For about a month now, I have been compiling a list of the land snails endemic to Turkey. A taxon (for example, a species, a genus or a family) is endemic to a particular area if it occurs only in that area. My next step and the ultimate aim is to determine if there are any centers of land snail endemism in Turkey. A center of endemism could be defined as an area where there many more endemic species than there are in adjacent areas of equal size. So to determine if an area is a center of endemism, one needs an objective way of counting endemic species.
An internet search yielded the full text of quite an informative paper1 titled "Endemism in the Australian flora". I have no specific interest in the flora of Australia, other than reading the relevant posts on A Snail's Eye View, but I figured the theory and methodology used in this paper would be as applicable to snails as they were to plants.
The authors mapped the distributions of 8468 plant species on a map of Australia using a grid of 1° latitude X 1° longitude. The resulting map (Fig. 1 in the paper) immediately gives one an idea of where the endemic species are concentrated. (One complaint I have: the color codes used in the maps are not explained in the paper unless I missed it, but from the context it appears that yellows and reds are for low numbers of species, the blues are for intermediate numbers, while the reddish colors are for the highest numbers of species.)
Three ways of measuring endemism are discussed in the paper. One is a simple count of the number of endemics per grid cell. But the patterns of endemism are scale-dependent. The authors set an upper limit of 4 cells and calculate for each cell the total number of endemics whose ranges are 4 cells or less (Fig. 2 in the paper).
The second measure is the weighted endemism (WE), which is the sum of the reciprocal of the total number of cells each species in a grid cell is found in.
What makes the paper really useful is the third and the a new measure of endemism it introduces, the corrected weighted endemism (CWE). The corrected weighted endemism is simply the weighted endemism divided by the total number of species in a cell.
These measures are discussed in detail on p. 186 of the paper. (The abbreviations used here are mine.)
Using one (or more) of these methods, a measure is calculated for each grid cell and indicated on a map. The comparisons of mapped CWE values gives a pretty accurate idea of where the centers of endemism are; it does at least for the Australian flora, according to the authors (Fig. 3 in the paper).
As I develop my database further and start analyzing it, there will be more posts on this subject.
1Crisp, M. D., Laffan, S., Linder, H. P., and Monro, A. 2001. Endemism in the Australian flora. Journal of Biogeography 28:183-198. pdf