Geography Reference
In-Depth Information
unique geobiotic phenomenon. Tracks and area cladograms are appropriate
instruments to develop such a common language.
Biogeographers have classified geographic features in terms of their im-
pact on dispersal and vicariance (Cox and Moore 1998; MacDonald 2003;
Rapoport 1975; Simpson 1953, 1965; Vargas 1992b). The most important
are barriers (geographic features that hinder dispersal) and corridors (geo-
graphic features that facilitate dispersal). Barriers are easily identified with
geographic elements such as mountains, rivers, and seas. In the marine en-
vironment, in addition to land barriers (e.g., the Isthmus of Panama), there
can be more subtle barriers, represented by changes in physicochemical
properties (Cecca 2002). Corridors include such a variety of habitats that
many of the organisms found at either end of them have little difficulty tra-
versing them (Cox and Moore 1998). These terms are relative because, for
example, a cordillera may act as a barrier for certain species but be a cor-
ridor for others. Instead of barriers, cladistic biogeographers usually refer to
vicariance events.
In some instances, physical or biological conditions make it easier or
more difficult for certain species to cross a certain barrier. Features that are
not equally favorable for dispersal of all species are called filters. For ex-
ample, before the rise of the Isthmus of Panama, a chain of small islands
(stepping stones) on a shallow sea of about 150 m in depth occupied its
place. These islands facilitated dispersal of some species (e.g., mice) but
acted as a barrier for other species (e.g., flightless birds). After the Isth-
mus of Panama developed, during the Pleistocene, most of Central Amer-
ica was occupied by dense tropical forests, which allowed the dispersal of
forest species but acted as a barrier for biota from the savannas (MacDon-
ald 2003).
There aresomeareascompletely surrounded bytotally different environ-
ments, such as islands, caves, and high mountain peaks, where chances of
dispersal are very low for most taxa. They are known as sweepstake routes,
and they differ from filters in kind, not merely in degree, because almost all
species that traverse them cannot survive (Cox and Moore 1998).
In dealing with long-term changes (
table 8.1
) in the biotic distributional pat-
terns,continentaldriftmaybearelevantfactor(Briggs1987;CoxandMoore
1998). Not only do the splitting and collision of landmasses affect distri-
