A new study revealed that the invasive European earthworms are altering the physical and chemical properties of soil in North American forests, resulting in the decline of species diversity in the forest.

The study, published in the journal Global Change Biology, showed a general pattern between the decline of species diversity in the North American forests and the spread of European earthworms.

"The earthworm invasion has altered the biodiversity and possibly functioning of the forest ecosystems, because it affects the entire food web as well as water and nutrient cycles," explained Dylan Craven, a scientist at the German Centre for Integrative Biodiversity Research (iDiv) and at the Leipzig University and lead author of the study, in a press release.

For the study, the researchers gathered and analyzed data from 14 studies. The researchers discovered that since the reintroduction of earthworms in the North American forest, they make their way through the forests at approximately five meters annually.

Despite its known beneficial nature, earthworms are considered to be destructive in the North American forests. This is due to the fact that trees in the area were accustomed to a life without earthworms.

Earthworms mix earth and build extensive burrows, loosening the soil in the process. Some earthworm species can also alter the soil pH. Many native trees can't adapt to such conditions and eventually die.

As an example, the plant goblin fern has become rare in forests areas with abundant earthworms. Other plants threatened by the earthworm invasion include the largeflower bellwort, Japanese angelica tree, forest lily, Solomon's seal and tormentil.

Conversely, these earthworms prepare the soil for non-native exotic plants. Grass is also known to prosper in areas with high abundance of earthworms. The fine roots of grass make it possible for them to absorb nutrients from the soil quickly and tolerate summer drought.

Because different types of earthworms live in different layers of the soil, the researchers concluded that their impact would be cumulative and the more earthworms living in a certain area, the more plant species will vanish.

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