Agriculture Reference
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find necessary, including comparison of the distributional ranges of phylogenetically related groups.
This step also includes analyzing the fauna of a geographically, ecologically, or historically uniform
area. The third and final step,
, tries to explain the reasons for the present distribution
as identified. It is from this step that the principles of biogeography emerge. At present, in North
America, with megadrile earthworms the situation is probably somewhere between the first two
stages (Reynolds, 1975a, 1976b, 1994a).
Studies of the
causal analysis
of animal distributions and of the grouping of these distributions
according to different principles are investigations of momentary phenomena. Faunistic and
regional zoogeography is basically static zoogeography because present distributions are the
result of processes that have moved the animals in space, resulting in change of the distributional
picture with the passage of time. Thus, causal zoogeography can be identical to dynamic
zoogeography because causations of distributions are dynamic processes. As long as the inquiry
probes the reasons for the arrival and colonization of a species in a certain area, the investigation
is within the field of dynamic zoogeography. For example, the lack of an animal species in a
certain area may be caused by ecological (it cannot exist there) or zoogeographical (it has not
arrived there) reasons.
The answers pertaining to dynamic zoogeography are often ecological, but the ecological cause
is followed by movement of individuals or populations and of the subject organism. When the
ecological cause is followed by a positive or negative response of the organism or population, and
when a spatial shift is not involved, the whole phenomenon falls within the sphere of ecology. This,
however, is usually assigned to the taxonomist because evolutionary changes are most easily
discerned by noting morphological changes. Lindroth (1962) expressed the relationship of these
two disciplines as follows: Zoogeography always depends on taxonomy to know
status quo
what
to study,
but taxonomy also depends on zoogeography because geographic speciation is the accepted norm
of the formation of its basic working unit.
Every animal species originated from a few ancestors in a limited area; if a particular species
is now widespread, it must of necessity have reached parts of its present range at an earlier period.
The first aspect of dynamic zoogeography pertains to
. If the details of dispersal processes
are known, much about the presence or absence of animals can be explained. Dispersal may result
as a by-product of other important phenomena, belonging to the biological habits of the animal,
or it can result from distinct, adaptive characteristics of the species that directly assist dissemination
into wider areas. Although every animal species has a capability to migrate, dispersing individuals
must find suitable areas in which to settle and reproduce through many generations. When the
process of settling or colonization is studied, the ecological factors that make the existence of a
species possible in a given area must be scrutinized, as well as the adaptations and limitations of
a species, such as structural, physiological, behavioral, or population dynamic properties that enable
it to initiate a new population and survive (successfully) in the newly colonized area. Factors of
dispersal as well as factors of existence in an area can influence the size, extent, and dynamism of
the distributional range of the animal.
dispersal
James (1995) (see
Chapter 3
this volume) uses a coincidently similar approach when he
elaborates on the concepts of Ball (1975). Ball described three phases of biogeographic method-
ology: (1) empirical or descriptive, (2) narrative, and (3) analytical. James concentrates his discus-
sion on two main points: (1) deoxyribonucleic acid (DNA) sequence information and (2) biogeo-
graphical research related to glaciation. I have very limited personal experience with DNA research,
but for more than 2 decades (Reynolds 1973a) I have advocated this approach but have never had
the opportunity for follow-up. It now appears that James, his colleagues, and their students have
the means to do so, and earthworm ecology and biology should benefit greatly from the results of
their work.
I have devoted considerable time and activity to the second topic over the years. My earthworm
research has concentrated on presenting wide-ranging formal surveys in North America and
