Every year around Wold Animal Day, we go on safari in our own gardens looking for the important but often overlooked soil animals. 2021 will mark the 7th edition of the Soil Animals Days. This year, the annual event organised by NIOO and a growing number of partners will take place between 24 September and 6 October.

First held in 2015, the Soil Animal Days are an annual citizen science project organised by the Netherlands Institute of Ecology (NIOO-KNAW) together with the Centre of Soil Ecology (CSE) and a growing number of partners. Soil animal experts from NIOO, Wageningen UR and Amsterdam's VU University are responsible for drafting the special soil animal chart that participants can use for their observations. 

Every year in or around October, people all over the Netherlands go out into their gardens, and entire classes go exploring their schoolyard. So if you're in the Netherlands and have access to a garden, park of schoolyard, you can join our citizen science project by downloading the chart from this website and looking for soil animals.

The project's results help scientists establish how well soil animals are doing in Dutch cities and towns. Without a healthy number of soil animals, there can be no healthy soil - and that would have grave consequences for food production and biodiversity, among many other things.

Earthworms vs woodlice

Last year turned out to be a year of extremes, especially weatherwise: first dry for a long time, then very wet and on average always warm. This may have been to the earthworm's advantage and the woodlouse's detriment, suggests woodlice expert and Soil Animal Days stalwart Matty Berg from Amsterdam's VU University. "The weather has indeed played tricks on the woodlice this year. They cannot tolerate drought very well."

Earthworms, therefore, were the overall winners after a number of years in which the woodlouse was the odds-on vafourite. Meanwhile, the number of human participants last year was significantly higher than usual: 1188. "These were truly Soil Animal Days XL", says organiser Gerard Korthals from the Centre of Soil Ecology, which was set up by NIOO together with Wageningen UR. In previous years, many people went looking for soil animals in groups. This year, a higher percentage went solo - which may well have been a result of the pandemic.

Between them, the participants counted an unprecedented 17,500 soil animals: "another record" according to Korthals' colleague, Ron de Goede from Wageningen UR. After six editions of the Soil Animal Days, this now brings the total number of soil animals found and counted to nearly 53,000.

On a less positive note, the average number of soil animals per garden was only 36: down from 43 in 2019. With the exception of the ant, all the main groups of soil animals were found in a smaller percentage of gardens last year than during previous editions. Millipedes were affected in particular: in 2019 they were found in 54% of gardens, in 2020 in only 41%. Spiders also declined significantly.

Overall conclusions

So are there any general conclusions to be drawn after six years of Soil Animal Days in the Netherlands? Gerard Korthals and his fellow soil researcher Ron de Goede answer in the affirmative. "Green and half-green garden, and parks and public gardens, are eldorados for earthworms, snails, spiders and woodlice in particular." These groups of soil animals are found in more than 80% of the gardens belonging to one of those types.

Another conclusion is that the weather is indeed an important factor when it comes to the survival and level of activity of soil animals. In 2017, arachnids came out on top after a wet season while in 2018 - a dry year - there were lots of woodlice and few slugs. "We can conclude that in dry years, in particular, the type of garden and the way in which it is maintained are key factors for the survival of soil animals in the city."

So how about the future? Will 2021 be another unusual year for soil animals? And will woodlice be able to buck their downward trend? Find out with us between 24 September and 6 October, when the Soil Animal Days return for the seventh edition!