阅读下列短文，从每题所给的四个选项（A、B、C、D）中，选出最佳选项。
As with every other invasive（入侵的）species, introducing worms to an ecosystem（生态系统）that developed without them has an effect on everything that lives there.
This concern is very clear in the Great Lakes region. Non — native earthworms（蚯蚓）have been in North America for a couple hundred years, brought over from Europe with soils and plants. More recently, Asian species, sold as fishing bait（饵料）, have started to spread in the Great Lakes region, where there are no native earthworms.
In these earthworm-free forests, when leaves fall onto the forest floor and plants and animals die, they build up into what’s called the duff（腐殖质）layer. In this area, it was just bacteria that broke down the organic material for plants to consume and take up. Earthworms come in, they do their job and do it extremely well — they take that forest floor and turn it into the rich black soil you find in your garden.
