The tropics are home to huge earthworms, but a new study suggests they host fewer species.

Iñigo Virto

First global map of earthworms reveals which places are chock-full of them—and why

Earthworms are the unsung heroes of the planet’s ecosystems: Unnoticed below our feet, they grind up soil and dead matter, recycling essential nutrients and moving air and water deeper into the ground. Without them, soil health would suffer and plant productivity would falter. Now, for the first time, researchers have mapped where these humble invertebrates live, identifying wormy hot spots around the globe. The project, which pooled earthworm data from more than 140 scientists and 6900 sites, has cataloged hundreds of species and revealed trends about where each plies the soils—and under what conditions they thrive.

“The results … provide a comprehensive global perspective on one of the most important animal groups,” says Stefan Scheu, an ecologist at the University of Göttingen in Germany, who was not involved with the work. Scientists can now start to come up with conservation plans for worms and other organisms that integrate life above and below ground, he adds.

During the 1800s, intrepid explorers collected and cataloged many of the world’s plants and animals, providing range maps for different species that launched further study. But that wasn’t true for subterranean life. “We’ve been lacking basic information [for a long time] about what earthworms live where,” says Noah Fierer, a soil ecologist at the University of Colorado in Boulder.

So, soil ecologist Helen Phillips from the German Centre for Integrative Biodiversity Research in Leipzig and her colleagues contacted all the earthworm researchers they could track down to ask for data about the animals living in their study sites. Ultimately, 141 scientists provided numbers and species names from more than 6900 sites across 57 countries. “There was about three times as much data as I was expecting,” Phillips says.

Compiling and analyzing those data, many of them in different formats, must have been a challenge, says Katalin Szlavecz, a soil ecologist at Johns Hopkins University in Baltimore, Maryland. For example, earthworms have been studied long enough in Europe that most of the species are known. (The United Kingdom has 33 kinds.) But in the tropics, “Every time they dig a hole, they find a new species of earthworm,” Phillips says. And that uneven amount of study had to be taken into account to use the data effectively.

She and her team evaluated the data to make sure they were as comparable as possible from site to site, and then used computer modeling to generate their global map. They were surprised when their analysis showed that temperature and rainfall seem to have a greater influence on where earthworms do best than soil type, they report today in Science.

“It’s surprising that soil properties weren’t the most important driver,” says Tami Ransom, a community ecologist at Salisbury University in Maryland. Szlavecz, too, was astonished how little soil type mattered. The effects of temperature and rainfall suggest climate change will have a far greater influence on below-ground life than expected, they say. Consequently, life above ground might also be affected in ways not previously anticipated.

The distribution of different earthworm species was also surprising. When it comes to life above ground, the tropics have the greatest biodiversity. But underground, these constantly warm regions are far less diverse, at least at a local scale: The rich soils of Europe, the northeastern United States, the southern tip of South America, and the southern regions of New Zealand and Australia seem to have more earthworm species in a given area. Those temperate zones also host more earthworms overall, according to the model, with up to 150 per square meter versus just five per square meter in the tropics.

It’s vital to know what earthworms exist where, says Kevin Butt, an ecologist at the University of Central Lancashire in the United Kingdom who was not involved with the work. That’s especially true, he adds, “as we are in an era when large global upheavals are at play.”