When I was a kid I was taught that earthworms were good. Lots of worms was a sign of a healthy soil.
As I got older, I discovered that isn’t entirely true–some midwest soils didn’t have earthworms until Europeans showed up. Some soils had a whole fauna of American worms that were displaced by the invasive, introduced earthworms. There are at least 45 different species of non-native earthworms in the US right now.
It leaves me rather conflicted about earthworms, as a gardener.
Earthworms cause basic changes in the structure, biology, and chemistry of soil. In gardens and (many) agricultural lands, worms are a good thing. They increase soil drainage and mix organic matter down into the soil.
But…hardwood forests in the Midwest are not used to having worms. The last glaciation killed the native worms off. Several different researchers have documented that as biomass of earthworms increases, the amount of forest understory growth decreases, and fewer trees had seedlings. That means the forest is less likely to regenerate itself as trees age and die. Not good.
“The earthworms that the team studies were brought to North America by early European colonists, probably in the ships’ ballasts or in plant soil….In some areas of the forest, more than 350 worms can be found in one square meter. “The impact of that many worms is huge for the forest ecosystem as from spring to fall they actively consume litter and mix it into the soil, leaving only a bare surface by year’s end.” Filley said.
In contrast, sites that have no earthworms have many years of accumulated litter and organic matter above the soil. This has implications for plant seed germination, water holding capacity and infiltration of the forest floor, among other things.
Decomposition of leaves and twigs by bacteria and fungi is normally the primary source of nutrients in the forest. Cycling of nutrients from leaves and other materials that fall to the forest floor is critical to maintaining the health of the forest.
This study concluded that part of the problem is that earth worms poop out lots of lignin–a very difficult compound for soil bacteria to break apart. Interestingly, the net effect of this could be to latch onto more carbon, (“carbon sequestration”), rather than to release it for use to plants or in the atmosphere. So while worms are not so good for the forest, they could potentially be good for keeping carbon out of the atmosphere.
Which to choose? In some places, there is no choice. The worms are here to stay.
If you live in the Midwest, please check out the Great Lakes Worm Watch before you order worms for composting, or let that fishing worm go!
Filley, T., McCormick, M., Crow, S., Szlavecz, K., Whigham, D., Johnston, C., & van den Heuvel, R. (2008). Comparison of the chemical alteration trajectory of leaf litter among forests with different earthworm abundance
Journal of Geophysical Research, 113 (G1) DOI: 10.1029/2007JG000542
Hale, C., Frelich, L., & Reich, P. (2006). CHANGES IN HARDWOOD FOREST UNDERSTORY PLANT COMMUNITIES IN RESPONSE TO EUROPEAN EARTHWORM INVASIONS Ecology, 87 (7), 1637-1649 DOI: 10.1890/0012-9658(2006)87[1637:CIHFUP]2.0.CO;2