From generalized tracks to ocean basins—how useful is Panbiogeography?

In 1993, in a brief review of different methods of analyzing patterns of distribution (Cox & Moore, 1993), I commented upon the methodology of panbiogeography. These comments were later criticized by Grehan (1994). In writing a reply to Grehan, I found that it was necessary to construct a wider analysis and review of panbiogeography, so that my original comments, as well as Grehan's criticisms, can be seen in the context of Croizat's original views as well as in that of the more recent elaboration of his theory by Grehan and others. There is probably no other field of biological enquiry that is as bewildering as that of biogeography. Evidence there is, in abundance, but it consists of an immense diversity of patterns; from essentially world-wide, to taxa found in only one isolated locality; from wholly contiguous, to taxa found today in several continents separated by hundreds or thousands of miles of open ocean. The biogeography of islands adds another layer of complexity, in the need to make sense of their varying patterns of diversity and of the ways in which their biota has been affected by the area or location of the island. By both instinct and training, scientists are drawn to attempt to reduce apparently meaningless patterns to an ordered series that can be seen as the result of the interaction of a restricted set of characteristics of the world of nature. In the case of the comparison of island biotas, this impulse resulted in McArthur & Wilson's Theory of Island Biogeography (1967) which, though initially welcomed, now seems to lack the wide applicability that its authors had suggested. In the larger sphere of biogeography in general, Croizat's theory of Panbiogeography (Croizat, 1952, 1958, 1964) can be interpreted as a similar, but perhaps more successful, attempt at generalization. Some scientific theories are proposed simply as a reaction to unexplained facts. The Theory of Island Biogeography is a good example of this, for no-one had previously attempted to provide any similar synthetic explanation. Other theories are, in addition, a reaction to a previous theory that the author finds unacceptable or inadequate. Croizat's Panbiogeography is an excellent example of this, for it is a formidable expression of his (justified) belief that Darwin's theory of local evolution followed by dispersal over the pattern of geography seen today (except where it had been modified by comparatively minor changes in sea-level or climate) was inadequate to explain many of the facts of world biogeography. If Darwin's theory was correct, then the only way in which one could explain the apparently irrational, disjunct patterns of distribution seen in many living organisms, as well as of some fossils (such as the Permo-Carboniferous Glossopteris flora) was by the invocation of chance dispersal on a massive scale. Croizat saw this as a convenient way of escape, which allowed biogeographers to suggest an apparent 'explanation' that was in fact merely an unargued statement of belief, and which provided no basis that could be tested. Its widespread use, then, relieved them of any need to provide any other, more rigorous and integrative pattern of explanations for the problems they faced. Two factors may have combined to hinder the evaluation of Croizat's ideas. Firstly, the extreme forcefulness of his rejection of dispersal as the mechanism of all spread from location to location led him to the extreme (and unnecessary) rejection of dispersal as a possible mechanism under any circumstances. Secondly, his ideas are expressed in a terminology different from that used in other theories of biogeography. I shall, in this paper, attempt to 'translate' the theory of Panbiogeography, as expressed by Croizat and elaborated by his supporters, into terms that are similar to those used in other theories, and then to assess the extent to which they are acceptable and useful, while answering Grehan's (1994) criticism of the views expressed in my brief review of Panbiogeography (Cox & Moore, 1993). In his methodology, Croizat followed van Steenis (1934–35) in first mapping the locations in which an individual taxon is found, and then linking these locations by a series of lines that he called 'tracks' or 'graphs of geographic distribution'. He found that, in many cases, the tracks of many taxa, belonging to a wide variety of organisms, could be combined to form a 'standard' or 'generalized' track. This concordance of pattern convinced him that these generalized tracks were a reflection of an underlying reality. However, these tracks did not conform to what might have been expected if the distribution of the organisms reflected the prevailing assumptions that each had evolved in a limited area, and had dispersed from there over the modern pattern of geography. This was either because