Earthworm invasion in Canadian forest linked to insect decline, study suggests
CBC
In many parts of northern North America, there have been no earthworms since before the last ice age. Now, with help from humans, invasive earthworms are colonizing new areas. And in areas of a forest with more invasive earthworms, there are fewer insects, a new study has found.
The study authors say that along with climate change, land use change and pesticides, earthworm invasions might be an "underappreciated driver" in a widespread decline of insects that some scientists have raised the alarm about.
Invasive earthworms have already been linked to changes in soil organisms, plant communities and forests' ability to store carbon.
So Malte Jochum, who led the new study in an aspen forest west of Calgary, expected worms might have some impact on insects. Still, he was "awestruck" by the size of the impact, said Jochum, a biologist and ecologist at the German Centre for Integrative Biodiversity Research and the University of Leipzig.
In areas with the highest mass of earthworms, there were on average 61 per cent fewer individual insects, 18 per cent fewer species and a 27 per cent reduction in the total mass of insects, the researchers reported this week in the journal Biology Letters.
While competition for food and habitat clearly played a role for some organisms, the ripple effects could be seen through the food web, even among organisms with no known direct predator-prey or competitive relationship to earthworms.
"It's fascinating how below-ground invasions can have such an impact on above-ground animals," Jochum said.
Earthworm invasions are widespread on the continent, and increasing in the north as permafrost thaws.
"The scale of it is huge and will probably expand with climate change," Jochum said. He noted that there's no reason to think other ecosystems being invaded by earthworms would respond differently. And he thinks it's something people should be mindful of as they garden or fish, potentially transporting worms to new places.
"It seems that people living in North America don't know that earthworms don't belong there … and I think it's always important to remind people of that."
During the last ice age, most of North America was covered by a massive ice sheet. When the glaciers retreated, organisms such as insects and plants started to recolonize, explains Ed Johnson, a University of Calgary professor emeritus who co-authored the study.
"The system has reorganized itself without [earthworms]," he said. "And when they show up, the system gets reorganized again."
Researchers first noticed earthworms invading the aspen forest surrounding the University of Calgary field station in the 1980s, Johnson said. He suspects they've been in the area — the Kananaskis Valley in the front ranges of the Rocky Mountains alongside Barrier Lake — since the 1960s. The worms, many of them invasive species not native to North America, first show up near cabins, fishing spots or places horses have been — a sign they've been brought by humans.
Over decades, generations of researchers have watched and tracked the invasion. "Sometimes you can literally see it" in the changes to the leaf litter, Johnson said.
