Friday, May 19, 2017

Reading up on biogeography part 2: Panbiogeographic Track Analysis

The second paper in this little series of posts is
Romano MG et al, 2017. Track analysis of agaricoid fungi of the Patagonian forests. Australian Systematic Botany 29: 440-446.
What I appreciated about reading it was first that it was concisely written, and second that it gave me insight into the Panbiogeographic methodology of Track Analysis. It had so far been merely a bunch of arcane terms to me, which of course makes it impossible to judge its meaning. And in contrast to the previous paper, which left out most the details of its methodology and instead referenced earlier papers, this one gives a clear explanation. This kind of stuff is exactly why I am reading through the journal issue.

So, how exactly does Track Analysis work?

First, you need species with disjunct areas of distribution - or at least species that are poorly enough sampled that they appear to be disjunct. Then you draw a line along the shortest distance between any two of their occurrences. Let's assume we have a species occurring on two islands of this little landscape I just generated in GIMP:

Panbiogeographers call this red line, with the occurrences of the species forming the end points, a Track.

If you have more than one species showing the same Track, you promote that line on the map to a Generalised Track:

To cite the present paper, in panbiogeographic logic "a generalised track ... allows inference of the existence of an ancestral biota widely distributed and fragmented by vicariance events, suggesting a shared history."

Now you may come up with other tracks in the same study group that do not run parallel. Where generalised tracks cross each other, panbiogeographers draw a circle with an X in it and call that place a Node, like this:

In this case, their interpretation is that this is "a complex area, where different ancestral biotic and geological fragments interrelate in space-time as a consequence of terrain collision, docking or suturing".

Aaaaand... that was it, really. Draw some lines on the map, conclude vicariance and "complexity". The rest of the conclusions in the present paper are largely about the need for more sampling, and that fungi can also be used as a study group.

Does this approach make sense?

Unfortunately, I don't really see it. The logic behind the panbiogeographic interpretation of Generalised Tracks is that patterns of disjunction shared by several taxa are evidence of vicariance, presumably because they assume that chance dispersal would have to be utterly random and create different distributional patterns in each and every species.

But a little contemplation should blow that idea out of the water. There are several other good reasons why disjunct ranges can be shared across taxa. One would be an a priori lack of alternative habitat - if you have two wet patches and otherwise only steppe, then all wetland species will be restricted to those two patches, even if one of the two wetlands was colonised from the other entirely through long distance dispersal. And that restriction alone will produce a shared history, without vicariance. Another option would be prevailing wind or ocean currents, which make long distance dispersal decidedly more probable in some directions even as it is still a stochastic process (dice, but a bit loaded) and, more importantly, not vicariance.

The interpretation of Nodes as showing things like terrain collision also seems to be missing a few crucial steps, at least in my eyes. Don't get me wrong, I am as aware of fossil ranges being an important part of evidence in geology as the next biologist, but still I'd actually prefer to consult a geologist instead of trying to deduce geological history from patterns of distribution alone.

Finally, this whole approach appears to have a weakness that seems quite critical. Science does not proceed by knowing how to confirm, it proceeds by knowing how to reject a hypothesis. Now the question here is this. Yes, panbiogeographic track analysis is apparently designed to conclude vicariance and an area being "complex". But if a disjunction really has not been caused by vicariance, how would a panbiogeographer conclude that? Would they ever do so?

That, alas, is left unexplained, at least in this paper.

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