More than half of all species live in the soil, according to a study that has found it is the single most species-rich habitat on Earth.
Soil was known to hold a wealth of life, but this new figure doubles what scientists estimated in 2006, when they suggested 25% of life was soil-based.
The paper, published in the journal Proceedings of the National Academy of Sciences, found it is home to 90% of fungi, 85% of plants and more than 50% of bacteria. At 3%, mammals are the group least associated with soils.
“Here, we show that soil is likely home to 59% of life including everything from microbes to mammals, making it the singular most biodiverse habitat on Earth,” researchers write in the paper, which is a review of existing literature. The actual figure could be even higher as soils are so understudied, they say.
Before this study, scientists did not know what the most species-rich habitat was, says the lead researcher, Dr Mark Anthony, an ecologist at the Swiss Federal Research Institute for Forest, Snow and Landscape Research. “In my research circle, many suspected it should be soil but there was no evidence.”
He added: “Organisms in soil play an outweighed impact on the balance of our planet. Their biodiversity matters because soil life affects climate change feedbacks, global food security, and even human health.”
Soil is the top layer of the Earth’s crust and is composed of a mixture of water, gases, minerals and organic matter. It is where 95% of the planet’s food is grown yet it has historically been left out of wider debates about nature protections because we know so little about it. One teaspoon of healthy soil can contain up to a billion bacteria and more than 1km of fungi.
Researchers used the rough estimate of there being about 100bn species in total. They then used theoretical estimates and data analysis to work out what fraction of those species were found in the soil. They defined a species as living in the soil if it lived within it, on it, or completed part of its lifecycle in it. Other habitats they looked at include marine, freshwater, the ocean floor, air, the built environment and host organisms such as humans.
There is a large error range of 15% with the estimate – so the average prediction could in theory be as low as 44% or as high as 74%. For some groups the range was large – for bacteria, estimates ranged between 22% and 89% living in the soil.
Anthony said: “What actually surprised me the most was the sheer challenge of this undertaking, and how much variation there is to our estimates for many large groups, particularly bacteria and viruses, the two most diverse forms of life on Earth.
“Keeping that in mind, our estimate is really a first attempt to organise existing global richness albeit with quite a large error to many of the estimates. While true diversity lies somewhere within this range, our effort is the first realistic estimate of global diversity in soil, and we need it to advocate for soil life in the face of the biodiversity and climate crises.”
A third of the planet’s land is severely degraded and 24bn tonnes of fertile soil are lost every year through intensive farming alone, according to a UN-backed study, the Global Land Outlook. Pollution, deforestation and global heating also damages soil. Adopting less intensive agricultural practices, greater regulation of non-native invasive species, and increasing habitat conservation will all help increase soil biodiversity, researchers say. Practices such as soil transplantations could also restore microscopic lifeforms in soil.
Dr Roy Neilson, an ecologist from the James Hutton Institute in Dundee, who was not involved in the research, said: “It is extraordinarily difficult to enumerate soil biodiversity … The approach taken in this study arguably generates the current best estimate of global soil biodiversity.
“However, as the authors note, generating these estimates has been challenging and they are transparent as to the level of robustness of their data, which in turn highlights areas for future scientific investigation,” said Neilson, who is an author on the British Ecological Society’s upcoming report on regenerative agriculture.