the taxon is found in scattered locations (for example, around the margins of the Pacific or Atlantic, or also on several Pacific islands), or because it is absent from areas that were thought to have been on its dispersal route. Croizat felt that it would be surprising if any single taxon had managed by chance to cross the intervening gaps, and incredible that a considerable variety, with different ecologies and methods of distribution, should have been able to do so. This led him to reject both the concept of origin in a limited area, and that of dispersal. Croizat's methodology does not, therefore, include any use of the concept of 'dispersal' in the commonly-accepted sense of extension of range across a barrier—what Pielou (1979) refers to as 'jump dispersal'. He fleetingly recognizes that it may take place, as where he rhetorically poses the question of the origin of the biota of Krakatoa after the 1883 eruption. But he then avoids answering the question by asserting that there are many examples of vicariism, and that 'immobilism—as the cause of vicariism—enjoys preeminence above mobilism' (Croizat, 1964 p. 216), and 'evolution has vicariant form-making for its fundamental law' (Croizat, 1964, p. 625). In place of the concepts of local origin and dispersal, Croizat suggested that any widespread but disjunct distribution was merely the relict of a formerly continuous distribution. So, for example, the bird genus Sasia'is today in Borneo and Nepal because generalized picumnine birds in pre-Sasia level of evolution were once distributed to include within their range Borneo and Nepal' (Croizat, 1964, p. 208). (This quotation also illustrates his belief that evolution could take place in parallel, independent fashion from more generalized ancestors.) Croizat suggested that these patterns were also often of great antiquity. Thus, after discussing the disjunction of Oenothera and of many other animals and plants that are found in localities scattered along the western margin of both North and South America, 'The ancestral populations of all these plants and animals were 'on the spot' long before the rise of the Rocky Mountains and the Andes' (Croizat, 1964, p. 546). Having dismissed dispersal (in the conventional sense) as the possible cause of any disjunction, Croizat was then logically free to accept the date of any disjunction that is due to geographical or geological events as indicating the date by which any taxa separated by that event were already in existence. So, for example, having accepted a mid-Jurassic date for the appearance of the Mozambique Channel, he concluded that the Euphorbiaceae, which are found in Madagascar as well as in Africa, had already differentiated by that date (Croizat, 1964, p. 360). Similarly, the ancestors of the Proteaceae, Compositae etc. were already in Australia between the Permian and the Triassic (Croizat, 1964, pp. 173–4), and the explanation of disjunct distributions involving North America and the Old World required the origin of the angiosperms in the Triassic/Jurassic, and the origin of genera such as Jasminum and Menodora in the Early Cretaceous (Croizat, 1964, p. 25). Even if it had taken place in the very distant past, however, the extensive areas occupied by some taxa required explanation. Croizat (e.g. 1964, p. 211) explained this as the result of alternating periods of 'mobilism' and 'immobilism'. If the environment of a species living in one area became favourable, the species would be able to spread to neighbouring areas using its ordinary powers of survival, such as flight, setting up breeding territories, etc. (Croizat was here excluding the exceptional or 'extra-ordinary' methods that had been invoked by earlier biogeographers to explain the crossing of barriers between areas of distribution.) This would represent a period of mobilism, during which the active movement precluded the spatial isolation that is necessary for local differentiation into new taxa. This would be followed by a period of immobilism, during which the stability of range provided an opportunity for evolutionary change. Croizat referred to this as 'form-making', which he defined Croizat (1964, p. 6) as 'the process responsible over space in time for the appearance of a certain taxonomic group at a certain point of the map.' He expressed the whole process as 'dispersal=form-making+translation in space'. Croizat did not, however, believe that there was a rigid distinction between the periods of mobilism and of immobilism: 'At no time will dispersal be so fixed as to exclude slight migration, or so unfixed as to exclude a measure of form-making' (Croizat, 1964, p. 711). It is important to note here that Croizat uses the word 'dispersal' as referring to a pattern of distribution, and not in the sense, that he had rejected, as referring to a process of extension of range across a barrier. His formulation