Earthworms have a deleterious impact on the insect fauna above ground in northern North America. Soil ecologists discovered this effect on insect population, biomass, and species richness.
Their findings suggest that changes in insect communities may have causes that have previously gone unnoticed. There have been essentially no earthworms in the northern section of North America since the last ice period, roughly 10,000 years ago.
They were, however, introduced throughout the previous few centuries, most likely through soil and plant transfer from Europe. They have diffused and altered the soil extensively since then, with far-reaching implications for the soil ecology.
The influence of these invaders on the world above ground has rarely been examined up to this point. The research was carried out in a forest in Calgary, Canada, that has regions where earthworms are either present or absent.
The researchers collected aboveground insects using insect vacuum samplers and compared the results. They discovered that the quantity, biomass, and species richness of insects differed dramatically between places with invasive earthworms and those without.
For regions like Europe, where natural communities have always co-developed with earthworms, comparable negative effects due to new earthworm species are very unlikely. Quite the opposite. Here they are important ecosystem engineers, which many important ecosystem functions depend on.
Dr. Malte Jochum
The number of insect individuals was reduced by 61%, insect biomass was reduced by 27%, and 18% where earthworm biomass was highest reduced species richness.
Insect above ground affected by invasive earthworms underground
“We had expected that earthworms would have an impact on aboveground insects,” says lead author Dr. Malte Jochum from iDiv and Leipzig University. “Even so, I was surprised at how pronounced the effects were, and that not only the abundance but also biomass and species richness were affected.”
However, the processes by which earthworms effect insects are still unknown.
“It’s possible that the earthworms eat the food and reduce the habitat of those aboveground insects, such as beetles and fly larvae, which break down dead plant material,” says Jochum.
Given that the majority of insects are herbivores, it’s possible that the observed insect drop is attributable to changes in vegetation produced by altered soil conditions. The researchers were unable to discover any substantial changes in the number of plant species or plant coverage in this example.
“Still, this doesn’t rule out the influence of the plants,” says Jochum.
The data on species composition and other functional aspects of plant communities, on the other hand, has yet to be assessed. There was also a noticeable rise in predatory bug species and spiders. These appear to be gaining ground as a result of the adjustments.
Underestimated causes for biodiversity loss to be considered in conservation
“Up to now, only a few causes have been used to explain global changes in insect populations; mostly alterations in habitats above the ground,” says senior author Prof Nico Eisenhauer from iDiv and Leipzig University.
“These new results show that biodiversity loss can also have other causes which have, so far, received little attention and that these should be taken into consideration when developing management and conservation strategies for biodiversity.”
Introduced earthworm species can be found on practically every continent, not only North America. However, because earthworms have been scarce in northern North America for a long period, the impact of these invaders is particularly severe.
“For regions like Europe, where natural communities have always co-developed with earthworms, comparable negative effects due to new earthworm species are very unlikely,” says Jochum. “Quite the opposite. Here they are important ecosystem engineers, which many important ecosystem functions depend on.”
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).
