Rhipsalis Riddle - or the day the cacti came down from the trees
Copyright, Dr. Phil Maxwell, Bathgate's Road, Waimate, South Canterbury (Copyright November 1998. Reprinted with permission of the author. This article originally appeared in the New Zealand Cactus and Succulent Journal.) email: email@example.com
"Rhipsalis is a highly specialized genus, unlikely to have developed during the Mesozoic and certainly neither ancestral to the rest of the family nor even related closely to the ancestral stock." Benson 1982: 115
"The Rhipsalidinae certainly yield in antiquity to no other cactus.
That they are 'derivative' is plainly impossible." Croizat 1961:
Most books or articles on cacti mention the fact that members of the
family are almost entirely restricted to continental North, Central and
South America and the islands of the Caribbean. There are only two exceptions
- a couple of endemic genera (Brachycereus and Jasminocereus) and several
endemic species (or subspecies) of Opuntia on the Galapagos Islands (about
800 km west of Ecuador), and the genus Rhipsalis, which as well as being
present in the rain forests of much of South and Central America, occurs
throughout a large part of central Africa, Madagascar, the Comores, Seychelles,
Mascarenes and Sri Lanka. What is remarkable is that most publications
devote little or no space to a discussion of the occurrence of Rhipsalis
in the "Old World", or if they do write it off as dispersal
by birds. Rowley's discussion (1978) is rather more critical than most,
but it is far too brief (and inconclusive). Benson (1982: 114-116) devoted
more space to the problem than other authors, and although I don't accept
his conclusions he did at least discuss the competing ideas in some detail.
However, I think it is time to look afresh at the problem in the light
of modern ideas on historical biogeography.
I certainly don't want to play down the importance of ecological biogeography, but my own interest is in the other branch, "historical biogeography", which seeks to explain how organisms come to have the distributions they do (and by extension, the reasons why extinct groups had particular distributions). It is a study of fundamental importance in evolutionary theory, even if it has been downplayed by some contemporary biologists.
A LITTLE HISTORY
What proved to be the most important of all the intellectual voyages of discovery didn't even have the pretence of being for scientific purposes. This was of course the voyage of HMS Beagle from 1831-6, which set out to map the coast of South America as accurately as possible for the benefit of the Royal Navy. Charles Darwin was invited to take part as a gentleman companion for Capt. Robert Fitzroy (later to be Governor of New Zealand from 1843-45). He set off with two major pieces of intellectual baggage - the fixity of species, and a stabilist view of geology. One of the books he took with him was the first volume of Charles Lyell's "Principles of Geology", one of the most influential scientific works of the 19th century.
In Patagonia he unearthed the skeleton of a giant sloth and wondered
how it was that such a large animal had become extinct so recently. He
also collected cacti, including one later described as Opuntia darwini,
but in general he was less interested in plants than in animals and geology.
His experiences in the Galapagos - where the governor of the islands told
him that he could tell just by looking at a giant tortoise which island
it came from - eventually led him to abandon the widely held idea that
species were immutable and to formulate his theory of evolution by natural
Darwin, however, never seriously questioned geological stabilism. Geologists
in the 19th century certainly didn't deny the evidence for vertical movements
of the Earth's crust (ie in earthquakes and by extension, mountain building),
but maintained that the continents had remained in the same relative position
throughout time. This was to pose all sorts of problems for biogeographers
throughout the 19th century and beyond. There were only three options
open to those with a stabilist view.
First, there was the idea that the present ocean basins were formerly
occupied by land-masses which for reasons unknown had later foundered.
Oceanographers have searched in vain for evidence for sunken continents,
although this has never daunted the more extreme proponents of lost continents
such as Atlantis and Mu.
Second, the continents were connected by land-bridges which allowed plants
and animals to disperse (or "migrate") before becoming submerged.
It wasn't a preposterous idea of course - there is a perfectly good land
bridge between North and South America, and the remnants of one between
north-west North America and north-east Asia. However, there simply is
no evidence for most of the other land-bridges proposed by biologists.
