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Woodlice back on top, slugs deterred by drought

November 08, 2018

The 4th edition of the Dutch Soil Animal Days saw 856 'citizen scientists' comb through more than 200 gardens and parks to find some 7500 soil creatures. Findings that stand out after this year's long, dry summer: woodlice have regained their ubiquitous status, while slugs were spotted in fewer places. The drought may also explain why so many people had difficulties finding all the soil animals, although having a balanced garden does help.

After the wet early autumn of 2017 saw them slipping from our Soil Animal top three for the first time, woodlice returned to the top step of the podium this year. They were found in no fewer than 89% of gardens, followed by earthworms (85%) and arachnids (82%).

Effects of the drought: fewer slugs

The summer of 2018 was extremely hot and dry. According to Gerard Korthals, soil ecologist at NIOO-KNAW and Wageningen University and initiator of the Soil Animal Days, slugs are more vulnerable to drought and heat than other soil animals. "This was borne out by the fact that the more than 800 citizen scientists who participated this year didn't see as many as them as before."

Consequences in the longer term could be serious, says Korthals. "After a number of years of this kind of extreme weather, the balance in the soil could change." Slugs are important because they break down bits of dead organic matter, and are themselves a food source for hedgehogs and other animals.

More ants, recovery underway

Meanwhile, the participants in the Soil Animal Days spotted more ants than they did last year: ants are now in 4th place, as they were after the Indian summer of 2016. In addition, other soil animal species showed signs of a speedy recovery.

"Green gardens are resilient", Gerard Korthals adds. "So recovery is indeed possible, and is in fact already underway. I was struck, for instance, by the number of young millipedes I saw."

Prone to stress

In other types of gardens, however, soil animals were more prone to heat stress. "Just think of gardens that don't have any cool and damp spots for them to retreat to, or balconies or green roofs." Consequently, the results varied quite widely this year: a substantial number of observers found it harder than in previous years to find soil animals. "It's really important to have a healthy, balanced garden."

Green roofs scored a narrow pass (5.5./10), while Tiny Forests - mini-forests set up in urban environments by the Dutch Institute for Nature Education and Sustainability (IVN) - were added as a new type of garden this year, and immediately garnered the highest score overall: 9.2/10.

Individual gardens graded

These grades only indicate how suitable particular garden types potentially are for sustaining a wide range of soil life. Experimentally, the gardens of individual participants were also graded this year for the first time. In many cases, this resulted in scores that were significantly lower than the general one for a particular type of garden.

The individual scores were obviously affected by the drought. "But also by the fact that it's difficult to have and locate all the soil animals in one garden", says soil ecologist and earthworm expert Ron de Goede. "Still, it's a valuable test, which will help us make the grades for soil life in individual gardens more accurate and more widely applicable."

Unusual observations

In addition to the standard results, observers submitted a number of unusual and interesting findings this year. Here are some of the things they included:
  • "A blue and a white woodlouse" (the blue colour is caused by a virus, the white woodlouse was moulting).
  • "A crayfish underground", "a viviparous lizard" and "two shrews".
  • "Six newts and some really speedy critters" (we had several reports this years of newts that were trying to find cover, and the "speedy critters" were probably springtails, which jump away when your hand comes too close).
  • "We found most of the animals under flowerpots or stones. Even there, it was quite dry now."
  • "It was great fun. We'll do it again next year!"
Anniversary edition

Will the soil animals most affected by the drought and heat have fully recovered by next year? To find out, we need more multi-year data. And so the Soil Animal Days will return in 2019 for their fifth anniversary. Any organisations and interested parties in the Netherlands are welcome to join us and organise activities for this special anniversary edition!

If you're outside the Netherlands, you're warmly invited to contact us and see how you can help:

Meanwhile, to see where observations were submitted this year, and how they were distributed across the Netherlands, you can consult our online map: For a more detailed analysis, there is an interactive page (in Dutch) at

What are the Soil Animal Days?

During the Soil Animal Days, we put a spotlight on the crucial but often overlooked life in the soil. A key element is the effort by 'citizen scientists' to look for soil animals in Dutch cities and towns. Healthy soil can't exist without healthy soil life, and therefore neither can we!

The main organisers of the Soil Animal Days are the Netherlands Institute of Ecology (NIOO-KNAW) and the Centre for Soil Ecology (NIOO & Wageningen UR). Experts from NIOO, Wageningen University and Vrije Universiteit Amsterdam have composed a special soil animal chart - together with our media partner, Vroege Vogels.

The initiative is also supported by the Dutch Institute for Nature Education and Sustainability (IVN), NL Greenlabel, the Dutch Institute for Biology (NIBI) and the National Science Weekend.
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.

Netherlands Institute of Ecology (NIOO-KNAW)

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