells and dog whelks.
On the exposed shore there wasn’t much competition as there were plenty of resources available. There are plenty of minerals for micro algae to grow successively and in great quantity. There is space for limpets as not a lot of creatures are adapted to survive on the coast. Still wave actions can cause major exertion force, killing any organisms present. There is also high predation on the exposed shore.
Limpets are well adapted on the sheltered rocky shore because by having smaller circumference, they also have a smaller surface area. I have come to know that this is an adaptation to conserve water. Having less surface area means less water is lost through dehydration. It is important that limpets stay moist where there is less wave action and therefore it makes it harder for them to recover any water loss.
Limpets prefer to live on sheltered rocky shores. Even thought the abiotic conditions are harsher they are well adapted to survive these conditions. There is less wave action on the sheltered shore and also there is hardly any predation. The main thing that patella vulgata are competing for is space.
Evaluation: -
Even though the Mann-Whitney U test was a success and showed us that there is a difference in patella vulgata across two shores, there were limitations that had an affect on the results.
If there were a more concise way to measure out the transect line from chart datum, my result could have been a lot more accurate. The way I had to measure out the distance from chart datum was very difficult and there was a great chance that I could have made an error.
The 30 meter transect line that was placed across both shores might not have been 3.5 meters from chart datum all the way through. Again this would make my results inaccurate and the experiment an unfair test. To check if the transect line was 3.5 meters from chart datum all the way through would have been very time consuming, and time was a limiting factor. Also the lack of apparatus, if I had more advanced apparatus that could calculate the distances to greater accuracy this would have benefited my results.
Another source of error was my ability to identify limpets. There were organisms that look limpets but are not limpets and if I counted these in my results, it makes my results inaccurate. The only way that I could have got around this problem if there was an expert that came along with me and could have helped me identify patella vulgata. But this is not a practical thing to do.
Lack of apparatus had a limitation on my results, we were given no thermometers and pH probes would have been useful. These apparatus could have let me take measurements of the environment, so I could have concluded my results more evidently.
There were anomalies in my results. I did get some random coordinates that were next to each other. When I counted the amount of limpets on one coordinate the number was quite high and then when I placed the quadrat on the next coordinate the number of limpets counted was not high. I was expecting that if the sheltered rocky shore was more abundant in limpets then all my sample numbers would have been high, but this was not the case. I think this was because even on the sheltered shore there are micro-habitats that patella vulgata do not prefer to live in. this is why the difference in numbers occurred.
