[Taxacom] Dispersal clarifications

[Michael Heads]

Hi Jason, 
 
There was something else in your letter I meant to comment on. You said:
 
'since the Atlantic ocean has been expanding for 100my you´d expect early dispersal to be vastly more common than now, progressively becoming rarer until it reaches the current situation, where only the most powerful fliers on a lucky break make it (i.e. D. plexippus). The point of course is that the window of opportunity to disperse closes at different rates for all organisms so many (maybe the majority) rarely get the chance to do it'. 
 
This is a good description. To start with, a clade might have been a small-range endemic found in what became Brazil and Ghana. It dispersed around its range by normal dispersal. When the range was split it carried on dispersing but at a certain point, depending on its normal means of dispersal, ecology etc., it broke apart. The same thing happened to all the groups in the community. A few organisms have not diverged at all but have maintained a large range on both sides of the Atlantic (whether populations on both sides are permanent or whether there is episodic recolonisation).  This lack of divergence is probably a result of genome architecture - not enough 'evolvability' - as well as ecology.  None of this denies active range expansion of some groups, either due to natural causes or because of human habitat change or introduction. But the underlying division of Brazil and Ghana was vicariance. The one thing I'd disagree with in your
 description is the word 'lucky'. None of the processes involved here are driven by luck.    
 
Michael 
 
 
 


Wellington, New Zealand.

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

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


From: Jason Mate <jfmate at hotmail.com>
Subject: [Taxacom] Dispersal clarifications
To: "Taxacom" <taxacom at mailman.nhm.ku.edu>
Received: Saturday, 11 June, 2011, 12:04 PM



Sorry for the delay Michael.  In regards to your last email:
I am still waiting for a working definition of your ´simple movement´.  Til now you haven´t really defined it, rather you have offered a gut-feeling of what you think it is. Movement, IMO, is a range of distances travelled and probabilities, for each individual organism out there. So why make a special, compartmentalised category for the long distance ones? You seem to define it based on your personal observations, e.g.: "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." I have no doubt that if you were to look at the movements of all the Tui´s in your neighbourhood you would find, with a sufficiently large dataset, the odd long range trips.
Dispersal per se is not a single step but is dependent on several factors, both internal and external to the organism. Internal ones would be the vagility of the species or population and the physical condition of each individual. External ones would be the size of the source population, the distance to the target (not intentional of course, there is no grand plan to disperse to greener pastures) the size of the target (hitting Madagascar vs the Canary Islands), atmospheric variables (main currents, weather conditions, etc). And even if you get there there are yet more variables: lack of mates, habitat heterogeneity, competition. So colonising a new area (an island for example) involves a great number of variables on which ultimate success depends, surely a small chance. The probability for any given species if the aggregate of all the individual probabilities, not just the behaviours that you may observe outside your window, although in a way it is a
 beginning. On the other hand you mention: "I don't think 'chance dispersal', with speciation, sensu Mayr etc., exists." Do you mean that a species that colonizes an island cannot diverge, in time, to the point of becoming a distinct species, even an endemic? What particular mechanism would prevent this from happening? To which you add "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." Well obviously not, dispersal to your garden is easy, genetic exchange has been hardly disrupted. But still you continue "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." There are so many ad hoc assumptions in this remark it is
 difficult to discuss it. In the first place, communities are not monolithic blocks that respond equally to external pressures. I think that the biodiversity conservation end ecological literature has sufficient examples to show that although closely related species often respond the same to external pressures communities don´t. As for how many individuals are needed to start a colony, tramp species don´t appear to need many, but I guess you can never tel exactly how many. Nevertheless I agree that the vast majority of dispersals fail, simply because they end in some sort of sink (ocean, non-vegetated atoll) or an island where the ecological conditions of the species are not met. But we can make some guesses as to which dispersals are more likely. Polar bears would have a tough time settling in the Bahamas but I doubt Drosophila would be as challenged. Hence some groups will be better at dispersal than others. In fact if you look at the faunas of
 oceanic islands you see some groups, i.e. mammals, that are simply no good at it whereas others like birds are. Still, you bring out your big guns and argue that dispersal is a lame duck because, "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."  Why would you expect this? In fact, since the Atlantic ocean has been expanding for 100my you´d expect early dispersal to be vastly more common than now, progressively becoming rarer until it reaches the current situation, where only the most powerful fliers on a lucky break make it (i.e. D. plexippus). The point
 of course is that the window of opportunity to disperse closes at different rates for all organisms so many (maybe the majority) rarely get the chance to do it. In fact some can´t do it even when it seems impossible not to disperse:"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." I cannot speak for the flora of New Zealand but I can vouch for the dozens of butterflies and scarabs species  that span over the Wallace line. The point is we can find examples to support both mechanisms. Some organisms can disperse, others can´t. I am sure that if we looked at the natural history of one group and another we would find wildly different reasons (competition, disease, temperature...).
"Nevertheless, a common argument goes: 'I saw species A in my genus move, therefore the biogeography of the species and the genus can be explainedby movement'.  This is like the sun going around the Earth - the appearance may not be the reality." Agreed, but it works both ways. Just because you have a pattern that appears vicariant it doesn´t mean it is the case. Remember, you can´t always extrapolate.                             

Finally, you also mention that "dispersal theory" (I prefer mechanism) does not accept allopatry as evidence for vicariance. In truth allopatry is a pattern of distribution. Explaining this pattern requires other facts and a hypothesis (mechanism) that fits the facts. Using a pattern to support a mechanism used to explain said pattern is circularity at its best.
Best                           
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