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The last ice age wiped out earthworms in the northern US states and Canada. Over thousands of years, without these worms, a rich, deep soil full of organic matter was able to build up.
Gardeners know that earthworms devour organic matter and this does make them useful in the garden when you want quick decomposition.
Unfortunately, earthworms have escaped to forests and natural areas where their ravenous apatites have actually depleted the deep humus of the forest floor at an alarming rate. Be careful not to move wormy soil to natural areas in the northern US and Canada.
This confuses me a little CompostR. Earthworms are indeed ravenous, but what comes out the other end - earthworm castings - is one of the richest, most nutritious organic substances you will ever come across, which surely can't be bad for the soil...? (Are you referring to imported, non-native earthworms, maybe?)
Earthworms are already "north" due in part to fishermen transporting and discarding them. In fact, the nightcrawlers (it's commonly spelled as one word) which are sold as bait are likely labeled "Canadian" nightcrawlers.
While Charles C. Mann is a good journalist, author and science writer, he is too Jamestown-centric (or too unfair to John Rolfe) in the National Geographic article. After all, many immigrants brought plants directly to their new homes all over America and millions of plants have been commercially imported over the years.
A Google search found numerous articles on the topic of non-native worm species . A few that were found:
Opening up a can of (exotic) worms - Wisconsin Council on Invasive Species - http://invasivespecies.wi.gov/awareness/docview.asp?docid=30... (a PDF file) from which I quote:
"There are up to 14 non-native worm species in the northern half of North America, eight of which have been documented in the field study. During the last glaciation, ice sheets covered Canada, Minnesota, Wisconsin, Michigan, North and South Dakota and parts of Iowa. In all of those areas the native earthworms were wiped out. South of that ice sheet there are native North American earthworms, but they’re very different from the worms we Northerners bait our hooks with. It’s just been in the last two to four decades that the European earthworms have begun to find their way into remote areas that were previously worm-free."
TexasTam - Where you are I think there are some native earthworms, but they are smallish. You're right, I think the larger ones are "imports."
I'm in Pennsylvania, so I we don't have ANY native earthworms, ONLY the "imports" (either from the southern US or elsewhere on the globe).
Yes, while worm castings are good fertilizer, the problem is worms can digest organic matter in a few weeks that normally decompose in YEARS. This is OK in the compost pile, but on the forest floor, it can mean the depletion of humus. Without humus that acts like a sponge, fertility and soil moisture can wash away.
Anyway, that's my understanding. I sure was flabbergasted when I learned for the first time that earthworms can be harmful!
