Agriculture Reference
In-Depth Information
It is certain that humans have been aware of earthworms for a very long time, yet only in the
past several millennia have people had the motivation or wherewithal to alter global distributions
of earthworms in any ecologically significant way. This required the birth of agriculture and thus
an incentive to transport live plant materials, perhaps with or in soil, from one place to another. It
may be speculated that the Polynesian peoples, or other seafaring groups with some reliance on
agriculture, were among the first to cause the long-distance transport of earthworms, although there
is no reason to suspect them of doing so intentionally. As transportation technology advanced and
trade routes, road networks, and convenient means of moving heavy loads over long distances
became readily available in Europe and Asia, earthworms were likely to move as well.
To be sure or suspect that a species of earthworm or any other organism is outside its natural
habitat range, there has to be a concept of natural range and some knowledge of the diversity and
phylogenetic relationships of the organisms concerned. Otherwise, there is no basis for inferring
that an occurrence at a particular site is unnatural; after all, until the late 19th century
., any
such situation could be explained as the will of the Creator. Even well into the post-Darwin age,
describers of new earthworm species found outside what is now believed to be the natural ranges
of the species often used the foreign place names as specific epithets. There are numerous cases
of earthworm species that were either first discovered out of their indigenous ranges or were given
synonyms based on specimens collected outside their natural range (Reynolds and Cook 1976).
For example,
.
A
D
Kinberg 1867 is an East Asian species that is found commonly
well out of its normal range and in Hawaii was given the junior synonym
Amynthas gracilis
Rosa 1891.
On the other hand, certain more familiar species of European Lumbricidae (virtually all earthworm
taxonomists were European until the 20th century) kept turning up in odd places, such as Australia
(Blakemore 1999) and New Zealand (Lee 1959), such that by 1900, it was generally believed that
certain earthworm species had been widely distributed by human activity (e.g., Michaelsen 1900);
these are collectively referred to as
A. hawayanus
species. Once some basic outlines of earthworm taxa,
including families and their generic distributions, were worked out (1895 to 1930), the extent of
artificial earthworm distributions could be much better appreciated.
Because certain earthworm species turn up in unexpected places, early observations often
considered that native indigenous earthworms of many areas were declining in numbers and species,
and that exotic earthworms were increasing in abundance. Eisen (1900) observed the phenomenon
in California, and in the central United States, Smith (1928) noted a transition from an abundance
of the native
peregrine
L.
over the period 1900 to 1925. Significantly, this occurred in urban lawns and gardens, so the
Diplocardia communis
Garmann in 1888 to domination by
Lumbricus terrestris
Diplocardia
had survived a habitat conversion. Much later, Ljungstrom (1972) noted replacement
of the South African native earthworm fauna by exotic species.
The mechanisms of replacement of native earthworm species by exotic species are complex,
and there has been little experimentation to identify the key processes involved. These mechanisms
could include the intolerance of the native earthworm to altered habitats, a loss of key biotic
relationships present in intact ecosystems, competition pressures from exotic species, and an
inability to reestablish populations after partial recovery of ecosystems (Stebbings 1962; Kalisz
and Wood 1995).
Earthworms are still traveling, although recent increased stringency of border controls may
have reduced international traffic to some extent. Gates (1982) received thousands of specimens
that had been intercepted at the U.S. borders over several decades in the 1900s. The result of the
many years of earthworm transport is global homogenization of earthworm communities, in agri-
cultural and urban lands, modified by climate conditions. The U.K. is now populated by the same
species of Lumbricidae that were present in the rest of glaciated Europe. The same set of species,
for the most part, occupy soils in the temperate zone of North America and the cooler regions of
Australia, New Zealand, North Africa, South America, and South Africa and are part of a mixed
earthworm fauna in temperate East Asia.
spp. have been seen in irrigated highway
rest areas in the Humboldt Sink region of Nevada, a cold desert with salt alkaline soils, where they
Aporrectodea
