Giant Ginkgo

The other day I was walking down my street, and I watched the huge number of American Robins foraging for worms and other tasty morsels. The thought crossed my mind about the connection between the current large robin populations and the alien and extremely common European earth worm (or Night Crawler). Urbanization has helped increase the robin population, there are many more today than before the arrival of the European settlers. In fact, urbanization has lead to over-populations in some areas. Do earthworms benefit from the same sorts of european style urban landscaping that helps Robins? Is the monstrous robin population a direct result of the earthworms?

Robins eat some 200 different insect species, including beetles, millipedes and even cockroaches. They will even eat small snakes, and they also eat berries and fruits on a regular basis. (When worms and insects become scarce in the fall, they will switch to a berry only diet.) Worms are the staple to their diets, which had a devastating effect on their populations when DDT was common. Worms would absorb large amounts of DDT which robins would consume, and when they burned off their fat during migration the toxic poison would flood through their bloodstream, killing them.

Robins migrate along the 37 degree isotherm, traveling with frontal storm systems so that they can feed on the worms that have been forced to the surface by the rain or melting snow. (Worms avoid direct sunlight because they are paralyzed by ultraviolet light, but they will come out if they face drowning.) Earthworms meanwhile migrate vertically each year, moving eight feet below the surface to be below the frost layer during winter. They will sometimes gathering into large balls containing hundreds of worms to keep warm and moist through the winter. Some of the alien species have a ten to fifteen year lifespan. They reappear on the surface once the ground temperature reaches 36 degrees, just in time for the returning robins to eat them! When the worms return to the surface, they leave the dirt 'castings' from their tunnel at ground level. Charles Darwin measured as much as 36,000 pounds of castings per acre from them, back in 1881 - this information helped convince people that worms were a great thing to have around.

Native worm species in North America don't occur above the glacial line. The glaciers killed them off during the last Ice Age, and in the millennia since then they haven't returned even a hundred miles into the worm free zones. The European Earthworm is a prolific breeder and moved into the northern climates quickly due to potted plants, fishermen, ships ballast and many other ways. Did robins piggy-back onto this invasion, pushing farther north because of the huge increase in available food? I would guess that the largest shift would be in the breeding and wintering territories.

Worms are very efficient at removing the leaf and organic debris from the surface, called duff, and pulling it deep underground. This has had the effect of killing off the native plant species, insects, and small animals that live underneath the trees in northern forests in the United States. The topsoil loses the important nutrients formed by the slowly rotting leaves and other debris, the missing cover deprives fungus and microbes the fuel needed to keep the earth healthy.

Fishermen apparently have both been key in the spread of worms into forests that had been free of them. The worms invade forests when fishermen throw their remaining bate away on the land, quickly causing the destruction of the ground vegetation and understory layers of the forest nearby. The worms radiate out from the fishing grounds, removing the duff along the way.

In the Pacific Northwest, the worms take over the land of clear-cut forests, permanently altering the landscape. Currently very little is known about the long term effects of these changes, but the signs aren't terribly positive. I just think about the incredible undergrowth in the rain forests in the Olympic National Park, and whether the forests survive the destruction of these layers. The deforestation occurs all the way up to the edges of the parks, giving the worms an edge into the virgin forests. Could this be a part of the problem of re-establishing old growth 'style' forests, which apparently can take up to 500 years to regenerate?

So far I haven't had much luck finding the answers to my original questions; it seems likely that the connection hasn't been researched yet or I really need to spend a day in a good research library to find the answers I am looking for. But I have learned a lot more about worms than I expected. Hopefully this has been a marginally entertaining read for you...