Agriculture Reference
In-Depth Information
permafrost in periglacial areas, and the earthworms have been very slow to diffuse northward again.
I hesitate to use the word ÑdisperseÒ because there appear to be few events in the normal course
of the earthworm life cycle that promote dispersal, other than coming out to crawl over the soil
surface after heavy rains. Using estimates of rates of spread obtained from the Netherlands (Mari-
nissen and van den Bosch 1992), 10,000 years is enough time for some peregrine earthworms to
advance 60 to 100 km. Thus, range expansions of 100 to 200 km would be expected because of
the maximal extent of the icecap in the Northern Hemisphere. Bouch (1983) reported on the
earthworm recolonization of glaciated France along the Rhne River, with some ecological inter-
pretations of species interactions.
Range expansion rates may differ among earthworm species for various reasons, including modes
of reproduction and ecological niche. For example, epigeic species may have a higher natural rate of
diffusion than do oligohumic endogeic species. If this is true, this would result in something analogous
to a chromatographic fractionation of the earthworm fauna. Should such a pattern emerge, earthworm
ecological functions would be represented differently in the natural vegetation of locations along
north-south transects crossing the limits of native earthworms. This should have some observable
impact on ecosystem processes. The extreme case would be in the northern regions, where no
earthworms are present and forest litter layers are quite deep. The contribution that I have tried to
make to the question of how glaciation (or any other historical factor important to a species distribution)
affects earthworm distributions has been to remove the question from the narrative domain and to
formulate and test hypotheses. Those hypotheses tested were either the predictions of the central
hypothesis (in the present case, that glaciation made impossible the occupation of ice-covered lands)
or the alternatives to the central hypothesis. Although negative data are never as satisfying as positive
data and can be overturned by a single positive datum, broad-scale events are often amenable to
investigation only by a process of elimination of alternatives.
Much earthworm biogeography (e.g., Reynolds 1995) (see
Chapter 4
this volume) has been
directed to the distributions of peregrine species. Completely different historical agents, such as
patterns of human migration and horticultural and agricultural trade routes, would seem to be
involved. Totally different ecological concerns emerge: Are absences of earthworm species because
of historical factors alone or because of incompatibilities between sites and species? Are the present
distributions of the peregrine earthworm species determined by their transport history or by eco-
logical factors? To what extent is habitat disturbance involved in successful establishment of
peregrines? Does it make sense to talk of species associations when the species found together
may have nothing more in common than a collection of travelers in a train station? Are there forms
of data, such as molecular data, that will allow the origins of populations of peregrine species to
be traced to their areas of origin? The task of bringing rigorous scientific methodology to this topic
will require much insight and may yield further insights into the connections between ecology and
human history.
Earthworm systematics has been making primary contributions to the work of people whose
interests lie in ecology, agriculture, and other fields in need of a coherent classification of earthworm
species. This is caused in part by the service functions of the field and a shortage of earthworm
taxonomic specialists. However, using the new types of data and analytical methods discussed here,
it should soon be possible to move toward providing the robust phylogenies required for good
biogeography. The potential for contributions to the understanding of the EarthÔs history and its
interactions with the history of the biota is great. There may be few other terrestrial taxa so widely
distributed, so ancient, and so amenable to collection and study as earthworms. When the high
degree of endemicity of earthworm species also is considered, it is surprising that scientists working
on biogeographical questions have not already taken more advantage of the marvelous research
opportunities provided in earthworm systematics and biogeography.
