The first global survey of abundance of soil roundworms shows a staggering number of the tiny, transparent animals – more than 400 billion billion, weighing about 300 million metric tonnes. In sheer numbers, that is comparable to estimates of the number of stars in the observable universe.

And that’s just in the top six inches of soil.

Roundworms, or nematodes, are typically less than a centimeter long and live in the film of water around soil particles. They are by far the most abundant animals on Earth and fill roles at all levels of food webs. Different species eat bacteria, fungi, algae and plants, or other worms. A few species can cause disease in plants and a few are parasites of humans. But for the most part, nematodes live peacefully in the soil where they play vital roles in recycling nutrients.

The survey was led by Johan van den Hoogen and Stefan Geisen, Institute of Integrative Biology, ETH Zürich, Switzerland and included a long list of authors around the world, including Professor Howard Ferris at the UC Davis Department of Entomology and Nematology.

Analyzed with machine learning

The researchers used machine learning to analyze published data and unpublished data collections from 6,759 sites around the world to predict the global abundance and density of different types of worms. Perhaps unusually, the survey was based on counting worms under the microscope, rather than a DNA analysis. They also calculated how much carbon soil nematodes use and release on a daily basis.

One surprise from the survey is that soil nematodes become more abundant and diverse in the boreal forests and tundra of the far north. This is an exception to the usual role of biodiversity, which typically increases as you get closer to the tropics.

The northern and Arctic regions are also those most immediately impacted by climate change. How a warming climate will affect soil nematodes and microbes – and what effects that will have on soils, ecosystems and the climate – is unknown.

More information

Soil nematode abundance and functional group composition at a global scale (Nature)

Global maps of soil-dwelling nematode worms (Nature News & Views)