[Taxacom] Just one species of Bromeliales in Africa

Mon Oct 17 17:41:48 CDT 2011

In case Jason is being serious, he might like to read a commentary by
Phil Maxwell excerpted from his website at
http://www.rhipsalis.com/maxwell.htm

(b) Dispersal by Birds or by Rafting
The most popular explanation, if the books available to me are any
guide, is that birds dispersed the seeds of Rhipsalis across the
Atlantic then to Madagascar, Comores, Seychelles, Mascarenes and Sri
Lanka. This certainly has a superficial plausibility - Australian birds
often get blown across the Tasman Sea to New Zealand (a distance of
about 1700 km) but I don't know of any evidence that they bring seeds
with them. One who explicitly attributed the disjunct distribution of
Rhipsalis to long-range transport by birds was R. Roland-Gosselin whose
original paper appeared in 1912 and was reprinted in 1947. Anthony
(1948, cited by Benson 1982: 115) supported this theory in a paper
published in the same issue of the Journal of the New York Botanical
Gardens as Camp's. Similar ideas have been espoused - invariably with
little or no critical discussion - by many subsequent authors, although
Benson (1982: 115) was commendably cautious when he commented
"transportation by birds is a somewhat plausible explanation, though
there is no evidence that it is the correct one". The leading worker on
the genus, Wilhelm Barthlott (1979: 23) stated "migrating birds must
presumably have brought seeds to West Africa, some thousands (or
millions) or years ago, and these plants spread from there". In their
generally excellent book "The Cactus Primer" Gibson & Noble (1986: 250)
mentioned the fact that some biogeographers had pointed to the
distribution of Rhipsalis as evidence for continental drift, then
stated:

"This example was thoroughly discredited by biologists, who noted that
the species involved, R. baccifera, is widespread in the New World and
the Old and because birds feed on the small, sticky, whitish fruits and
then deposit the seeds with their feces a long distance from where the
fruits were eaten."

There are two points to make about this claim - the first of course is
that the New World Rhipsalis differ from the typical R. baccifera (see
above). The other point is that Gibson and Nobel do not provide any
documentation for the efficacy of bird dispersal of Rhipsalis seeds.
Exactly how far is "a long distance" - 10 km, 50 km or 1000 km? I for
one would like to know if any experiments have in fact been carried out
to answer this question. Rowley (1978: 4-5) was one of the very few
botanists to have devoted more than a few words to the Rhipsalis problem
- he mentioned the three possibilities and commented that
"Roland-Gosselin rather too blandly accepted birds as the agency
responsible" for its presence in the Old World, but he did not come down
firmly in favour of any theory.

The first problem with the dispersalist scenario is to identify a
suitable vector. There are of course plenty of migratory birds, but most
of these fly more or less along the meridian (ie north-south and back).
After all they migrate to seek food and a suitable breeding site,
leaving the northern hemisphere in the winter to take advantage of the
southern summer, and vice versa. This is at right angles to the kind of
migration we are seeking. Some oceanic birds such as albatrosses of
course fly in a more or less latitudinal direction, but they are not
noted for eating berries of any description, let alone those on
epiphytes growing in rain forests. For the sake of argument, however,
let us imagine a small fructivirous bird eating Rhipsalis berries
somewhere in Brazil, conveniently near the coast, and then being caught
up in a violent westerly storm. This bird is flown across the Atlantic
(a minimum distance of nearly 3000 km) without voiding the ingested
seeds, alights in a tree in a rain forest in west Africa and promptly
regurgitates or defecates them into a convenient crevice. After a time
they germinate, other birds eat the fruits and disperse the seeds so
Rhipsalis eventually becomes established in the west African rain
forests after which it spreads across the continent to the east coast.
The whole process is then repeated, and Rhipsalis colonizes Madagascar,
the Comores, Mascarenes, Seychelles and Sri Lanka. Even if we overlook
the fact that the Old World Rhipsalis differ from those in the Americas,
it doesn't require much thought to realise just how unlikely this
scenario becomes; if the only occurrence of Rhipsalis outside of the
Americas was in west Africa then it might be plausible, but to account
for its presence elsewhere requires repetition of an already highly
improbable event! [As an alternative, it could be argued that the seeds
were stuck to the birds' feet or feathers, even though birds take great
pains to preen themselves to maximise their aerodynamic efficiency. The
problem is otherwise exactly the same - the process has to be repeated
several times.] Of course the Atlantic was much narrower in the past -
Africa and South America started to part about 130 Ma (million years
ago), with the equatorial region parting company a few tens of millions
of years later, but if bird-assisted dispersal took place when they were
much closer, exactly when was this - when they were 1000 km apart, 500
km, or 50 km? Why not while they were still in contact?

