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  • Level: GCSE
  • Subject: Maths
  • Word count: 3341

topshell populations

Extracts from this document...


Skill A-Planning:


Could there be a difference between monodonta lineata populations in sheltered and exposed areas at Saltern cove.

Hypothesis: I predict that there will be a higher population of monodonta lineata in the sheltered area of saltern cove than exposed area of saltern cove.

Nul hypothesis:I predict that there will be no difference in monodonta lineata population’s in the sheltered and less sheltered areas of saltern cove.

The investigation: My investigation will be conducted at saltern cove. Saltern cove has a very diverse range of species but I will only be looking at one species of animal. For my investigation I will be counting the population of monodonta lineata in different environments.

Map of saltern cove:

Location: United Kingdom image00.png

Grid ref: SX894578

Postcode district: TQ4 5xx (map centre)

X: 289500m

Y: 57800m

Lat: 50:24:36N (50.41)

Lon: 3:33:25W (-3.557)

Saltern cove: Saltern cove is a site of special scientific. This means that the government protects the site. Saltern cove is especially interesting because of its geology but it also has a diverse number of species. I will only be concentrating on the monodonta lineata populations in the cove.

Background Information:

Monodonta lineata belong to the trochidae family of the gastropods. The shell of a monodonta lineata is cone shaped or pyramidal. They have an inner-layer of an iridescence material often known as mother of pearl. image01.pngimage02.png

The opening of the shell is called the operculum. It is round, with a spiral pattern and composed of a horny material.

Monodonta lineata are herbivores they prefer to live on algae or algae covered rocks in shallow water.

Trochids have separate sexes and most species expel sperm and eggs into the water for external fertilisation.

The monodonta lineata is the largest of the topshells it can reach 30mm.

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Risk assessment:

When conducting this infield experiment it is important to be safe. So we must consider safety points.

The path to the beech is very steep and I will be carrying a lot of equipment so I must be very careful not to slip because it could lead to serious injury.

Also because I’m working on different sides of the beach I must make sure that I have enough time to sample at both sites before the tide cuts them off because I could get trapped on one side of the beach.


Type of Variable


My independent variable will be the area at which I’m sampling. One area will be exposed to large waves and harsh abiotic factors. The other will be a sheltered area t where the abiotic factors will be less harsh.


My dependent variable will be the population of monodonta lineata in my two areas. I will be measuring the amount of topshells in these areas.


My controlled variables will be the sample area.

The size of the quadrat.

The amount of samples taken.

By controlling the size and site of the sample area it will allow me to pick a suitable site to conduct the experiment and mark out a suitable sized area to collect enough samples to gain good reliable data.

By controlling the size of the quadrat I will be able to carry out my experiment in a suitable time frame whilst being able to gather reliable data.

By controlling the amount of samples taken I will be able to carry out my experiment in a suitable time frame whilst being able to gather reliable data.

All these aspects of my experiment will be controlled.

Skill B-Implementing:


After writing my plan I decided to make some modifications. These modifications made my experiment more accurate and allowed me to record very accurate results.


Reason for modification:

Did twenty quadrat samplings instead of fifteen quadrat samplings.

I think by using 20 quadrats I got more data which makes my statistical test more accurate and therefore my conclusion will be more reliable. I will also get a more reliable estimate for the total populations of monodonta lineata in each area. By sampling more the possibility that the results obtained weren’t by chance but show an actual trend.

Sample areas were 3metres, in vertical height, up the beach instead of 2 metres.

From my background research I discovered that monodonta lineata like to live higher up the beach in summer. So I decided to sample a bit higher up the beach meaning I would get more reliable data. I will also get more accurate data if I sample at the same vertical height in both my areas. This may affect my data because the overall populations may be reduced because there could be increased predation from organisms due to the increased time that monodonta lineata will be out of the water.

To ensure that the data collected was accurate I used various methods. When measuring the amount of monodonta lineata in each quadrat I made sure that all the monodonta lineata were collected and counted properly by double checking each time. I also made sure that the quadrat was carefully placed at the exact cordinates. By using a random number table I ensured that there was no bias in the choosing of the cordinates.

Skill C-analysing:

Chi squared test:

The number of topshells that were observed in two different environments were as follows:

Sheltered = 395

Exposed = 181

The nul hypothesis being tested is that there should be no difference between the amount of monodonta lineata in each area.


























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One major limitation is the fact that monodonta lineata are always on the move; they never stay in one place for too long so you can never be quite certain that the results that you obtained are that accurate. For example a large amount of topshells could have migrated out of the exposed area on the day that I took my samples. Which would have cause my results to be slightly biased in the way that the exposed area would be less populated because of mass migration instead of the harsher abiotic factors. So I use a 5% probability of a chance result to be considered in my statistical test. This allows the possibility of mass migration or the effect of other factors beyond my control to be considered when doing my statistical test.

Another limitation is the fact that I must make assumptions when conducting my experiment. Assumptions such as, that there will be no mass migration, mass predation and mass deaths or births. All these scenarios are a possibility however unlikely they could be so I must consider them when writing my conclusion.  

With the resources at my disposal I think my experiment was very accurate and gave a reliable interpretation of these results but if I had greater resources and time I could have made my experiment more accurate. By expanding the sample area to encompass the whole of the sheltered and exposed areas and doing lots more samples the data that I would have collected would have been very accurate. If I had a lot more time I could have done sampling over many different days in summer and winter this would have given me reliable population data and I could have measured the population fluctuations between seasons.





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