Species-area curve

In ecology, the Species-Area Curve is a graph recording the cumulative number of species of living things recorded in a defined area of a particular environment (usually by using quadrats) as a size of the area examined. It is related to, but not identical with, the species discovery curve.

The species-area curve is normally plotted by using quadrats of successively larger size, so that the area enclosed by each one includes the area enclosed by the smaller one (ie. areas are nested). Given this technique, the curve will necessarily be increasing, and it will normally be negatively accelerated (that is, its rate of increase will slow down). After plotting the curve, it is usual to estimate the area after which using larger quadrats results in the addition of only a few more species. This is called the minimal area. A quadrat that encloses the minimal area is called a relevé, and using species-area curves in this way is called the relevé method. It was largely developed by the Swiss ecologist Josias Braun-Blanquet.

Estimation of the minimal area from the curve is necessarily subjective, so some authors prefer to define minimal area as the area enclosing at least 95 percent (or some other large proportion) of the total species found. The problem with this is that the species area curve does not usually approach an asymptote, so it is not obvious what should be taken as the total).

Reference
Barbour, M. G., Burk, J. H., & Pitts, W. D. (1980). Terrestrial plant ecology. Menlo Park CA: Benjamin/Cummings. Pp. 158-160.