As a keen gardener, I am always pleased to see lots of big earthworms in my garden.

Growing up in a family of gardeners in England, I learned how important they are for the health of the soil and its productivity. Earthworms aerate the soil allowing for oxygen uptake, carbon dioxide release, and better drainage. They recycle nutrients by consuming and breaking down dead organic materials into simpler nutrients that are available to plant roots. Their activities contribute to a more stable soil structure and an increase in crop productivity.

So, imagine my surprise and chagrin to discover years ago that, in North America, they are an introduced, invasive species. In Canada, it is generally believed that most native earthworms did not survive glaciation. Invasive earthworms make up 77 per cent of all Canadian worm species and their impacts on the soils of natural ecosystems are only just coming to light.

A study to be published shortly has found that invasive earthworms can cause a decline in the numbers, biomass, and diversity of native worms, insects, and spiders in eastern Canadian forest soils. Also, they can cause declines in some plants and have even killed trees of the sugar maple.

Research in western Canada shows that earthworms alter the forest floor, especially the duff (humus) layer. Here they change the distribution of carbon, nitrogen, and other chemicals as well as roots. The distribution of microbes, other soil creatures and understory vegetation also change.

Western and northern coniferous forest soils were thought to be too acidic for introduced earthworms to survive. However, the spread of worms is increasing in northern forests. As the climate warms, soils may dry out and become less acidic. So, the spread of invasive worms may increase even more with a potential dramatic outcome.

It is estimated that introduced worms are now present in 97 per cent of the areas in North America where soils have been examined, whereas native worms are only present in just 72 per cent of examined soils.

The problem of invasive worms presents a dilemma for us. On the one hand, they greatly increase crop productivity and food abundance. In addition, collecting earthworms for fishing bait (sold as dew worms, night crawlers, or angleworms) is quite a big business in Canada.

On the other hand, we are facing a potential major disruption of forested ecosystems and their ability to grow the trees on which our economy largely depends.