The study looked at 60 plots in an aspen and poplar forest in Alberta, Canada, and found that as the numbers of earthworms wriggling in the soil and leaf litter increased, the diversity and abundance of invertebrates aboveground decreased. These results might sound surprising, since earthworms are widely considered to be helpful garden residents. Worms earned their reputation by aerating and mixing soil with their burrows and releasing locked up nutrients in their castings, all of which can help certain plants thrive.
But this new study is part of a growing body of research suggesting that at least in the forests of northern North America, earthworms may not be the slimy angels of the underworld we tend to think they are. “When people talk about insect decline, they rarely talk about the soil,” says Nico Eisenhauer, a soil ecologist at Leipzig University in Germany and one of the authors of the new study. “Many of the insects and invertebrates that are in decline have life phases in the soil. What you don’t see flying around now has first disappeared from the soil, and earthworms can fundamentally alter soil conditions.”
Earthworms’ subterranean engineering isn’t a problem in their native ecosystems, but in the northern half of North America, the glaciers of the last ice age wiped out virtually all soil-dwelling worms more than 10,000 years ago. The ice sheets covered nearly all of Canada, most of the northeast U.S., and much of the upper Midwest. When the ice receded, forests returned but the worms did not because they can only expand their range by a maximum of about 30 feet a year. These northerly ecosystems evolved for millennia in the absence of earthworms. Without worms munching through fallen foliage and churning the soil, these forests accumulated thick layers of leaf litter, which came to support a vast array of animals, fungi, and plants. Eisenhauer says even non-scientists can appreciate the difference.