might be 'translated' as 'the pattern of distribution of a taxon is the result of natural extension of range, that did not involve the crossing of barriers, followed by independent evolutionary change within populations that have become, to a greater or lesser extent, separated from one another'. Croizat's tracks do not, therefore, imply any concept of movement: 'A track is expressed by a line on the map along which forms of closely related affinities are found. A channel of this kind should of course not be understood as tantamount to a line of flight or the like . . . . a track stands for a chain of centers of form-making and distribution, each center arisen by fragmentation of the range formerly held by ancestors at a lower level of evolution than their descendants' (Croizat, 1961, p. 1615). Having, then, replaced Darwinian dispersal with a concept that he found more convincing, Croizat still had to explain those patterns of distribution in which the different areas occupied by the taxon are widely separated by environments in which they could not ever have existed—as in the case of the ocean that surrounds terrestrial taxa found in such Pacific island groups as the Hawaiian or Galapagos archipelagos. Because biogeography is, obviously, the result of observations in both biology (systematics) and geography, and because modern geography is clearly inappropriate as the canvas upon which to present the results of modern systematics, Croizat believed that systematics was the only reliable basis for his theories. If the systematics was correct, the patterns must then be the result of alterations in geography, and he accordingly suggested that land had once extended much further westwards from the Americas. He thus writes of a 'land limit' (I would not identify it as a fast, immovable shore, of course) which departing from Revilla Gigedo interests next Galapagos, eventually to reach Peru. This 'land limit' can further be extended to the islands off Chile . . . . and also without difficulties to Hawaii. . . . . to this limit (and possibly beyond) ran the rims of geosynclinal 'foreland insular galaxies and the like which life could use to further its movements.' (Croizat, 1958, p. 799). Similarly, 'The western longitudinal half of the New World, which is now a thin 'Andean' or 'Cordilleran' strip, was in epochs bygone a considerable bulge of land into the Eastern Pacific' (Croizat, 1964, p. 90). He viewed both North and South America as composite continents, formed by fusion of land that had originated in the Pacific with land that had originated in the Atlantic (Fig. 1). . ''Croizat's (1961) plate tectonics model for the formation of the Americas by fusion of different geological and biological systems out of what are now (a) the Pacific and (b) Atlantic basins. This origin is sometimes reflected in organisms with (a′) western and (b′) eastern main massings having trans-Pacific or Atlantic baselines respectively (redrawn after Croizat, 1961).'' Figure and caption from Grehan (1991), p. 351. Croizat seems to have been intent on minimizing the extent to which dispersal (in the conventional, non-Croizatian sense) had to be invoked in order to explain patterns of distribution. So, even though he stated that the rise of the Himalayan Mountains had caused discontinuities in the distributions of races and species of plants and birds there (Croizat, 1964, p. 63), he did not assume that the gaps between the locations of some taxa around the Pacific were the result of extinction after the taxa had dispersed to these locations. Instead, he suggested that the biota had arrived in each of these locations as part of the biota of an island that had become fused to the mainland, by what he called 'horstian dispersal'. 'In horstian dispersal involving the insular festoons off a mainland . . . the islands in the festoon interplay in distribution both among themselves and with the mainland in the criss-crossing manner . . . In the case where the nascent Andes 'sucked in' pre-Andean cores, the Andes also received life from said cores in the horstian manner.' (Croizat, 1958, p. 492, footnote; his italics) Similarly, 'Onagraceous plants later to become modern Oenothera, Fuchsia, etc., were dwelling both north and south of the equator ages before the present geography began to take shape. We can still form pretty concrete and critical ideas of where these ancestors mainly stood . . . whether strewn in origin along an axis Baja California/Peru, of which the Galapagos and Revilla Gigedo are surviving mites, or clustering in ages long past around cores of land that still survive here and there embedded and inlaid into the geography of our times from Mexico to Argentina' (Croizat, 1964, p. 547). Croizat also suggested that the biota of the Cape region of South Africa, and that of Madagascar, had arrived there in 'horstian' fashion (Croizat, 1964, p. 