The Rocky Intertidal Experiment
Content:
In Nahant, I worked from the high intertidal down into the low using random quadrat sampling. It was easier to work from high to low because we usually arrived before low tide on any given day. I would randomly throw my quadrat (a 1/2m2 of piping), remove all of the snails into a Tupperware container, and separate and count them before putting them back.
*The pictures to the right are from our first class trip to Nahant where we worked in groups and counted the number of Littorina littorea, or common periwinkle, in different heights in the intertidal.
Figure 1 is a graphical representation of the different densities of L. obtusata and L. littorea with respect to elevation. The results in table 1 illustrate a significant p-value of 0.0038 showing that there is a significant interaction between both L. obtusata and L. littorea and elevation. Table 2 shows there is a significant interaction with the number (of snails) and species of snail due to a p-value of 0.0271. Table 3 also shows the same interaction as table 2, but it includes the sum of squares as well as the F-ratios from the experiment. Figure 2 is a square root regression comparing the densities of both L. littorea and L. obtusata. There is a negative interaction between the two species. Table 4 is the significance of the regression shown in figure 2. The F-value of 0.08 is nearly significant, and reinforces the idea that the species prefer different elevations. Figure 3 is the difference between the least square means of the color morphs of L. obtusata. I also tested different color morphs of L. obtusata including light brown, dark brown, green, and yellow to see if there was a difference between the colors of snails and elevation and there was not.
The tables show that there is a significant interaction between the different snails and elevation. The results from table 1 show a significant p-value which means there is an important interaction between the species and elevation. It shows that the two species respond to elevation in a different way. In addition, table 2 also shows significant results with the interactions between the number of each snail species throughout various elevations and the species itself. The results show L. littorea are found at a lower elevation, which is consistent with the data that I gathered. In addition, the results in figure 3 illustrate a square root regression showing the two species react differently to elevation. Although the following table (table 4) does not show an f-value that is statistically significant, the interaction shown in the regression itself is important. The hypothesis could be possible based on the results of the regression because it is almost significant, but the results would be stronger with more data. The two species may have a negative interaction because they have different habitat preferences. Based on the data that I gathered in Nahant, not counting the regression analysis I accept the alternative hypothesis, that the two different species of snails live at different elevations in the rocky intertidal.
I also tested the data that I gathered of different color morphs of obtusata. The results are shown in figure 2 and table 4. I tested between yellow, light brown, dark brown, and green to determine if there was any difference between the colors in respect to elevation and there was not. The results showed a p-value of 0.054, showing the data to be almost significant. At first I thought there might be a difference between elevation and different color morphs of obtusata, but then I realized there probably wouldn’t be a large difference because obtusata live primarily on or near Ascopyllum nodosum and therefore wouldn’t be found far from their food source. With more data, it could be possible to test this hypothesis further.
Overall the results of this study could be improved with more data. The near significant F-value in Table 3 suggests that with more data, it could be possible to have a more significant regression between the two snail species further reinforcing the data that the two species are found at different elevations within the intertidal. In addition, this study could also be improved by studying another intertidal site and combining the data to make sure the results are consistent.
Click either of the pictures above to see a larger view of the charts