[Taxacom] Dispersal

[Jason Mate]

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? 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: 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. 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. 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: 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. 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?