There is another aspect to this scenario that hasn't been addressed to
my knowledge. A bird flying or blown across the Atlantic would have the
advantage of a large "target", ie it couldn't avoid making landfall
somewhere on the west coast of Africa (assuming it could last the
distance). Madagascar is of course a lot smaller than Africa but is as
little as 425 km from the African coast, and bird-assisted dispersal
across the intervening Mozambique Channel is not too implausible. [It is
worth noting that until humans arrived there (probably less than 2000
yeas ago) Madagascar was home to a pigmy hippo. Its presence there
suggests either that its ancestor swam from Africa when Madagascar was a
lot closer or that there was some sort of short-lived land connection to
Africa.] It is the other occurrences of Rhipsalis that pose a real
problem for the bird dispersal theory. Consider first of all the
Comores, tiny islands about 300 km east of Mozambique. They certainly
don't present much of a target for any bird heading eastwards across the
Indian Ocean. The same can be said for the Mascarenes (Mauritius and
Reunion) which are 850 km east of Madagascar, and the Seychelles which
are 1800 km east of Kenya and 1200 km north-east of the northern tip of
Madagascar. Sri Lanka is a lot larger than these tiny islands, but it is
a staggering 3600 km north-east of Madagascar.

Yet another question must be asked: Why, of all the cacti that have
juicy fruits (and are therefore potentially attractive to birds) should
it be Rhipsalis that has this wide distribution? Gibson and Nobel,
quoted above about the fondness some birds have for Rhipsalis fruits, go
on to say a mere two pages lager (1986: 252): "even though cacti
generally have juicy fruits, long-distance dispersal of most tribes of
Cactoideae has apparently not been effective in causing these cacti to
range widely between the continental areas or even within a continent."

Quite! Croizat's comment is particularly relevant to this discussion:
"it remains to be seen why 'casual dispersal' could not fill with
Rhipsalidinae the Andes throughout, where ecology to suit is certainly
not wanting" (Croizat 1961: 759).

If there is one cactus genus that might plausibly be subject to
trans-oceanic dispersal by birds it is surely Melocactus, some species
of which conveniently live in coastal areas, and of course have fruits
which are not only juicy but brightly coloured as well and are therefore
presumably even more likely to be eaten.

The alternative method of dispersing Rhipsalis across the Atlantic ocean
is by rafting on vegetation washed into the sea by storms. This has had
much less attention that bird dispersal, but Benson (1982: 115) thought
that "Rhipsalis would be more likely than most plants to cling to a tree
trunk or to its branches, above water". Well, maybe, but I think it
highly improbable. 

In any case, as Benson admits, there are not suitable ocean currents
that would take Rhipsalis into the Indian Ocean to Madagascar, let alone
Sri Lanka and assorted islands.

It should be clear from the preceding discussion that I consider the
bird-dispersal scenario to be as dead in the water as any rain forest
bird that tries to fly the Atlantic. This really leaves only the
vicariance explanation as a viable option, and I will discuss this
scenario and its implications for cactus evolution in the second part of
this article. 

-----Original Message-----
From: taxacom-bounces at mailman.nhm.ku.edu
[mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Jason Mate
Sent: Monday, October 17, 2011 6:18 PM
To: Taxacom
Subject: Re: [Taxacom] Just one species of Bromeliales in Africa

I am unfamiliar with Pitcarnia but I do recall that Rhipsalis has a
rather interesting distribution. Most species are found in the
Neotropics but a few are found in tropical Africa, and Asia. It is the
only Cactaceae found outside the Neotropics, and as it produces sweet
fruits which are attractive to birds, maybe it hitched a ride?
Best
Jason

> From: kennethkinman at webtv.net
> To: taxacom at mailman.nhm.ku.edu
> Date: Mon, 17 Oct 2011 04:19:10 +0000
> Subject: Re: [Taxacom] Just one species of Bromeliales in Africa
> 
> Hi Geoff,
>       I think you are correct, and that Pitcairnia feliciana was 
> collected somewhat inland (not coastal).  But I am still surprised 
> that it has not been found in other countries in western Africa. If 
> the Rapateaceae species dispersed earlier as suggested, perhaps
competitive
> exclusion could be a possibility.   In any case, I just found a paper
by
> Dick et al., 2007 (Molecular Ecology 16:3039-3049) which suggests that

> this Pitcairnia species and the Rapateaceae genus dispersed across the

> Atlantic by very strong wind currents (perhaps along with a few other
> plant species).     
> 
>       Not sure if dispersal by strong winds is more likely than 
> dispersal by birds in these cases, but more cases of plant species 
> being dispersed from northern South America to this area in equatorial

> west Africa are slowly being discovered.  And these are relatively 
> late events (within in the last 12 million years), completely 
> different from the vicariance event which resulted in the damselfly 
> genera discussed earlier (which diverged FAR FAR earlier, on the order
of 150 million
> years ago).      
>       So I continue to believe that vicariance (which is supported in 
> many cases for genera and higher level taxa) has been invoked too 
> often for some species level events.  Thus my earlier frustration that

> some would jump on a species level dispersal in the opposite direction

> (Nile crocodiles from Africa to South America), suggesting vicariance 
> in that case merits serious consideration (due to their excessive 
> distrust of molecular clocks and molecular data in general).  Although

> I frankly don't think molecular clocks are nearly as regular as some 
> might think, I also think that the opposite view (dismissing and 
> trashing such data as often as possible) is even less helpful.
>         -------------Ken
> 
> 
> 
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