Foiled Again

If you recall some of our first posts, you may remember a few whines and complaints about 25 hour trips to Wales etc… Well, it turns out that the airports have done it again, and some of the members of our group finally made it home 3 days later.  This has put a significant damper on our blog-writing; there are a few more entries that cover the end of our second week in Wales — look for those soon.

In the meantime, mull over these graphs showing the data we collected on cockles.

Let’s take these graphs apart step by step.  First, note that the x-axis is the same for both graphs; it shows tidal elevation.  High is closest to the marsh and Low is closest to the water.


The y-axis for each of these graphs shows the scale of the response variable (what we are measuring).  The graph on the left is showing the number (#) of live cockles that were found at each site, and the graph on the right is showing the age (years) of cockles at each site.  Finally, the titles of these graphs indicate that they are showing us the mean (average) values from a data set.  The round dots indicate the mean, and the blue bars are showing 95% confidence intervals (ignore this jargon if statistics is not your gig).

Ok, so we’ve mentally labeled our graphs.  What are some broad observations?

1. The number of living cockles found was highest at the Medium site.

2. The mean age of cockles appears to increase as the tidal elevation decreases.

(Note: Here is where the confidence intervals come in.  Although the mean age at the Low site is higher than the mean age at the Medium site, notice that their confidence intervals overlap.  This indicates that these two means, statistically speaking, may not actually be different (or different enough).)

Before we move on, here are the graphs again so that you don’t have to scroll back and forth:

You are such a good sport for sticking with us so far! Here are some inferences and conclusions:

1. There is something limiting the population of cockles at High tidal elevations.  We know that cockles are filter feeders (they filter seawater for plankton).  Therefore, when the tide goes out, they can’t eat.  This could explain why the number (and age) of cockles found at the High site was low. We all need food to grow!

2. The age of cockles at the Medium and Low sites are similar, but the number of cockles found at these two sites is vastly different (Medium =102 and Low=18).  Because growth does not seem to be inhibited (ie no food limitation), perhaps we can infer that there is a waterborne predator that is munching on the population of cockles at the Low site; crabs are definitely a potential predator.

In order to draw more conclusive inferences, we would need to run some more experiments.  Have any ideas for some simple experiments or observations for next year that would help us draw more informative conclusions?  Leave us a comment!

About David Malmquist

David Malmquist is the Director of Communications at the Virginia Institute of Marine Science, College of William and Mary.
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One Response to Foiled Again

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