When it comes to habitats that are particularly rich in species, we usually think of rainforests or coral reefs. But most of the biodiversity on earth is hidden in a habitat that is often neglected: the soil. It is home to almost 59 percent of all known species - from microbes to invertebrates and plants to mammals, making it the most species-rich habitat on earth, as a study has now revealed.
The soil – the layer between the bedrock and the surface of the earth that is enriched with organic material – is home to a large number of organisms and plays an important role in the material cycles of our planet. Because the soil stores large amounts of organic matter in the form of plant residues, animal excretions and carcasses and other relics and thus the carbon contained in them. At the same time, the microbial degradation of organic matter by soil organisms releases copious amounts of greenhouse gases such as carbon dioxide and methane. The living environment of the soil is therefore important for the climate, atmosphere and biosphere.
Inventory in the organism kingdom
But how many organisms live in the soil? And what part do they play in the diversity of species on earth? Because the inhabitants of the soil have not yet been fully researched, there is little reliable data on this. An earlier estimate, which included only terrestrial animal species, came to a share of around a quarter of the earth's biodiversity. However, microorganisms, viruses, fungi and plants were left out. "Our work is now a first but important attempt to estimate what proportion of global biodiversity lives in the soil," explains lead author Mark Anthony from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.
For their study, Anthony and his colleagues first collected information on the total number of species in the large groups of organisms - from the smallest bacterium to trees or mammals. They also deliberately included extrapolations and estimates. "If we only included the known species of all groups, then we would miss the vast majority of small and occult life forms - such as arthropods, nematodes, fungi, protists, bacteria, archaea or viruses," the scientists explain. Based on this information, they used the known ways of life to determine the approximate proportion of species living in or on the soil for each large group. They also included species that spend only part of their life cycle in the soil.
Species richer than any other habitat
The evaluation showed that on average more than ten billion species live either completely or partially in the soil - not including viruses. If you put this in relation to the total possible number of species on earth, this corresponds to around 58.5 percent of all species. This means that biodiversity in the soil could be higher than in any other habitat on our planet. "Our results demonstrate that the soil is the most biodiverse habitat on earth," say Anthony and his colleagues. But even the new values could still be too low: "In fact, our values are probably an underestimate for many groups, because the above-ground living environment has so far been studied far better than that in the ground," says the team.
However, the proportion of terrestrial species differs depending on the group of organisms. It is lowest in mammals at only 3.8 percent, in molluscs it is 20 percent and in arthropods it is at least 30 percent. Most plants also belong to the soil dwellers: After all, up to 67 percent of their biomass is underground as roots and tubers, and the seeds also germinate in the soil. However, the large group in the family tree that is by far the most adapted to life in the soil are the fungi: around 90 percent of the more than six million species of fungi live on and in the soil. On the other hand, it was difficult to estimate the number of bacterial and viral species living in the soil. This is because only a fraction of the true diversity of these groups is known to date, and it is often difficult to delimit the species. The researchers estimate the proportion for bacteria to be around 22 to 89 percent and for viruses to be six to 43 percent.
Taken together, the results underscore that soil is an important but often overlooked part of the Earth's biosphere. For example, soil-dwelling organisms are far less frequently found in red lists, and they often fall through the cracks in species protection efforts. "Our study now shows that the diversity in the soil is large and correspondingly important and that it should therefore be given much more consideration in nature conservation," says Anthony. This is particularly important because the habitat soil is particularly badly affected by human intervention.
Source: Swiss Federal Institute for Forest, Snow and Landscape Research WSL; Specialist article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2304663120