Biology 201
Ecology of Marine Atlantic Shores
Kasia Baca Fall 2008

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Nahant Class Exercise:

In order to practice different research methods that I learned in class, we took a trip to the Northeastern University Marine Science Center in Nahant, MA on September 12, 2008. We tested to find if Littorina obtusata associates more often with Ascophyllum nodosum compared to either Fucus vesiculosus or Chondrus crispus along the rocky intertidal. A quadrant sampling method was used by recording the percent cover of algae and counting the number of each snail for each quadrant sample. A total of twelve quadrants were sampled with four of each algae species.

                                    
                                                                     C. crispus                                          L. Obtusata

The quadrant size was 0.28 m^2 and snail density is defined as the number of snails per meter. In general, the quadrants that had A.  nodosum as the dominant species was apart of the mid-intertidal. While one C. crispus sample was taken in the mid zone, the other three were from the lower intertidal. Finally, the F. vesiculosus had three samples taken in the lower zone and one in the upper. Percent cover was estimated for each quadrant and each snail was identified and recorded.



Chart one: This chart gives the density (count/meter) of each snail species for each algae species. L. obtusata does not seem to prefer Ascophyllum over the other algae.

 

In general it looks as though Nucella does not associate with C. crispus and very little with F. vesiculosus (see chart one). In this study, L. obtusata does not prefer Ascophyllum over the other species. ANOVA tests were also done to evaluate these relationships to find if there are any statistical significance in any of them (L.littorea F= 0.20, P>0.82; L. obtusata: F=0.59, P> 0.57; Nucella: F= 1.13, P> 0.37). None of the ANOVAs were significant so no relationships are in this data.

Since the ANOVA for L. obtusata and Ascophyllum were not significant, the hypothesis iss rejected. However, a different trend was found in the data. Looking at the above chart, there seemed to be a positive correllation between the presence of L.obtusata and L. littorea(see figure two) (y=1.10x + 7.70, R^2= 0.5536). A t-test was done to further analyse this possible trend and found that it is not statistically significant (t= 1.80, P>0.073).



Figure two: A correlation between two snails that show a positive association with each other. The points contain error bars.


There is no preference of L. obtusata to Ascophyllum in this study. A reason why a snail might associate with a particular cover is that it may offer better biological protection like by decreasing desiccation. Ascophyllum was more dominant in the mid-intertidal region while Chondrus crispus was more common in the lower region. Even though A. nodosum is in a higher region, I would not expect more events of desiccation since this alga seems to create better cover for snails than C. crispus. Even though I have to reject my hypothesis, I still found that L.littorea and L. obtusata may have some kind of positive relationship which can be further studied to understand. More sampling of each alga is also required since four samples of each alga are not significant. Maybe more significant trends will be seen with more of these samples.

To test this possibility, I used the class pooled data to find if any of the relationships I found in the data holds true with more samples (see chart two below). The ANOVA again were not significant (L. littorea F = 0.50, P> 0.61; L. obtusata F = 0.46, P> 0.63; Nucella F= 0.73, P> 0.49). There was no postive correlation between L. obtusata and L. littorina.


Chart two: This is similar to chart one but instead uses more samples.