10 June 2010

Do Cochlicopa lubrica aggregate?

This is the 4th post in this series about the snails, Cochlicopa lubrica, on my garage door. As long as the snails are there, I will write about them.

The question I have had in my mind is this: is the distribution of the snails on the garage door random or is there a tendency of them to be near each other, in other words, to aggregate?

What is meant by aggregation could be tricky to conceptualize. My garage door is 240 cm long and 210 cm high. If there were 1000 snails on it, they would have no choice but to aggregate to some extent. On the other hand, the distribution of 1000 snails placed randomly on the garage door could be distinguished by using appropriate statistical tests from that of the same number of snails piled on top of each other in one corner. In the 1st instance, there would be no aggregation; in the 2nd instance, there would indeed be aggregation.

So, on 2 occasions within the last 12 days, I have measured the location of each dormant Cochlicopa lubrica on my garage door*. Because I also marked the location of each snail with a number, I know that each snail moved between the 2 sets of measurements. Therefore, there were no duplicate measurements. However, one problem I have is that there aren't that many snails on the garage door; during the 1st set of measurements, there were 36 snails and during the 2nd set 24. Therefore, unless the snails have a very strong tendency to stay close to each other, it may be difficult to see clear spatial patterns. Here are 3 of the snails I photographed tonight. Was their distribution random or not?

I don't have time to run any statistical tests tonight. But here is one simple analysis of the combined data from 2 days. This graph shows the distributions of the heights of the snails above the ground (in 20 cm increments).

Most snails remain below 120 cm. I don't know why they tend not to climb higher. This tendency to stay close to the ground increases their chances of being near each other or their chances of aggregating.

To be continued...

Part 3 was here.

*No, I didn't have anything better to do with my time. Besides, I was having fun.


Fred Schueler said...

"Random distributions always appear to be clumped" -- we knew we'd succeeded in home-schooling our daughter when she came out with this. Maybe with the inset panels on your garage door, it won't be possible to truely test for a really random distribution.


Yes, the panels are indeed a potential problem. The snails may or may not prefer the overhangs. But then again, there are no smooth, featureless vertical surfaces in nature either.

xoggoth said...

Hey that looks like my garage door. Have you nicked it? Bet that's my snail too.


Yes, I nicked it, whatever that means :)