278). Croizat did, therefore, see past geological events as having had possible effects upon the biogeographical patterns of today. In 1993 (Cox & Moore, 1993, p. 242) I commented that '. . . . even after plate tectonics became widely accepted, Croizat for a long time refused to accept that theory, preferring other geological mechanisms'. Grehan (1994) feels that this is misleading. He states that Croizat did not object to the theory of plate tectonics, that he compared tectonics and tracks, that he discussed the spatial relationships between mid-oceanic ridge systems and the distribution of organisms, that he produced a plate tectonic model for the Americas, and that he only rejected 'Wegener's Theory' in so far as it conflicted with the patterns of Pacific dispersal. However, these comments confuse plate tectonics both with tectonics in general, and with Wegener's theory of continental drift, which came long before plate tectonics. 'Tectonics', in the geological sense, is merely a general word relating to movements of parts of the Earth's surface, either vertically or horizontally, whereas 'plate tectonics' refers to the precise theory of division of the Earth's surface into separate areas, or plates, that move as a whole relative to one another. So neither the fact that Croizat compared 'tectonic lines' in the West Indies with the distribution of Columba, nor his acceptance (or otherwise) of Wegener's theory is relevant to my comments on his attitude to plate tectonics. Similarly, what Grehan presents as Croizat's 'plate tectonic model for the formation of the Americas' (Fig. 1) is nothing of the kind. It instead illustrates Croizat's view that the margins of the Americas had either received areas of land, with their biota, by 'horstian dispersal' from the neighbouring ocean basins, or had themselves once extended further out into those basins. As will be argued below, neither of these concepts is supported by our modern, detailed knowledge of plate tectonics, and the fact that the much later concept of terranes parallels horstian dispersal is irrelevant to Croizat's attitude to plate tectonics. Croizat thus believed that the apparent movement of organisms across the map was in fact merely the result of the movement of segments of the Earth's surface. These had originated at the periphery of one continent and, bearing their biota, had traversed the ocean before becoming attached to the margin of another continent bordering that same ocean. Those of his 'tracks' that linked the resulting areas of distribution of the taxa involved therefore crossed the ocean basins (even if over a geography different from that of today) rather than crossing the pattern of land-masses seen today (Fig. 2). Where tracks Croizat therefore believed that the which he a or a biogeographical and geological Because they had received of land, with their living from more than one and across different Croizat viewed continents as well as . The main of biogeography by The lines represent tracks of the The lines and The involved with tracks are the Americas are to the different Pacific and Atlantic Figure and caption from Grehan (1991), p. Croizat believed that his extensive and of the resulting of generalized tracks an Where the pattern of distribution of a taxon did not conform to this, he therefore felt it to this in order to out whether a of the original could the So, for example, on that the biogeography of did not conform to his he a considerable of time (and of in Croizat, in the systematics of the he was to a new that to its biogeography. the same he an extensive review of the biogeography and systematics of the plants and their (Croizat, Croizat believed that his on the biogeography of living organisms was so extensive and that there was point in from the distributions of life living (Croizat, 1964, p. and, because the so much more it a dispersal much dispersal than the other way (Croizat, 1964, p. In my brief 1993 I stated that Croizat to the of the Grehan (1994) that this is and comments that Croizat at separate on fossils and However, it is to suggest that the are an of Croizat's detailed use of fossils in his Croizat the of a as in an example of that Grehan and does he make a detailed between the of a and therefore its and the distribution of the same group today. of these to general such as out the of the of the Permo-Carboniferous and the appearance of the first fossils of birds and or to an that the of the are a biogeographical for those of South America, because in both the extinction was due to many of the are Croizat the same point several example, of them (Croizat, 1964, p. to the that the of the is not the same as the time of origin of the group in Grehan also a quotation in which Croizat states that both the and the have the same track because both are found in as a in the and living in the as well as in the In this is the only example in