The third approach ignores geological explanations entirely and explains
what are known as disjunct distributions by passive dispersal of organisms
by such means as wind or ocean currents, or by other organisms such as
birds. This is usually known as "jump dispersal" or "long-range
dispersal". Not an unreasonable approach of course - there are many
seeds with adaptations that allow them to be blown considerable distances,
and there are small freshwater bivalves that attach themselves to the
legs of waterfowl. Large land-dwelling vertebrates present more of a challenge,
but this has not stopped the more extreme dispersalists from coming up
with a solution - the favourite is to envisage them being transported
on rafts of vegetation carried by convenient ocean currents. This according
to some is how giant tortoises and the ancestors of the land and marine
iguanas got to the Galapagos. A few years ago a locally produced TV documentary
used a similar explanation to account for the presence of an iguana in
Fiji - its ancestor was alleged to have drifted on such a raft for several
thousand kilometres across the Pacific from South America! Darwin himself
spent much time immersing seeds in salt water to see how long they could
remain in the oceans until they found suitable landfall. Some seeds have
an impervious coating but others soon became water-logged. [Nothofagus
seeds fall into the latter group.] Dispersalists admit quite freely that
some of the processes they envisage are highly improbable but argue that
given enough time almost anything is possible - not a very satisfactory
explanation. After all the molecules making up my computer keyboard could
in theory all move upwards at the same time and take it through the ceiling,
but I have no intention of attaching it to the bench with superglue! The
dispersalist theory is often called "centre of origin" biogeography:
a species evolves in some restricted area, then disperses far and wide.
Of course, there is a very viable alternative to stabilist geology. The
story should be familiar by now, how the German meteorologist Alfred Wegener
proposed the idea of Continental Drift in 1912 partly to explain the similarity
in shape and geology between the Atlantic coasts of South America and
Africa; how it was vilified by most earth scientists for about five decades
until the mid-60s when some inspired geophysicists came up with the idea
of sea-floor spreading and later, plate tectonics, all to the immense
discomfort of traditional geologists. Of course things were never quite
that simple - in fact it was a group of geologists in the southern hemisphere
who kept Wegener's theory alive; Alexander du Toit in South Africa, Warren
Carey and the New Zealand-born Lester King in Australia, and John Bradley
in New Zealand. In Britain it was Arthur Holmes, who even came up with
a mechanism for continental drift. Geophysicists in the meantime were
more noted for "proving" continental drift was impossible. The
theory held on in the southern hemisphere because the evidence has always
been strongest here - the concept of Gondwana and its break-up have remained
the cornerstone of the theory. It is also worth noting that some biologists
were aware of the implications of continental drift long before plate
tectonics became accepted. One who is relevant to our story is the American
botanist W.H. Camp who published a paper in the Journal of the New York
Botanical Gardens in 1948 titled "Rhipsalis - and plant distributions
in the Southern Hemisphere", in which he explicitly attributed the
distribution of this genus to continental drift. Another early proponent
of the idea that Rhipsalis is a "Gondwanic" genus was Croizat
(1952:362, cited by Hunt 1967:433).
A BRIEF DIGRESSION
Croizat was born in 1894 in Turin, Italy of French parentage; his parents separated when he was 6, and he spent the first half of his life in poverty. He emigrated to the United States in 1923, made a living selling his watercolours until the Crash of 1929, moved to Paris where he found the life as a penniless artist less than satisfying, and then moved back to New York. He was eventually employed as technical assistant to the Director of the Arnold Arboretum at Harvard. He is of particular interest to cactophiles because many of his early publications dealt with succulents and were published in the American Cactus and Succulent Journal. One publication, however, is a 141 page booklet "De Euphorbio antiquorum atque officinarum" (A study of succulent Euphorbiae long known in cultivation), dated 1934, which seems to have been privately published, a harbinger of what was to come. [For the record he proposed the cactus genus Navajoa in 1943.] He had the temerity to publish a paper critical of a leading Kew botanist and was in time sacked, which I suspect left him with an outsized chip on his shoulder. He then emigrated to Venezuela, had several jobs in botany in academia between 1947 and 1952, divorced his first wife and married a Hungarian refugee who ended up owning the most successful landscaping firm in Caracas. This was important because his major works, including "Panbiogeography" (3 large volumes), "Principia botanica" (2 volumes) and "Space, time form: the biological synthesis" were privately published. These books, totalling several thousand pages, are not for the faint-hearted; in fact, it is fair to say that Croizat's vitriolic attacks on his opponents have put many readers off. Attitudes to Croizat have not been helped by some of his disciples - many of whom happen to be New Zealanders - who have adopted his tactics. Croizat, who died in 1982, coined the slogan "earth and life evolve together".
