Feature Paper: DOWNLOAD Grehan (1994) The beginning and end of Dispersal: the representation of 'Panbiogeography'. Journal of Biogeography, 21(5):451-462.
Author Abstract: The contrast in theory and method between Croizat's panbiogeography and traditional approaches to biogeography presents traditional biogeographers with a significan hurdle when attempting to portray panbiogeography. This epistemological barrier can result in panbiogeography being misrepresented when attempting to categorize panbiogeography within traditional frameworks that are not applicable in a panbiogeographic context. The conflict between the traditional contexts presented in Cox & Moore (1993) and the alternative methods and concepts developed in panbiogeography are illustrated here with respect to the following issues: (1) standard tracks as vicariance events rather than biogeographic (spatial) homologies, (2) Croizat opposing tectonics rather than integrating tectonics with biogeographic patterns, (3, 4) Croizat giving little attention to fossils and climate rather than extensively discussing their relationships with biogeography, (5) ocean baselines being inappropriate to terrestrial patterns rather than representing the spatial homologies, and (6) treating main massings as Darwinian centres of origin rather than spatial criteria for orienting tracks.
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 are now into week 3 of our 12-week course in biogeography, and this week we'll look at the biogeography subdiscipline of panbiogeography, which was created by the biogeographer Croizat (1950s) and has been controversial over the years because panbiogeography started out just as plate tectonic theory was being accepted, but before a full knowledge of continental movements and plate tectonics occurred. As the author of this week's paper notes, much confusion surrounds panbiogeography because it is concerned mostly with geographical connections, which we'll discuss, rather than emphasizing "ecological and evolutionary approaches to biogeography," resulting in "the unintentional result [of] a mis-representation of panbiogeographic concepts that conflicts with actual panbiogeographic texts." In other words, ecologists and evolutionary biologists approach panbiogeography from the context of their fields, causing some errors.
Today we'll start a discussion on what exactly panbiogeography is as well as ensuring that misunderstandings don't occur.
To start, panbiogeography deals with certain terms. As the author notes, "the track method represents one of the most well known aspects of panbiogeography. Individual locations are linked together as line graphs or 'tracks' and those tracks sharing the same biogeographic homologies (baselines) are grouped together into standard or generalized tracks."
Croizat was concerned primarily with "the geological background upon which the ancestral range [for organisms] was established, not Darwinian barriers with respect to the current isolates." In other words, Croizat was not concerned with reproductive barriers or natural selection but rather where individual groups of organisms originated ancestrally.
Another point where panbiogeography typically differs from other branches of biogeography is that "major geographic features widely regarded as 'barriers' in conventnal biogeography (e.g. oceans) become 'centres of origin' in panbiogeography. These panbiogeographic centres of origin are not of the Darwinian kind from which taxa migrate according to their different means of disperseal, but are the baseline that orient or center the geographic sectors involved with the evolution of particular taxa." Because panbiogeography is "interested in biogeographic centers and biogeographic origins" it differs from vicariance biogeography, which eschews "any concept of center of origin."
Now that we understand the concept of a "track" (a biogeographic connection between locations) we can discuss the concept of a "node," which is a geographic area that has served as an originating point for two or more other geographic locations (with their respective groups of organisms). In Croizat's terms, a node is a "region where more than [one[ biogeographic system had converged." However, when starting a panbiogeographic analysis, one looks at "plant and animal distributions" first in attempting to reconstruct historical patterns rather than looking at geological theory. This is likely a result of Croizat working concurrent with the beginnings of plate tectonic theories.
The author then goes into a discussion refuting a number of the negative claims against Croizat, such as whether he paid enough attention to plate tectonics, the fossil record, or climate change. As such a discussion isn't pertinent to defining panbiogeography, I'll leave it to interested readers to follow up in the full paper. Suffice it to say however that the author provides multiple examples where Croizat did give due attention to all three areas.
The third key definition for panbiogeography is the "baseline." A baseline is, in essence, a large track, particularly through an ocean basin, that connects the "biogeography of the group so that it may be compared and analyzed in relation to other distributions that may or may not share the same spatial homology." Baselines are typically drawn through ocean basins to connect the majority of common organisms on landmasses, as illustrated below.
When baselines, tracks, and nodes are represented graphically, a kind of "minimum spanning tree" (MST) is created. The MST is a mathematics problem (combinatorics and graph theory) to represent the shortest graphical space (most conservative representation) needed to connect multiple points. From a panbiogeography perspective, a MST represents the most conservative "route" by which organisms traveled geographically to become separate populations today. Panbiogeography then adds the concept of a "main massing" to try and determine the base of the MST such that it becomes a "directed graph." In other words, it isn't important merely to connect the floras and faunas of various geographical regions, but also to attempt to determine the origins of disparate groups today.
After defining the key terms of panbiogeography and giving several examples, the author notes that while "Croizat established these methods and concepts over 40 years ago [50 years ago today], they do remain relevant in the context of modern research on geology, paleontology, ecology and systematics." The author also notes that "panbiogeographic methods can lead to novel predictions about evolution that can be tested by independent disciplines such as systematics."
The author concludes the paper by noting that many biogeographers are concerned with "the dichotomies of ecology vs history, and dispersal vs vicariance. The imposition of binary oppositions requires the selection of one alternative as if this choice represents the real, natural, world. Panbiogeography represents a research program that is working to deconstruct the strict demarcation between these artificial dichotomies."
This paper is from 1994. Since then, biogeography has continued to grow as a science and many environmental scientists are recognizing the importance of multidisciplinary research and approaches in order to solve ever-complex biological problems. No theory is static in science. Only laws are such. As a result, panbiogeography should be recognized for its value and not just as a precursor to vicariance biogeography.
The rest of this week, we'll continue our examination of "panbiogeography" (the remaining papers on the reading list are noted below). As usual, I encourage downloading the articles in advance and reading them before the summaries. Panbiogeography is a complex and misunderstood branch of biogeography, so I wanted to provide several papers that note the growth of the science over the last 20 years.
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