THE SOIL FAUNA - THE OTHER LAST BIOTIC FRONTIER

Different approaches to biodiversity yield global totals as small as 3 million or as large as 80 million species. Erwin's calculation and es timation leads to an estimate of ca 30 million species and relies on f our assumptions of which one concerns the ratio between the number of canopy insects and those found elsewhere, especially in the soil. A sh ort survey of the microarthropods living in coastal sand dunes and col lected with a new flotation method yielded amazing results. In spite o f the severity of the habitat (low organic matter content and extreme dryness), the density of microarthropods varied between 175 000 and 1 400 000 individuals per square metre, i.e., densities 3 to 10 times hi gher than densities usually observed in any other type of soil. A tota l of 31 species was recorded, most undescribed and smaller than 200 mu m. The consequences of these findings on the estimation of the number of species are discussed. It is suggested that the soil, including the deepest horizons and the rhizosphere, might constitute a huge reservo ir for biodiversity.