[Taxacom] Dispersal

[Michael Heads]

HI Jason,
 
I've added my new comments (with an asterisk) below.


Wellington, New Zealand.

My papers on biogeography are at: http://tiny.cc/RiUE0

--- On Tue, 7/6/11, Jason Mate <jfmate at hotmail.com> wrote:


From: Jason Mate <jfmate at hotmail.com>
Subject: [Taxacom] Dispersal
To: "Taxacom" <taxacom at mailman.nhm.ku.edu>
Received: Tuesday, 7 June, 2011, 7:41 AM



Dear Michael,
I have grouped your comments to avoid needless repetition. If I have misquoted you in any way I apologise.
MH: Many mainland populations expand their range onto an island, e.g. weeds after a volcanic eruption. This can be observed and studied. On the other hand, simply assuming that a distinctive island endemic 'dispersed there' from the mainland by 'biogeographic', 'jump' or 'waif dispersal' is not acceptable. It could be there for many other reasons (e.g. it could be a relic inherited from a previous island in the vicinity)....MH: I see dispersal happening all ther time. Every organism (including everyone reading this) has dispersed to their current position.... MH: It depends which concept of dispersal you're referring to. I don't think 'chance dispersal', with speciation, sensu Mayr etc., exists. On the other hand, all plants and animals move, and their pattern of movement is related to their means of dispersal. This movement may have little or nothing to do with the geographic distribution of their clade. A migratory bird moves in a very different way to
 an earthworm, and yet major clades in both groups can have very similar distribution patterns.  
JFM: So if dispersal is so common, doesn´t it follow that the chance of dispersal to an island is inversely proportional to the distance of the source to the island? 
 
*MH: of course, for dispersal in the sense of simple movement.  
 
An organism that expands its range and makes it to an island has bought itself a ticket (not guaranteed) towards becoming an endemic, either by survival comapared to the source or speciation by the fact that genetic exchange will be highly restricted (the extreme case being no exchange).
 
*MH: all clades are endemic to some area or other, but the weed that makes it to my garden (an island of suitable habitat, just like an island in the sea) doesn't become a local endemic there. Range expansions don't usually involve one-off dispersal events by single individuals, but populations or, usually, whole communities. Range expansions occur for a reason - change in climate or whatever - and these factors affect the whole community.   
 
MH: No, of course not. I think you're aware of examples. But the principle is the same - dispersal here is just a word that is invoked, a sound you make with your mouth. You don't have to actually study anything. You just say: 'oh, it must have been dispersal, although strangely enough we don't know of any possible means'. Editors seem quite happy with this.  ...MH: I don't think dispersal is mysterious. It is the dispersalists who refer to means of dispersal as mysterious ('we can only speculate how it got here...')....MH: Of course. The weeds in my garden tend to come from the neighbors, not from the other side of town.JFM: Good, had to check just in case. Yet 
JFM:Normal range expansion can occur over an area of unsuitable habitat. 
 
*MH: Of course.
 
JM: The severity of the obstacle will determine the probability of overcoming this obstacle. ranging from 0 (impossible) to 1 (certain) probability of dispersal. I guess you consider nearly 0 as chance. 
 
*MH: No, I consider it as 'probability nearly zero'. 
 
JM: Also please note how in your example of garden weeds you employ the word ¨tend¨. Sometimes weeds can come from further just by chance (maybe your local pigeon population goes to a particular forest, but you don´t know this, hence you speculate).And, unless you introduced the organism, you can only speculate as to how it got there.
 
*MH: You use the phrases 'just by chance' and 'you can only speculate'. But you can actually investigate this local-scale biogeography. One of the 'weeds' that visits my garden is a native honeyeater (meliphagid) bird, Prosthemadera. There is a seasonal migration on a local scale, probably over 3-4 km. I haven't worked out the details yet, but you could. Wikipedia says: 'The movements of honeyeaters are poorly understood. Most are at least partially mobile but many movements seem to be local, possibly between favourite haunts... It seems probable that no single explanation will emerge: the general rule for honeyeater movements is that there is no general rule'.  I don't really believe this (it sounds like chance dispersal!), the problem just hasn't been worked on enough. A meliphagid in southern New Zealand (Dunedin) seems to migrate in a similar way to the Prosthemadera.    

MH: Vicariance might be falsified by finding that the distributions of taxa in a region don't share similar phylogenetic/biogeographic breaks. But this could be due to original allopatry followed by range expansion. Unlike the text-book examples which everyone learns, most broad theories don't fall over instantaneously because someone finds a single fact (see the detailed critiques of 'naive falsificationism' by Lakatos, Feyerabend etc.). Usually there is a slow accumulation of 'anomalous' data and eventually there is a broad shift of opinion. If all the molecular phylogenies showed no geographic structure and instead taxa tended to sort on colour or size or hairiness, rather than locality, no-one would be looking at  vicariance and it would just fade away. ...MH: Why do you think I'm only interested in vicariance and not dispersal? I'm very interested in ecological movement in groups, range expansion, daily and annual migrations, and, above all,
 dispersal in the most general sense: 'any and all changes of position' (Clements, c.1930). Allopatry is caused by vicariance, overlap is caused by normal range expansion (not chance dispersal).
JFM: Regarding the role of vicariance in allopatry, it would depend on your operative definition of vicariance. Suffice to say that habitat heterogeneity and the limitations of dispersal can create geographic structure, hence it is no proof of vicariance. 
 
*MH: Of course, dispersal theory does not accept allopatry as evidence for vicariance. My phrase 'Allopatry is caused by vicariance' was offered as a conclusion, not a proof. 
 
  In addition just because a particular biota´s origin is of one kind or another it should not be assumed that it all originated in this way.
MH: I'm not sure what happened to the rest of your sentence here. You can work out the probability of dispersal easily by making observations. A Rhipidura fantail appears outside my window once a day. Zosterops is there maybe three or four times a day.  
JFM: Lost in cutting and pasting. What I was trying to say is, could I extrapolate? i.e. D. plexippus or N. virginiana have made it across the Atlantic, ergo if you are a good flier and have air currents in your favour, successful ocean crossing has a ¨high¨ probability (can´t think of a good contrast to judge this by). Or is the extrapolation limited to taxa closely related to the taxon of interest?
 
*MH: I don't think you can extrapolate - many genera have a widespread weed and I guess the same is true for beetles. There are plenty of good fliers on both sides of the Atlantic - spore plants (bryophytes, fungi, ferns), flies, birds, plants with wind- or bird-dispersed seeds, lepidoptera, many beetles, etc. - and there are plenty of storms etc. that could blow organisms across the Atlantic. So after a few centuries, let alone a hundred million years you would expect all these groups to be more or less the same on both sides of the Atlantic - but they are not. The same thing happens at narrow breaks like Wallace's line, or between very windy mountains in New Zealand with endemic daisies etc. In my experience, trying to correlate distributions with what is known about means of dispersal doesn't work. Nevertheless, a common argument goes: 'I saw species A in my genus move, therefore the biogeography of the species and the genus can be explained
 by movement'.  This is like the sun going around the Earth - the appearance may not be the reality.                             
 
Michael
_______________________________________________

Taxacom Mailing List
Taxacom at mailman.nhm.ku.edu
http://mailman.nhm.ku.edu/mailman/listinfo/taxacom

The Taxacom archive going back to 1992 may be searched with either of these methods:

(1) by visiting http://taxacom.markmail.org

(2) a Google search specified as:  site:mailman.nhm.ku.edu/pipermail/taxacom  your search terms here