Gardeners consider earthworms as friends as they are known to help create humus, the fraction of soil organic matter that provides important nutrients for the plants. It is also commonly believed that these creatures are beneficial to the natural ecosystem.

Findings from a new study, however, suggest that these invertebrates, which are known to have been introduced by the Europeans, are not as harmless as they appear to be as too much of them could have adverse effects. An abundance of the annelids could have negative effect on the diversity of certain species of plants and may also have unwanted impact on the ecosystem.

The study, published in the journal Forest Ecology and Management, involved researchers visiting 40 parcels of land in five sugar maple forests, half of which were populated by earthworms.

The soil-burrowing earthworms appear to be beneficial to certain plant species such as grasses and ash trees. However, the researchers observed that new shoots of striped maple, red maple and American breech, and two species of fern became rarer as populations of the earthworm increase. 

According to Université Laval study researcher Line Lapointe, the most likely cause for this is the earthworms' consumption of the organic matter in forest litter. This alters the way the soil holds moisture and affects seed germination. It also hampers with the ability of some plantlets to survive.

Lapointe said that the threat posed by these segmented worms is not yet urgent but it still raises concerns since the creatures have already started to alter plant composition in sugar maple forests. If no actions are undertaken, these changes could become more severe and may eventually affect other forest communities.

"Most of our threatened and vulnerable plant species are in fact found in the forests of southern Québec," Lapointe said, adding that protecting these plants could be more difficult because of the presence of earthworms.

To prevent a population explosion of earthworms in the natural ecosystem, the researchers suggest that earthworms used for bait should not be thrown into the forest but instead be discarded into the lake.

The earthworms in farmers' fields, gardens, and lawns, as well as those used for bait, are all members of the species brought by European settlers. 

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