After the latest ice ages, earthworms were, by and large, absent from North America. Only in some sheltered regions that did not experience deep permafrost did the native species survive. It is a little known and seldom acknowledged and considered fact, however, that these earthworms didnt spread far after the glaciations. Thus, the prevailing earthworms found practically everywhere in North America and generally seen as natural, desirable, and self-evident components of the terrestrial fauna were more or less unintentionally brought in by European immigrants to become very successful invaders.
The Earthworms in Their Native Habitats
Earthworms are tube-shaped, segmented organisms that live in soil and feed on organic matter. They are invertebrates and thus lack a skeleton, but possess liquid-filled chambers which act as a hydraulic skeleton that can provide stability to whichever part of the body needs it at a given moment. They are very flexible, with longitudinal as well as transversal muscles. The digestive channel goes through the full length of the animal, and they are hermaphrodites, so each individual has both female and male sex organs. When mating, they reciprocally fertilize each other. Their coloration is mostly in the range of red-violet to gray-blue. There are globally some 2000 species of earthworms. In Europe, one of the most important groups is Lumbricidae, to which the common earthworm, Lumbricus terrestris , belongs. It can reach a length of 30 cm, which isnt much compared to the largest species, the 7 m long South African gigantic earthworm.
The workings of earthworms in the terrestrial ecosystem are well known. They live in the boundary layer between plant material such as decomposing leaves and grass on the surface and the mineral soil beneath. They live off of organic matter and act to speed up its decomposition by mixing more persistent parts of it such as humus with the inorganic soil components deeper down. In the process, the tunnels they dig facilitate aeration and drainage of the soil, which in turn further enhance microbial activity and release of nutrients.
Earthworms are generally well regarded in Europe and Asia, among farmers as well as scientists. Charles Darwin [], who published a book about them in 1881, put great emphasis on their importance. It may be doubted, he wrote, if there are any other animals which have played such an important part in the history of the world as these lowly organized creatures.
One hundred and thirty years later, BBC Nature sang their praise: Earthworms are the worlds unsung heroes. They loosen and mix up the soil, break down and recycle decaying plant matter and fertilize the soil by bringing nutrients closer to the surface. Birds often rely upon worms as a primary source of food.
The common earthworm Lumbricus terrestris
As education for children, Skansen, an open-air museum and zoo in Stockholm, Sweden, writes on its home page: Earthworms are small but very good farmers. They mix the soil and drag dead leaves down into the ground. Their tunnels go deep down and through them air and water can reach the roots of plants. They benefit other animals and plants.
Introduction and Spread
Although hardly any records exist, it is obvious that earthworms of many different species were introduced to North America on many different occasions and to many different places. It is also very easy to imagine how it happened. Settlers from Europe often brought all sorts of plants with them from their home countries and with them soil around the roots. In the soil, there would have been earthworms.
However, one person is first and foremost seen as the one who happened to introduce earthworms to the New World. That person was John Rolfe. His place in the history of European settlement of America is secure also for several other reasons, even if it could be argued that his introduction of the earthworm was the action with the most far-reaching and long-term consequences.
John Rolfe was a settler in Jamestown, Virginia, the first successful English colony in North America, and he was the person that more than any other made it so. He married Pocahontas, the daughter of the local Indian chief, Powhatan, thereby securing peace and food assistance that were vital for the survival of the settlers. He also introduced and started the cultivation of the tobacco plant Nicotiana tabacum . When he returned to England in 1616, he brought with him not only his Indian wife but also a large cargo of tobacco from his farm at Jamestown. Both were a huge success in London. John Rolfe and Jamestown became rich from the regular shipments of tobacco to England, and the small colony became a boomtown with a rapidly growing population. The ships that came to fetch the brown gold carried ballast of stone, gravel, and dirt, which was dumped onto the beach to make room for the bales of dried tobacco leaves. With the ballast came the earthwormsor so the story goes.
It is well documented that the settlers in Jamestown during the first decade found it hard to grow any of the plants they brought with them. Historians have attempted to explain this and mostly looked for socioeconomic explanations, one of them being that few of the settlers had been farmers back in England. That might well have been a factor, as was in all likelihood the severe drought of the summer of 1609, but these circumstances may not be the whole explanation. Whatever the reasons, the consequences were dramatic. During the winter of starvation 16091610, 440 out of the original 600 settlers died. Survivors were abandoning the colony, when a ship carrying new immigrants, among them John Rolfe, arrived with provisions and seeds. But also in subsequent years, harvests were minimal, not only of grain but also of cabbage, turnips, and other vegetables. The only crop the settlers planted that grew well from the start was tobacco. They could trade and get maize from the Indians, but for the settlers, it seems they didnt really attempt to grow it for themselves. Eventually, little by little, their vegetable gardens finally started to produce.
Lets return to the reasons for the settlers failure to produce adequate food . If the low crop yield in 1609 was really due to drought, why did they continue to be unsuccessful in the following years that enjoyed normal precipitation? If it was lack of farming skills, why did they immediately succeed with tobacco? One possible explanation is that tobacco and the maize that the Indians grew are American plants, the domestication of which had taken place in the absence of earthworms. The crops the Jamestowns settlers planted that failed to thrive were introduced from Europe and Asia, where earthworms have long had a decisive influence over the structure and quality of soils.
Back in Jamestown, earthworms slowly spread and started to give the soils in and around the colony a more European-like structure, which facilitated Eurasian plants to establish themselves there. This was, of course, the case not only for agricultural plants.
In a pattern resembling that of the highly suspect ballast in Jamestown, other, later, settlers came with plants and soil containing earthworms from their home countries. These became established and spread, in the slow way of worms, from their respective places of release. As a rule of thumb, earthworms on their own can extend their territory with a radius of maybe 10 m a year; so for their further spread inside North America, humans also played a key role. Besides gardeners, many of whom wanted earthworms to improve their soil and actively sought them out much of the spread was unintentional via transported plants and soil. Fishermen also played an important role, as can be concluded from the prevalence of earthworms around angling lakes and rivers. Leftover worms were simply released at the fishing sites. This resulted in that earthworms are now present in most North American habitats suitable for them. The different species, however, have a very uneven distribution, which has more to do with their haphazard pattern of introduction than their individual competiveness vis--vis each other in a given environment. One can conclude that as a group, earthworms have more or less conquered the continent, but the individual species have not yet reached their final distribution and, often enough, not yet met each other as competitors.
