Soils are the most species-rich habitats on earth

Coral reefs, the deep sea or the treetops of the rainforests are considered the main hotspots of biodiversity. However, they all trail behind the soils: According to a new study, soils are the most species-rich ecosystems worldwide. Their importance for human nutrition is enormous, and the proportion of soils worldwide that are considered degraded or destroyed is growing steadily. A trio of researchers led by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL has now made the first estimate of global soil biodiversity.

To do this, the researchers from WSL, the University of Zurich and the Agroscope agricultural research station searched the existing scientific literature or re-evaluated existing data sets on species that have been identified in soils. Their results indicate that two thirds of all species live in the soil, they report in the renowned scientific journal PNAS. This is more than twice as high as previous estimates of soil species richness. According to them, only 25 per cent of all species live in the soil.

The group with the highest proportion of species living in the soil is fungi - 90 percent of them live there. They are followed by plants and their roots with 86 percent. Earthworms and molluscs such as snails make up 20 per cent. "However, no one has yet attempted to estimate the diversity of the very small organisms such as bacteria, viruses, archaea, fungi and unicellular organisms," says the first author, Mark Anthony from WSL. Yet they are crucial for recycling nutrients in the soil, for carbon storage, and they are important as pathogens and partners of trees.

Since the data on soil diversity is extremely patchy - especially in the global South - the results of the study show huge ranges in some cases. For bacteria, for example, the mean value is 40 per cent of species living in the soil - but the range extends from 25 to 88 per cent. The uncertainties are also enormous for viruses, which are mainly studied as human pathogens. Accordingly, the authors are bracing themselves for some criticism of their methods and conclusions. "Our work is a first but important attempt to estimate what proportion of global biodiversity lives in the soil," says Anthony.

The goal, he says, is to provide the basis for much-needed decisions to protect soils and their creatures worldwide. "Soils are under enormous pressure, whether from agricultural intensification, climate change, invasive species and much more," Anthony points out. "Our study shows that the diversity in soils is great and correspondingly important, so they should be given much more consideration in conservation."