Wednesday, April 30, 2014

Leptokurtic distributions do not magically end at a water's edge

For some reason, my post on panbiogeography has become quite popular over the last two weeks or so; no idea why. Maybe it is accessed by a lot of people who want to obtain information on this school of biogeography, or maybe it has been discovered by panbiogeographers, although in the latter case I would expect them to be unamused and express themselves in the comments. Be that as it may, it may be useful to expand on an argument that I made in that original post.

As pointed out then, vicariance as the only explanation for current biogeographic patterns, or even just as the null model, is utterly self-defeating because dispersal is indispensable for the ancestor to cover the wide area that can subsequently be sub-divided by vicariance events. Another way of putting it is that if there were no dispersal, all of life would still be sitting in whatever single spot in the ocean it originated some three billion years ago. (And of course it is likely that that one spot has long disappeared under layers of silt, been subducted into the mantle of the planet, or undergone some other traumatic events.)

So really there is no way to tell an intellectually defensible biogeographic story without dispersal. The panbiogeographer now has two options to justify their rejection of dispersal: First, dispersal was only possible in the far past when various ancestors spread across the world, and then somehow became impossible. I hope it is obvious that does not make any sense whatsoever, and sincerely hope that no panbiogeographer would argue for it. Second, dispersal is admitted to be possible on the same land mass but suddenly stops completely once the land mass has broken up. (Subsequently I will focus on terrestrial organisms, but in the case of aquatic organisms simply swap "land mass" and "water body" and you have the same situation.) This option seems at least somewhat plausible.

In the previous post I cited numerous observable cases of organisms crossing from one land mass to another across appreciable distances. Today I want to make a more general point: Do we have any reason to assume that dispersal suddenly drops off completely at the edge of a land mass? Not really.

We would expect dispersal to follow a leptokurtic distribution: most of the propagules end up very close to the parent, and soon there is a steep decline followed by a long tail of increasingly rare dispersal events across longer distances. What we would not expect is a sudden and complete drop-off the moment a bit of water gets in the way; certainly not for seeds or flying animals, but even land animals can generally swim a bit, and tiny ones can be blow across by the wind.

Sure, the panbiogeographer could reply that the larger the distance, the less likely it becomes that a colonization event can take place across it. But that is just the point: less likely, not impossible. For their dogma of vicariance to make sense, we would have to assume that water forms a magical barrier at some point.

But where do you draw the line? If you accept that a group of organisms can sometimes cross 10 m of water, why would they be completely unable to ever get across 100 m? If you accept that a group of organisms can sometimes cross 100 km of water, why would they be completely unable to ever get across 500 km? There is simply no reason whatsoever to accept the magical barrier model unless you assume that some life forms turn to ash the moment they lose contact with land. Instead, it is all about probabilities, not about dispersal being impossible.

Of course, it depends on the organisms in question. Some groups definitely have a harder time surviving long travel, and thus their leptokurtic distributions may have a relatively short tail and they will not be able to cross 500 km of ocean with a sufficient probability of survival for a successful colonization event to happen within a realistic time window. This is especially true of large animals with high metabolic rates - where a lizard may survive rafting, a mammal of the same size is likely to fatally starve or dehydrate.

On the other hand, if a species is capable of powered flight, such as bats, birds and larger insects, it becomes reasonable to assume that the likelihood of them colonizing distant islands is actually higher than what we would expect from the leptokurtic distribution because individuals accidentally blown into the general area of such an island may actively seek it out. We would then get a bump on the distribution wherever there is an island. This ability would also extend to animal-dispersed plants whose seeds they carry in their stomachs or on their bodies.

A very illustrative example is New Zealand, ironically one of the strongholds of panbiogeography. The native vertebrate fauna before human arrival did not include any mammals except oceanic ones ... and bats. So we can assume here that its isolation has for a long time been too great far for a rat or possum to make the trip. But unless it is assumed that bats had evolved before New Zealand separated from Gondwana, which given their phylogenetic position means that many other groups of mammals would also have been there and then rather strangely all died out where only bats survived, the only possible explanation is that bats arrived in the country through long distance dispersal.

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