Majority of the world's most abundant creatures live in the far north

The first worldwide analysis of soil nematodes reveals that they are most abundant in arctic areas rather than in the tropics. The study was started by researchers from the Netherlands Institute of Ecology (NIOO-KNAW), and uses data from a global network of soil ecologists. They explain in the latest edition of Nature why nematodes could add significantly to the impact of climate change.

The study is the first to map the global distribution of nematodes. Also known as roundworms, they live mainly in the soil (as well as in aquatic sediments) and are the most abundant creatures on our planet.

The study focused on soil nematodes only. For each human, there are 57 billion (!) soil nematodes: that’s roughly 4.4 x 10²⁰ nematodes altogether, a much larger number than had previously been estimated.

Most nematodes are less than 1mm in length, absolutely tiny compared to humans. But add up their combined biomass, and it’s still a whopping 300 million tonnes. Moreover, it is estimated that between them, all those nematodes carry about half the amount of carbon that all humans carry, says shared first author of the study, Stefan Geisen (NIOO-KNAW). That’s a substantial amount of carbon.

More nematodes, more CO₂

“Perhaps the most remarkable finding is that most nematodes live in the soil of arctic areas”, says Wim van der Putten (NIOO), one of Geisen's co-authors. There's been much talk of the staggering numbers of organisms in the species-rich tropics. While that may be true for aboveground animals, it's not for these soil dwellers.

(Text continues below pictures)

About 38 % of all nematodes live in forests and tundras across North America, Scandinavia and Russia. Another 24 % can be found in temperate zones, and only 21 % in tropical and subtropical areas.

So far, the nematodes in the far north haven't been very active, because of low temperatures close to the North Pole. But if those temperatures rise due to climate change the nematodes will become more active, feeding on bacteria and fungi in the soil and freeing up nutrients.

Consequently the degradation of arctic peatlands will accelerate, resulting in the release of much more carbon. In the end, therefore, the large nematode populations in arctic and sub-arctic areas could contribute significantly to global warming.

"In other words", says Wim van der Putten, "this study reinforces the idea that we must do everything possible to halt climate change. If we don't, we will be setting in motion all kinds of processes that add to the rise in temperatures. That's a key conclusion of the study."

Soil researchers' roll call

The study focused on the top 15cm of soil, where most biological activity takes place. For their global dataset, the authors used 6,759 soil samples representing all contintents and a range of different environments from arctic tundra to tropical rainforest. They also looked at the functions of different groups of nematodes, which play a critical role in plant growth among other things.

First author Geisen collected nematode data from all around the world by enlisting the help of an extensive network of soil researchers. The Centre for Soil Ecology (CSE), a collaboration between NIOO and Wageningen UR, has built up a great deal of nematological expertise.

Together with the CSE, Geisen selected soil researchers from all over the world and asked them to contribute to the Nature study with their research data. The Crowther Lab based at ETH Zürich - set up by former NIOO-researcher Tom Crowther - processed this enormous amount of data.

With more than 300 staff members and students, the Netherlands Institute of Ecology (NIOO-KNAW) is one of the largest research institutes of the Royal Netherlands Academy of Arts and Sciences (KNAW). The institute specialises in water and land ecology. As of 2011, the institute is located in an innovative and sustainable research building in Wageningen, the Netherlands. NIOO has an impressive research history that stretches back 60 years and spans the entire country, and beyond.

More information:

• Researcher Prof. Dr. Wim van der Putten, head of Terrestrial Ecology NIOO-KNAW, tel. +31651193157, w.vanderputten@nioo.knaw.nl
• Science information officer Froukje Rienks, Netherlands Institute of Ecology (NIOO-KNAW), tel. +31-6-10487481 / +31-317-473590, f.rienks@nioo.knaw.nl

Article: Soil nematode abundance and functional group composition at a global scale, Van den Hoogen & Geisen et al., Nature, online 24 July 2019, doi: 10.1038/s41586-019-1418-6

NIOO-authors: Stefan Geisen (shared first author), Wim van der Putten, Gerard Korthals, Casper Quist and Rutger Wilschut.