[Taxacom] Dispersal clarifications

[Robin Leech]

Gentlemen,

I am reading too much philosophy and theory, and seeing too little science.

Matching up the mafik dykes in Africa with those of South America
will confirm continental splits. Age these dykes on both continents.  Relate
mammal and reptile groups that were together before the split, and which
now occupy different continents.

You will find that there are several related mammal groups in Africa
and South America. Continue your studies from there.

Think more of invertebrate organisms such as insects.

As for dispersal, check out the work by Linn Gressitt and Carl Yoshimoto
on the rate of dropout of organisms from the Orient to say the Hawaiian 
Islands.
Their work was in the early 60s.
In the first 500 miles, 95% of the organisms have dropped out, or are no 
longer
able to fly cuz they ran out of blood sugar. Some freeze to death at the
30,000 ft altitude in the jet streams.

After that, from the Orient, chance and luck get it to Hawaii, chance and 
luck
make it a gravid female, and chance and luck have it drop onto Hawaii on
vegetation that it can lay eggs on.  Chance and luck didn't freeze it to 
death.

Now think of spiders.  One of the reasons spiders are the first living 
things
found on new islands is that they do not have to expend energy to get there.
They use their silk "balloons".  So, when they land at such places as 
Suertsi
and Krakatoa, they scurry around eating the dead and dying insects. This
keeps them going till established vegetation and insects can get there and 
survive.
In the meantime, if no vegetation and insects establish, the spiders and 
balloon
away.

And spiders don't seem to freeze at the higher altitudes (protein sols in
many species).

After that, the explanation about a species not being there falls back to 2
reasons:
1. It hasn't got there yet.
2. Ones that get there cannot survive there.
Robin

----- Original Message ----- 
From: "Michael Heads" <michael.heads at yahoo.com>
To: <taxacom at mailman.nhm.ku.edu>
Sent: Saturday, June 11, 2011 10:04 PM
Subject: Re: [Taxacom] Dispersal clarifications


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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