21 September 2009

The vagaries of field work

Taking advantage of the nice weather we had yesterday, I spent several hours in the field continuing with our ongoing Cepaea nemoralis survey. The purpose is to determine how far the species has dispersed around the point where we think the original introduction took place some time ago.

But, how does one decide if a species of interest is absent at a locality, say, a square roughly 30 m along one side? Philosophically speaking, how can one prove the negative? One really can't*. And we don't actually claim to prove that a species doesn't exist at a locality where we couldn't find it. We only say that we couldn't find it. The idea is to carry out searches at as many localities as possible and then hope that some sort of pattern will emerge after we put all of the localities on a map. For example, we may see a boundary beyond which Cepaea nemoralis seem to be absent. Or we may see an association with an environmental variable; for example, most of our localities where we found the species may be near a water body or in forested areas.

But, the act of finding a species at a locality where it actually exists could be daunting, especially if there aren't very many individuals and if the visibility is obscured by the plants and the debris on the ground.

So, to make this short story even longer, I will give an example from yesterday.

At one of my localities, a steep, wooded and rocky slope, I spent, literally hanging onto the bushes and tree branches to stop myself from sliding down to the river at the bottom, about 15 minutes searching for Cepaea nemoralis. I found no Cepaea, but I did find several snails shells that belonged to other species, which made me think that my search technique was otherwise effective. Finally I decided this was going to be one of the "open" circles on the map, a spot where Cepaea nemoralis did not exist. I started climbing up the slope while still keeping my eyes on the ground. I reached the edge of the forest and was about to exit into the sunny field beyond, and then I spotted one Cepaea shell.

The proof that Cepaea nemoralis was present at station FR-19. The spire of the shell was missing.

If I had happened to be looking at a different spot, or a plant had totally blocked the view, if I had happened to climb up a different path, I would have missed it. Or, maybe I would have found not one, but 2 shells.

During the 3-hour field trip I ended up searching 10 localities and found Cepaea at 3 of them. Was the species really absent at the other places?

I will never know. And it doesn't matter, because a pattern is starting to emerge.

*Within the limits of reason, of course. I did not see any elephants yesterday at any of the places I looked. And that does prove that elephants did not exist at those places, mainly because elephants are unlikely to have been present.


Kevin Bonham said...

Could the spire on the photographed specimen be missing because of bird or other vertebrate predation? When all I have is a single dead shell in that sort of condition I tend not to count it as a confirmed presence hit on a fine scale, in case the shell was transported from elsewhere by a vertebrate and only eaten (or dropped) in situ.


Kevin: That's a good point, but I am tempted to ignore your argument. That particular shell may have been broken by a rodent that had captured it right there where I found it.

Kevin Bonham said...

Yes, it may very well have been found and killed right there. My question is whether there can be confidence that it was not killed somewhere else, which I think is necessary to treat a presence record as confirmed. However you would have a much better idea of what your local rodents, birds and other culprits do to snail shells (and perhaps how far they might carry them before doing it) than I would.

Curiously, evidence about the nature of bird vs rodent damage of Cepaea shells actually came up in a court case I was involved in eight years ago, but I wasn't the one who introduced it and am a little rusty on the details.

Tim Pearce said...

Even before I read Kevin's comment I thought the same thing. Predators often transport their prey before consuming it. The shell is evidence that the species lives in the general area, but not necessarily that it lived within that 30x30m square.

Also, we don't prove anything in science, so no, I reject your proof that there were no elephants. Absence of evidence is NOT evidence of absence, even if it makes a lot of sense.


If I went to an open field in Maryland (or any other place where elephants were highly unlikely to be found) & looked carefully & saw no elephants, that would prove that there were no elephants there.

cnmg said...

It would support your theory about the probability of elephants being present, but neither prove or disprove the presence of elephants.

You should probably search for live grove snails at night, during the rain - or after a heavy downpour

Also, on the branches of partly dead or decaying sumac - which they seem to prefer in this area.

Your friend from Boston