Biogeography's aims

Robert Mesibov mesibov at SOUTHCOM.COM.AU
Fri Apr 19 17:26:45 CDT 2002

Alec McClay:

"But it is surely also a legitimate question to ask
how a particular taxon came to have the distribution that it does. This is
a question about the geographical context in which particular biological
events took place. How does it not belong to biogeography? It's a bit like
saying that ecology is only concerned with the properties of whole food
webs or communities, and that therefore a study of the population dynamics
of a particular species is not an ecological study."

A widely used ecology textbook defines ecology (I'm paraphrasing) as the
science which seeks to explain why plants and animals live where they do.
This doesn't leave a lot of scope for biogeography as a specialist
discipline, and implies that if you know the environmental likes & dislikes
of a species you can explain its distribution and your biogeographical work
is finished.

More thoughtful biologists split biogeography into 'ecological
biogeography', dealing with distributions now and in the recent past, and
'historical biogeography', dealing with origins, extinctions, dispersal and
'connections' with long-term geological history. Ecological biogeography is
mostly pursued by ecologists. An example of their research would be
environmental-envelope modelling to predict ranges of single species, as
mentioned by Jim Croft here on TAXACOM a couple of days ago. Ecological
biogeographers regularly run up against distributions they can't explain
ecologically, and these are generally explained away as the result of
'historical contingency.'

Enter the historical biogeographers, whose motto (to quote the Australian
malacologist Bronwen Scott) is 'History first, ecology second.'
Unfortunately, although it's possible in principle to gather enough
evidence for a spatial history hypothesis for a single lineage, it's
usually extremely difficult. It's easier (although still not easy) to
gather bits and pieces of data about a whole range of taxa living in a
given area, and come up with a parsimonious notion about the history of
that biota and where it lives.

Let me give a specific example: the Australian orthopterist Ken Key
discovered that two grasshoppers in the endemic Tasmanian genus Russalpia
had parapatric distributions. In fact, their distributions neatly divide
Tasmania into two bits along a certain line. Key said there were two
questions to be answered: how did this happen, and why was the line where
it was? His hypothesis was that an ancestral, Tasmania-wide Russalpia
population was split by a vicariance event into (at least) two allopatric
populations. These differentiated, and when times improved the two
dispersed to form a line of hybridisation parapatry in the landscape. He
couldn't suggest a reason for the line's location. This is the kind of
research I assume Alec McClay is talking about.

What Key was unaware of was that a remarkably large number of Tasmanian
invertebrates and plants have range boundaries nearly congruent with the
Russalpia line of parapatry. In fact, there are numerous cases of
intrageneric parapatry in Tasmania where the parapatric partners meet just
where the Russalpia do. Some of these pairs appear to be older,
evolutionarily speaking, than Key's pair, while others are younger.

What's going on? We don't yet know, but the point is that Key's
single-taxon biogeography is unlikely to tell us. We'll need to use
molecular and other data from a whole range of taxa to work out a likely
history for (a) the two bits of Tasmania divided by the line and (b) the
taxa which respect that line in their distributions.

I don't think this example is atypical on a global scale. That's why I say
that biogeography's aim is to work out histories for 'inventories', not
single species. A hypothesised history for a single taxon may sound good
but be off the mark, because it doesn't embrace all the relevant
biogeographical information.

(A favourite analogy of mine for the latter: van Helmont's experiment on
plant growth. He grew a small tree in soil in an iron tub whose top was
perforated with holes for watering. The tree gained lots of weight. The tub
and its soil lost a few ounces. Conclusion: the added weight of the tree
came almost entirely from the added water. Van Helmont didn't appreciate
that gases have mass, and didn't know that one gas, carbon dioxide, was
absorbed in large quantities by his tree. He was missing information needed
for reaching his conclusion, although of course he didn't know that. It
worries me that biogeographers sometimes try for conclusions on limited
information. I still don't fully understand Croizat, but I have tremendous
admiration for his comprehensive approach to distributional data.)

Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 64371195; 61 3 64371195

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