Agriculture Reference
In-Depth Information
topography. This subject has not been thoroughly researched, and there is some evidence that
suggests that earthworms may actually reduce rates of soil erosion (Hopp 1946; Sharpley and Syers
1977) in agricultural land. Nooren et al. (1995) suggested that earthworms could increase rates of
clay loss from an African soil, thereby creating nutrient-poor sandy topsoils. Lavelle and Martin
(1992) hypothesized that, by protecting soil organic matter from oxidation, earthworms could have
an important influence on atmospheric carbon dioxide levels. Because rates of carbon loss from
soils make an important contribution to the elevation of atmospheric carbon dioxide levels, any
organism that is capable of contributing to reversing or moderating that trend should be investigated
closely (see
Chapter 10
this volume).
A third way in which earth history can play a significant role in field biology is in the
understanding of ecological interactions and of organism taxon distributions. For example, the
presence of a particular taxon and its unique ecological contributions to the biological community
of one site, but its absence from another, could be interpreted as caused by differences in history.
Perhaps the taxon could have existed in the latter site but did not get there. Thus, it may not be
some ecological or evolutionary necessity that determines certain characteristics of the system but
instead a historical accident. This is an important distinction because some simplistic interpretations
of community dynamics assume a long history of community optimization. It seems probable that
nature has not tried all possible combinations but has instead relied on the interactions and evolution
of what is present combined intermittently with unpredictable arrivals of new organisms to the site.
An analogy can be seen in communities of earthworms in which most or all the species are
introduced or exotic to a location. The organismal content of such communities is clearly accidental,
and any further interpretation of that content assumes that other potential invaders (i.e., those species
not currently present) have had an opportunity to invade, but only those ecologically compatible
with the community qualities have succeeded.
Of these three connections of earth history to modern biology, in terms of earthworms, I focus
on two: the first and the last. These may be stated briefly as using modern organisms to learn more
about the history of the Earth and viewing modern organismal distributions and ecology as the
outcomes of an interaction between evolutionary and large-scale abiotic processes. My purpose in
this chapter is to outline some of the ways that research into earthworm systematics and biogeog-
raphy can contribute significantly to the broader subject of earth history and vice versa. As I develop
these discussions, I suggest that researchers into earthworm systematics and biogeography can
profit from close attention to some of the recent developments in organizing their ideas and
analyzing their data.
PLATE TECTONICS AND EARTHWORM PHYLOGENY
The first subject is mutual enrichment between geology and systematics, from either geological
data informing on probable phylogeny or vice versa. An example of the first case is provided by
ongoing debates about the higher-level classification of the
. The earliest application of
earthworm systematics to earth history was in the connection with WegenerÔs theory that continents
move, which has since been transformed into plate tectonics theory. The vicariance events (splitting
of land masses) are quite ancient, and the resulting taxonomic divisions within earthworms are
generally at the family level, although some are within genera. Various family-level classifications
have been superimposed on paleoreconstructions of the past 250 million years of crustal movements
(Jamieson 1981; Bouch 1983; Omodeo 2000), but all those classification systems were contradicted
by the results of Jamieson et al. (2002), who were the first to apply deoxyribonucleic acid data to
Clitellata
studies of earthworm phylogeny (
Figure 3.1
). For instance, the Octochaetidae were shown to be
polyphyletic, the various concepts of the Acanthodrilidae were paraphyletic, and the Crassiclitellata
(earthworms as commonly understood minus the Moniligastridae) were clearly monophyletic.