Croizat's great insight came when he plotted distributions of closely
related organisms (eg members of a genus) on a map of the world. He soon
found that similar patterns ("tracks") emerged for quite disparate
groups, whether they were trees, lizards or insects. The conclusion he
came to, and the one which is the least controversial of his claims, is
that these patterns are not the result of chance dispersal but reflect
a far more basic underlying cause. Widely distributed organisms were likely
to be of ancient origin. He didn't rule out chance dispersal, but relegated
it to a minor role to account for the rare exceptions. Most plants and
animals in fact have limited powers of dispersal. In 1974 Croizat published
a joint paper with two biologists with the American Museum of Natural
History, Gareth Nelson and Don Rosen, called "Centers of origin and
related concepts". It was a critique of dispersalist scenarios and
introduced the world to what is usually called "vicariance biogeography."
This claims that after a species arises it spreads quite rapidly into
the available space, bounded only by ecological requirements. Any disjunct
distribution arises from "vicariance", splitting the original
range by climatic, geographical or geological processes. At the smallest
scale this may be the result of a river changing its course, by mountain
building, or by drowning of coastal hills to form isolated islands (eg
in the Marlborough Sounds). On the largest scale rifting of the range
is caused by continental drift. The paper appeared at a good time - plate
tectonics was accepted by nearly all earth scientists and biologists by
the early 70s, but nonetheless it proved very controversial. One implication
of the theory is that - other things being equal - the most widely distributed
member of a group of organisms with limited powers of dispersal will be
the oldest member of that group.
Croizat later repudiated his paper with Nelson and Rosen, claiming that
his particular kind of biogeography, which he called "panbiogeography",
was not the same as vicariance biogeography. In fact, vicariancism tends
to focus on the continental masses, whereas panbiogeography regards the
ocean basins as all-important. Croizat had pointed out that the margins
of continents were at least in some cases composite, that they seemed
to have been added to over long periods and retained their distinctive
biotas. One example was the Pacific coast of South America. During the
1980s geologists came up with the concept of "terranes". Work
in Alaska revealed a large chunk of the country (called Wrangelia) that
seemed to have a very different geological history from that of adjacent
rocks. It was suggested that Wrangelia had formed a long way (thousands
of kilometres) from its present position and had been "accreted"
much later. It was soon found that similar "exotic" or "suspect"
terranes are widespread, particularly around the Pacific. The terrane
concept has since become part of geological orthodoxy, and the adjectives
have long since been dropped. New Zealand itself is now thought to be
made up of several terranes added onto a small continental core.
THE RHIPSALIS PROBLEM
[The other subspecies are restricted to South America.] They do not give
reasons for this classification, and I prefer to regard these "subspecies"
as distinct species if only to avoid clumsy trinomials. In fact, R. mauritiana
is tetraploid (ie has twice the normal number of chromosomes, which is
22 in this group) whereas R. baccifera includes both diploid and tetraploid
forms; according to Barthlott & Taylor (1995:64) the former also differs
in "micromorphological epidermal characters and in having generally
larger fruits". R. horrida may be tetraploid or octoploid (with four
times the complement of chromosomes) and differs in having ribbed adult
stems with bristle-like spines.
There are three suggests that have been made to account for the distribution
of Rhipsalis. These are (a) introduction by humans, (b) long-range dispersal
by natural processes, and (c) vicariance.