Agriculture Reference
In-Depth Information
APPLICATIONS OF EARTHWORM BIOGEOGRAPHY TO THE
EARTH'S HISTORY
There are good reasons to apply earthworm systematics and biogeography to the study of the
geological history of the Earth, with particular reference to movements of the EarthÔs crust. Such
an idea began from two pieces of biogeographical data. First, earthworms (except anthropochorous
peregrines) are absent from midoceanic volcanic islands (Gates 1969; Nakamura 1990; Talavera
1990) and uplifted carbonate platforms, indicating that earthworms may experience great difficulty
crossing saltwater (Stephenson 1930). Second, there are endemic earthworm species on some
oceanic islands. The possibility of over-water dispersal has been debated extensively in the past,
particularly in relation to the endemic species of the subantarctic islands (Michaelsen 1911; Stephen-
son 1930; Lee 1959).
There is an opportunity to test the hypothesis of no over-water dispersal of earthworms in a
general way and to use it, if supported, to argue that earthworms are biogeographical model
organisms. By the term
in this context, I mean an organism with dispersal that
has been so poor that its distribution can be viewed as determined entirely by past land connections
and vicariance events that introduced saltwater barriers between land areas.
If earthworm transoceanic dispersal has been negligible in history, then earthworm phylogenies
can be used to unlock many earth historical riddles, such as the geological evolution of complex
areas like the Caribbean Basin (Maury et al. 1990; Pindell and Barrett 1990) and the archipelagoes
of Southeast Asia (Hall 1996, 1998). This application of phylogeny to earth history follows from
the simplest form of allopatric speciation. The biota of the separated areas will have evolutionary
histories that mirror the fragmentation history of the land. However, postfragmentation dispersal
muddles the land area cladogram that can be derived from the hypothesized phylogenetic tree of
the organisms. Therefore, a low- or no-dispersal taxon is preferable to one able to cross the barrier
(Noonan 1988; Sober 1988). In spite of this rather obvious conclusion, most of the work to date
in the field of historical biogeography has focused on relatively vagile organisms, such as reptiles,
birds, and insects.
Biogeography can be considered under many formats, and it is useful to define which kinds of
biogeography are concerned. Ball (1975) described three phases of the science of biogeography.
The first is the empirical or descriptive phase, in which basic data are collected. At this point, it is
known where the various taxa are located, and there may be some synthesis, such as the definition
of the classical biogeographic provinces (e.g., Nearctic, Ethiopian). Then, an attempt is made to
explain the distributions, and a narrative phase is the result. A plausible story is constructed, one
that seems to fit the evidence fairly well. Work to date on the subject of oceanic island earthworm
distributions has achieved this much, leaving room for an analytical approach.
As mentioned earlier, it appears that earthworms have rarely or never crossed saltwater (other
than the saltwater-tolerant species inhabiting seashores). This could lead to the statement of an
hypothesis that earthworms cannot survive, or do not have natural means of, transport across bodies
of saltwater. This is testable, although it would be difficult to release test earthworms or cocoons
by all possible means of conveyance (rafting vegetation, logs, on debris in violent cyclonic storms,
and so on) and even harder to track their fates. It may be easier to test the predictions of the
hypothesis.
The hypothesis that earthworms cannot survive natural means of transport across bodies of
saltwater predicts that land areas arising from midoceanic uplift of submerged rock or from volcanic
eruption should not have earthworms. Obviously, this hypothesis arose from the observation that
such islands either lack earthworms or harbor only earthworms that have been distributed widely
by human activity. It can be tested by examining islands conforming to the modes of origin just
mentioned and that have earthworm fauna about which little or nothing is known. The Lesser
Antilles fit this description. These islands constitute a fairly strong test because they are close to
one another and to potential sources of colonization, unlike the Hawaiian archipelago and the
model organism
