Centres of Endemic?

[Norbert Hahn]

Dear All,

I am sitting with a perplexing problem of trying to define the term "centre
of endemism"!
In a biological context, an organism restricted to localised area is
referred to as an endemic. If this localised area has a high occurrence of
endemics we generally refer to it as a "centre of endemism". Terrestrial
Centres of Endemism are most easily understood in the contexts of islands as
they are surrounded by the see and as such are relatively isolated. The
problem comes when one looks at islands in close proximity of one another.
As an example should one see the Galapagos archipelago as a centre of
endemism or should one treat every individual island as a separate centre
interacting with the other centres within the archipelago.

The above argument becomes problematic when one tries to apply the above
mentioned concepts of centres of endemism on to a continental scale. Our
fist problem would be to try and ascertain natural boundaries to delineate
centres of endemism. For example the Cape centre of endemism is well defined
as it occurs within the winter rainfall region of southern Africa which in
turn has created a unique environment known as the fynbos. The problem comes
once one enters the summer rainfall regions. As an example, the Soutpansberg
centre of endemism is a well-defied geographic area which in turn is well
delineated by it endemic fauna and flora. The problem comes once one looks
beyond the so call narrow endemic species (species restricted to the given
study area). As an example there is a strong interchange of plants between
the Soutpansberg and Maputaland (van Wyk 1996). Should one consider these
plants separately from the rest of the flora and thereby define them as
Soutpansberg-Maputaland endemics. Out of a biological point of view much can
be gained from treating these plans separately. They can give us an
indication to the probable evolution of the set biota. If one excepts the
Soutpansberg-Maputaland endemics, as separately delineated entitys one comes
up with the following perplexing problems. If the given two areas interact
in similar ways with other centres of endemism, does one define this group
as an archipelago like concept of endemism such as the Galapagos except that
the surrounding areas are terrestrial?

What does one call such an archipelago and where does on draw its borders?
White (1983) in his Vegetation Map of Africa named a specific floristic
region the Afromontane Region. He when on to define the Afromontane Region
as an archipelago-like centre of endemism extending from Lolma Mts. and the
Thingi Hills (11° W) in Siera Leone in west Africa to Ahl Mescat Mts (49° E)
in Somalia in the east to the Red See Hills (17° N) in the Sudan Rep. in the
north to the Cape Peninsula (34° S) in the south. The defining of floristic
regions as centres of endemism poses a problem as it excludes geographic
centres of endemism. The above exampel none the leas it shows that it is
possible for isolated regions scattered throughout Africa to interact.

What douse one call biological entities which occurs on two or more of these
centres. If one had to name these in accordance to their distribution things
would become quit interesting. Consider three centres of endemism named A, B
and C. The narrow endemics (Ed) would be called, A Ed, B Ed and C Ed, the
near endemics would be A-B Ed, A-C Ed and B-C Ed and the wide endemic would
be A-B-C Ed. This nomenclature equates to a formula of 2^n - 1 = no.
combinations. As can be seen this type of naming is quit unpractical. To
name the set entities in context to there floristic association could pose
an answer. The assigning of floristic elements has one draw back, they are
mostly based on hypothetical assumptions.

I have been tolling with the above problem for quit some time, all that it
has brought me was more confusion to unresolved questions.

I am eagerly awaiting you answer or possibly more questions.

Norbert Hahn
Director ICONS
P.O. Box 1734
Louis Trichardt
0920, South Africa
Tel: SA 015-5177176
E-mail: hahn1 at mweb.co.za

REFERENCES

VAN WYK, A. E. 1996. Biodiversity of the Maputaland Centre. The Biodiversity
of African Plants 198-207.

WHITE, F. 1983. The vegetation of Africa. A descriptive memoir to accompany
the Unesco/AETFAT/UNSO vegetation map of Africa. Paris: Unesco