0198566123.pdf

Renvoize’s (1979) analysis of the dominant
regional phytogeographical influences within the
Indian Ocean (Fig. 3.7) tells a similar story. His
paper illustrates that many island biotas are not the
product of simple one- or two-way filter effects, nor
do they enjoy full affinity with a particular conti-
nental region, but instead they consist of a mix of
species of varying sources and origins. For instance,
Christmas Island, close to Indonesia, is dominated
by colonists from South-East Asia, whereas the
Farquhar Group has an African component, a
reflection of the location of these low-lying islands
north-east of the northern tip of Madagascar.
Renvoize’s (1979) analysis of Christmas Island
was based on a list of 145 indigenous vascular plants
from the end of the nineteenth century. The break-
down of this flora was: 31 pantropical, 21
palaeotropical, 76 tropical Asian (Madagascar to
Polynesia and Australia), and 17 endemic species.
This contrasts with the indigenous flora of the
Farquhar Group, which consisted of 23 pantropical,
18 Indo-Pacific, and 5 African species—which seems

to be a fairly typical sort of mix for the low islands of

the Indian Ocean. Leaving aside introduced species,

the pantropical species constitute a far greater pro-

portion of the flora of the Farquhar Group than of

Christmas Island (respectively 50% and 21%).

Although several factors may be involved in this

comparison, the most obvious differences are loca-

tion and in the range of habitats available on the high

as opposed to low-lying islands (Renvoize 1979).

Christmas Island is an elevated coral-capped

island, having a plateau at 150–250 mASL, a peak of

361m, and a land area of 135 km^2. On low islands

few areas are beyond the direct influence of the sea.

Strandline species thus dominate the floras and,

since they are nearly all sea-dispersed and since the

most constant agent of plant dispersal is the sea, the

coastal habitats receive a steady flow of possible col-

onizers and a steady gene flow, not conducive to the

evolution of new forms. High islands do have

coastal habitats, but these habitats and their charac-

teristically widespread sea-dispersed species do not

dominate the floras as they do on the low islands.

DISHARMONY, FILTERS, AND REGIONAL BIOGEOGRAPHY 55

INDO-

CHINA

NEW

GUINEA

AUSTRALIA

Huxley's

line 1868

PHILIPPINES

Wallace's line 1863–1880

Wallace's line

J AVA

SUMATRA BORNEO

Weber's line

Indian

Ocean

Weber's line 1904

Weber 1894/Lydekker 1896

Pacific

Ocean

Figure 3.6Map of South-East Asia and Australasia showing some of the boundaries proposed by A. R. Wallace and others, dividing the two
faunal regions. The continental shelves are shown lightly shaded. In contrast to the sharp discontinuity first noted by Wallace in the distribution of
mammals between Bali and Lombok—marked by the original version of Wallace’s line—birds and reptiles show a more gradual change between
the two regions (see Fig. 3.3). (From various sources.)