Recently I ran across a science piece about the impact of earthworms in North American forests. I have always thought earthworms were good — they aerate and enrich the soil. But according to a new study published September 3, 2016 in the journal Global Change Biology, it’s not that simple.
In the last ice age, the northern United States and Canada were covered by an enormous ice sheet. Glaciers altered the landscape, destroying almost all the indigenous earthworms in the process. So when the glaciers finally retreated around 12,000 years ago and a forest ecosystem was reestablished, the earthworm component was missing. Settlers from Europe eventually reintroduced earthworms into the environment, but these were from a non-native species and have been creating conditions that decrease the diversity of native plants.
How? First, in the top layer of soil, earthworms convert leaves to humus. This is really good for gardens, but in a forest, this results in a fast-tracking of the release of essential nutrients instead of the leaves breaking down more slowly. Second, burrowing earthworms can disrupt the symbiotic relationship between plants and fungi. Some deep-burrowing worms can change the pH of upper layers of soil by bringing up alkaline soil from deeper in the ground. Third, burrows carved by earthworms speed up rainwater drainage, causing the soil to dry out faster. Fourth, earthworms can consume the seeds and seedlings of some plant species, affecting what grows in the forest. All of these changes negatively impact native plants that did not evolve in such conditions, thus the decrease in diversity.
Once again, nature is much more complex than it first appears, and environmental change can come from the most unlikely sources.
The original article is “Earthworm Invaders Alter Northern Forests” by Shireen Gonzaga, dated Sept 13, 2016. It can be found here — http://earthsky.org/earth/european-earthworms-change-u-s-forests.