Earthworm Invasion

Northern Maple forest without earthworms.

As odd as it might sound, earthworms are not native to the northern United States and Canada.  Why? Well, as glaciers receded from the northern portion of North America 11,000 years ago, they left behind a bitterly cold and extremely muddy waste land.  These glaciers reached from the north to their southern the extent of present day norther Iowa, Illinois, and Indiana.  Along this southern extent of the glacial line and northward there have been no earthworms until recently.  Worms simply couldn't survive the frigidly cold temperatures and frozen tundra when glaciers were present in these areas.  Further south however, where glaciers never reached, earthworms have been around for a long time. 

Within recent decades however, earthworms began showing-up in these formerly glaciated soils of the north.  This might not seem that weird until you realize that earth worms travel an average of 5 or 6 yards a year.  Over 11,000 years that equals only about 40 miles, which is a ridiculously slow rate that wouldn't have even allowed them to travel across an entire state.  Even if you double or triple that distance it doesn't even come close to the distance the worms would need to travel to show up in these northern forests.  So how did they move thousands of miles in just a few decades?  The only explanation is that humans carried them.  Fishermen and gardeners are especially notorious for carrying earthworms long distances.  As a result, worms were accidentally introduced to new locations hundreds of miles away from the nearest native worms. 

Northern Maple forest with earthworms.

Most people might think this is a good thing.  Worms are very good for garden soil after all.  The reality is though, worms are not very good for northern forest soils.  Worms are extremely efficient at what they do, which is break down organic materials such as dead leaves.  They do this extremely rapidly, moving nutrients from dead organic materials into the soil quickly.  As a result, plants cannot absorb the nutrients as fast as need and much is lost when water washes it out of the soil.  The burrowing action of worms also functions to compact forest soils, making it more difficult for plants to survive.  While some plants are well adapted to earthworms crawling around through their roots, other plants are extremely sensitive.  Sugar Maples, one of the dominant plants in these northern forests, is extremely sensitive to earthworms. Establishment of maple seedlings where earthworms are present becomes very difficult.  Northern forests with earthworms have far fewer plants than forests without earthworms.  Simply by changing soil and forest floor structure, the earthworm has a huge effect on the overall habitat. 

Fortunately, earthworms have not taken over every single forest in these northern areas.  Also fortunate is the fact that worms only travel about 6 yards a year.  This means, if people quit transporting worms to new areas in the north, populations of worms aren't going to expand much. 

Great Lakes Worm Watch