Hi, 

I am not very sure of what are you asking for; but if you are trying to estimate densities (individuals/surface unit2); then you can use the quadrat (square samplig device) method in combination with a transects sampling protocol.

Thanks Alfonso Nebra, It's exactly my request !

You can certainly use quadrats. There are some potential problems:

If you use the numbers you see in each as your data, they may be influenced by the weather at the time, or time of year. Many studies show that not all the population are active even in ideal conditions, and so "by eye" depends on how good you are at finding those that are resting.

At the same altitude, even in what appears to be the same habitat, the density can vary quite a bit. With a transect, and quite a lot of quadrats, this should not affect an altitudinal trend too much, but it will introduce "noise" if you are only after the effect of altitude.

With large snails like these it is possible to use another technique: mark and recapture. Lots of people have used this technique. If you want a quick estimate of the population at a point in time, then most people find that marks made with nail varnish last long enough. Of course, like all methods, there are problems here too, and it would involve more work, as each quadrat has to be visited at least twice, and the shells (with a dry surface) marked before returning them.

Depending a bit on the actual density, quadrats of up to 20 by 20 metres are good. probably not smaller than 10 by 10 metres.

Here are a couple of old papers involving mark recapture. standard textbooks deal with with methodology and complications.

Good hunting!

Thanks Robert for you're answer and the papers ! I'd thought of the mark and capture method. In fact the problem is more the access to the area (3-4 h walking) and the time I can pass on the field !!! The first snow fell last weekend... I think for this year the hunting is close !!!