Croizat's of a comparison between the of a and that of a modern even this is between Croizat's general to fossils was that the was very and that do not the and evolution of living organisms, so that fossils are of even in the of patterns (Croizat 1964, pp. He also states that fossils living forms (Croizat, 1964, pp. but that can use the of living forms to that of fossils (Croizat, 1964, p. In all of his comments on the to be of his lack of use of that Croizat also felt that and the comparatively recent changes due to the had been and used in order to explain of biogeography. he did not believe that active extension of range had been a in the patterns of he was in his rejection of any that such recent changes had been an for So and do limit the distribution of living forms to of survival, but do not explain and these forms (Croizat, 1964, p. He did not believe that recent changes could have caused the of areas in the Cape and states that even recent change may formerly widespread But it was not a recent change that was responsible for the surviving there this was the of very long geological (Croizat, 1964, p. his Croizat did not use changes as a possible explanation of disjunct patterns of distribution. Croizat his ideas in the and The biogeographers of those such as and had their theories during the first half of the and had accepted the evolutionary views of and the stability of modern geography and which also to have Wegener's It then, perhaps natural that the reaction of these biogeographers to Croizat's theories, which used biogeographical as a basis for to geography and was to (or to reject or it was all to such or Croizat not only rejected the use of dispersal as an explanation of all and example of rejected dispersal in any or with the of another explanation of the extensive areas of distribution found in many taxa, he merely that the whole area had been occupied by the taxon to the whole concept of spread from any scientific enquiry as 'dispersal' had previously could see no for Croizat's that the Americas had once extended far westwards into the or for his concept of 'horstian' dispersal. It was not the that Croizat's that the gaps between the locations of taxa might have within an original continuous pattern became one of the concepts that led to the theory of biogeography (Croizat, & see also Croizat in which he states that the his This has become accepted as an to the view that a new species might to dispersal over an barrier. such as also the of Croizat's generalized track as a for the biogeographical and of a biota, and it into an even more with the use of area The generalized track has also been used in the distribution of South & the biogeographers and of those Croizat's generalized tracks accept that Darwinian dispersal may also take place & These methods on the pattern of tracks or is for a of taxa, but in a for another then it is to that the pattern a different It is now that the pattern is the result of and that the pattern may therefore be the result of as & (1994) point the could be the pattern the result of a geography that dispersal and provided opportunity for The explanation of patterns in island groups as the result of can also be where these can be by patterns of change on geological The use of these methods led to the appearance of a to the problems of island & (1994) on the West (1994) and & on the These and plate tectonic to some of which also Croizat's that such as may be composite in both their and their The generalized track and area has also been used by in a of the relationships between New and the However, he then this analysis by the use of a series of concepts which involve new of Croizat's and and the use of new or and This has been by and Grehan in a series of and is the of their and their are the only their as and it is important to these new from Croizat's original because of the mid-oceanic of New and the areas, the New to the between the patterns and the as will be & p. a track as a or an that all the localities occupied by a taxon such that the of the of the each is the The of a track are the of that track. are with to a or ocean by an of localities for geographic within a p. The is also used in the same sense as However, for the New for a track does not represent a of but is a of relationships a geographical It the area occupied by the ancestral form to the evolution of the present p. A or generalized track therefore the pattern of distribution of an ancestral biota This of can also involve the concept of a which is a of or diversity p. comments that the concept is The and geographical of the taxa in a main are to provide an of the origin of the of the track. a example of this (Fig. He states that, if the taxa in New and New are taxa, a line is drawn between these groups of The (or of that line is by it to the next area occupied by the in The fact that New is to than to New the