A large international group of scientists, also including researchers from the Natural Resources Institute Finland (Luke), studied for the first time on a global scale, the diversity and abundance of earthworms and factors that regulate the distribution of earthworms. The local diversity and abundance of earthworms is the highest in temperate zones, not in the tropics, unlike with many other organisms. On a global scale, climate conditions, variation in precipitation in particular, have the highest impact on distribution of earthworms. The study was published in Science on 25 October 2019.
Soil dwelling earthworms are familiar to all of us. Their activities have a significant impact on soil properties, other organisms living in soil and plant growth. Earthworms break down plant residues and mix them with soil, produce a high volume of nutrient-rich excrementsand dig long and deep burrows in the soil. Earthworms are key soil organisms in many environments. In agricultural soils they are regarded as indicators of good growth conditions.
The new study aimed to identify basic reasons for global variation in earthworm communities. Climate conditions, variation in precipitation, in particular, proved to be the most important factor affecting earthworm diversity and abundance. Other factors such as variation in soil properties and plant cover may also be important on a smaller scale. However, their significance on a global scale was not as distinctive as the impact of climate.
Local diversity highest in the temperate zone
Results of field studies conducted in different parts of the world were compiled into an unprecedentedly large dataset on the global variation in the local number of earthworm species, density and biomass. The dataset encompasses nearly 7,000 locations in 57 countries. Even though the significance of earthworms as a part of the ecosystem has been understood increasingly well, their macroecology has remained less well known. Macroecology means the large-scale study of distribution, abundance and diversity of organisms.
The diversity of plants, birds and many insects typically increases towards the Equator. With regard to earthworms, the situation seems to be different. According to the study, the local diversity is the highest in temperate regions, such as Europe, northeast USA and New Zealand. This also applies to the abundance of earthworms.
The first comprehensive global maps of variation in the number of earthworm species and the abundance of earthworms published in the study are a significant achievement. “Researchers have known for decades that for any given area in the tropics, we would usually expect more species than in the same sized area in temperate regions. But until now, we had been unable to quantitatively investigate the same global patterns for earthworms, as there was no global earthworm dataset,” says Helen Phillips, researcher at the German Centre for Integrative Biodiversity Research (iDiv) and Leipzig University and the main author of the publication.
An abundance of earthworms in Finnish fields
Visa Nuutinen and Sanna Kukkonen, participated in the study from Luke. “Data collected in Finland was well-represented in this study,” Kukkonen summarises. “Luke’s previous inventory of earthworm communities in Finnish arable environments produced a large dataset suitable for the needs of this study,” Nuutinen says.
The typical presence of a couple or a few earthworm species in a Finnish field, seems to be characteristic for arable environments elsewhere, too. , The average earthworm density in Finnish fields is slightly more than 100 individuals per square metre. “Compared with global field soil averages, this seems even high. The global dataset created during the study will be an excellent starting point for future studies, too. The published work does not utilize all the information collected and the data will be published later,” Nuutinen says. In addition to Finnish fields, the dataset includes observations made in forests by the University of Jyväskylä.
The study reminds us that soil organisms should be taken better into consideration when assessing biodiversity. “Based on these strong climate effects, we conclude that climate change could cause shifts in earthworm communities and change the functions and services ecosystems provide. It’s time for a paradigm shift in the conservation of biological diversity – because they are mostly dwelling in the soil, we easily forget about the amazing creatures under our feet,” says professor Nico Eisenhauer from iDiv and Leipzig University.