Author Abstract: In a recent guest editorial Stace (1989) expressed concern at the preoccupation of vicariance biogeography with vicariance events at the expense of migration to explain distributions. I take this opportunity to draw attention to the emergence of panbiogeography as a programme of synthesis between ecology, geology, systematics, and biogeography that does not require a priori adherence to specific doctrines of vicariance and dispersal while making positive and productive contributions to biogeographic methodology and theory.
Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.
Review: We will continue our examination of panbiogeography this week with a discussion of how panbiogeography can be applied specifically to tackling biogeographic problems where vicariance or dispersalism don't seem capable of resolving distributions.
This paper is a little different that other papers we've reviewed in that it is written by one author (Grehan) as a response to another author (Stace) and therefore provides lengthy quotations from Stace with comments or rebuttals by Grehan.
Stace begins his view by stating that dispersal (or migration) and vicariance (separation of populations by barriers) are both important means through which disjunct distributions are created. A disjunct distribution is where certain taxa are found in multiple locations that are widely separated from each other but where those taxa are not (necessarily) found at all locations between known sites. In Stace's view, as with many biogeographers, either vicariance occurs in a given situation for a certain group of taxa, or dispersal occurs… but each are mutually exclusive of the others.
Grehan points out that panbiogeography does not consider vicariance or dispersalism in determining biogeographic affinities and therefore "should be of some considerable interest to Stace as it may represent an alternative solution to his stated concern." Grehan continues to note that "Stace's presentation of vicariance and dispersal places biogeography firmly within the jaws of a methodological dilemma that can be recognized as a 'binary opposition' where the main outcome is simply to present a negative image of the alternative." With panbiogeography "rejecting the terms of the debate (that vicariance and dispersal have equal and independent existence) as a false opposition" Grehan aims to prove in a short rebuttal that panbiogeography can provide a new basis as a major school of biogeography in resolving distributions.
Grehan discusses the late Joseph Hooker (a great past botanist), who recognized that in certain circumstances, either vicariance or dispersal could both be considered valid answers to individual distributions. Hooker (in the 19th century) realized that "the problem lay not in the lack of 'facts' but the absence of an appropriate method." Grehan argues that panbiogeography is the modern method that can resolve current biogeographic problems.
Grehan points out that Croizat (the founder of panbiogeography) "demonstrated that patterns of dispersal (= evolution of distribution) are correlated with neither the migratory abilities of organisms (their different means of dispersal) or current world geography. Croizat's analysis suggested that tracks [discussed in the previous two papers this week] were correlated on a global scale with ocean basins and he identified tectonics as the nexus between geology and biology. This finding resulted in novel geological predictions that have since been independently corroborated by geological research."
Grehan then discusses a specific example of a disjunct population and shows how the problem is solved using panbiogeography. Stace concludes the article by pointing out that many panbiogeographers are very dogmatic and more critical of other biogeographic schools than the alternative schools of thought are to panbiogeography.
Of course, any good student or researcher will examine all schools of thought in their field and make their own determinations based upon thoughtful analyses of different perspectives.
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