European earthworms reduce insect populations

image: Earthworms are considered essential engineers of the subterranean ecosystem. Researchers from iDiv and the University of Leipzig have now shown that they also play a major role in the formation of aboveground communities.
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Credit: V. Gutekunst

At least since the last ice age, about 10,000 years ago, there have been almost no earthworms in the northern part of North America. However, in recent centuries they have been introduced, probably through the transport of soils and plants from Europe. Since then, they have dispersed and significantly altered the soil, with far-reaching consequences for the soil ecosystem. The impact of these invaders on the world above ground has, until now, rarely been studied.

The study was carried out in a forest near Calgary in Canada, which has areas inhabited or uninhabited by earthworms. Here, the researchers used vacuum insect samplers to capture the aerial insects and compared the catches. They found that the abundance, biomass and species richness of insects in areas with invasive earthworms and in areas without them differed significantly. Where earthworm biomass was highest, the number of insect individuals was reduced by 61%, insect biomass by 27% and species richness by 18%.

Above-ground insect affected by subterranean invasive earthworms

“We expected earthworms to have an impact on aerial insects,” says lead author Dr Malte Jochum of iDiv and the University of Leipzig. “Even so, I was surprised how pronounced the effects were and that not only abundance but also biomass and species richness were affected.”

The mechanisms by which earthworms affect insects are not yet clear, however. “It’s possible that earthworms eat the food and reduce the habitat of these aerial insects, such as beetles and fly larvae, which decompose dead plant matter,” says Jochum. Since the majority of insects are herbivores, one could also hypothesize that the observed decline in insects is due to changes in vegetation caused by altered soil conditions. In this case, however, the researchers could not detect any significant changes in the number of plant species or the vegetation cover. “Still, this does not exclude the influence of plants,” says Jochum. However, data on species composition and other functional characteristics of plant communities have yet to be assessed.

The increase in predatory insect species and spiders was also striking. These seem to benefit from the changes.

Underestimated causes of biodiversity loss to consider in conservation

“Until now, only a few causes have been used to explain global changes in insect populations; primarily alterations to above-ground habitats,” says lead author Professor Nico Eisenhauer of iDiv and the University of Leipzig. “These new results show that biodiversity loss can also have other causes that have, until now, received little attention and that these must be taken into consideration when developing management strategies and conservation of biodiversity.

Introduced earthworm species are not only found in North America, but on almost every continent. However, since there have been very few earthworms in northern North America for a very long time, the effect of these invaders is particularly pronounced. “For regions like Europe, where natural communities have always co-developed with earthworms, comparable negative effects from new earthworm species are very unlikely,” says Jochum. “Rather the opposite. Here they are important engineers of the ecosystem, on which many important functions of the ecosystem depend.

The study was conducted as part of the EcoWorm project and was funded by the European Research Council (Horizon 2020) and the DFG (FZT 118).

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