Fig. 1: Illustration of aqueous habitat fragmentation and carrying capacity in relation to climatic water contents.

In regions where rainfall is frequent, the soil aqueous phase is largely connected and provides a common habitat for cells of different bacterial species. In soils of drier regions, the aqueous phase is increasingly fragmented and offers a large number of distinct habitats. When the soil becomes sufficiently dry, almost all aqueous habitats are physically isolated and might contain only a few species. Additionally, the total number of cells that can be maintained (potential carrying capacity) is reduced and smaller patches become uninhabited. The specific carrying capacity in a biome is based on carbon input flux and temperature that establish an upper bound on bacterial cell density (rarely realized in any particular location due to other limiting factors). The numbers below each panel indicate the number of cells per number of habitats. Diversity is expected to drop in dry regions with low cell abundance and in wet regions with high habitat connectivity.