0198566123.pdf

Those oceanic islands with large proportions of
endemic plants are typically high islands with
varied habitats (Box 3.2; Humphries 1979). Recent
work, based on molecular phylogenies, supports
the more traditional biogeographical analyses
undertaken by Renvoize (1979) in indicating multi-
ple influences on particular islands, and a variety of
stepping-stone routes back and forth across the
Indian Ocean (e.g. Galley and Linder 2006; Rocha
et al. 2006).
Renvoize (1979) declined to attribute the low
islands of the Indian Ocean below the level of
Palaeotropical Kingdom because they contain too
high a proportion of very widespread species. This
restrained approach to demarcation is realistic, and
although the categorization of islands into particu-
lar biogeographical provinces or regions is clearly
problematic (Cox 2001), such phytogeographical
and zoogeographical analyses have some value.
They establish the broad biogeographical context
within which the compositional structure of a par-
ticular island or archipelago needs to be placed
(cf. van Balgooy et al. 1996).
Distance from a larger source pool can be a poor
indicator of biogeographical affinity. First, there may
be historical factors, as in the cases of Madagascar
and the granitic islands of the Seychelles, which are

ancient continental fragments from Gondwanaland

and so began their existence as islands millions of

years ago, complete, we assume, with a full

complement of species. In the case of Madagascar

the split from Africa has been estimated to have

occurred as long ago as 165 million years (Davis et al.

1994), but the effective isolation date may be

considerably more recent (see Box 2.2). Secondly,

ocean or wind currents (or even migration routes)

may have determined colonization biases (Cook and

Crisp 2005), such as apparently in the case of the

butterflies of the Antillean island of Mona

(Chapter 2). Another classic example is that of

the flora of Gough Island (Moore 1979). The

predominant wind circulation system around the

South Pole provides a plausible explanation for

the affinities with New Zealand and South America

(Fig. 3.8). Changes in relative sea level, landmass

positions, and oceanic and atmospheric circulation

over geological time (Chapter 2) can mean that

such analyses are difficult to bring to a definite

conclusion. We illustrate the problems involved in

more detail below, with reference to Macaronesia.

As already mentioned, within historical biogeog-

raphy two opposing camps have formed up over

recent decades, the dispersalist andvicariance

biogeographers, each concerned with how disjunct

56 THE BIOGEOGRAPHY OF ISLAND LIFE

Gloriosa

AFRO-ARABIAN SUB-KINGDOM

PALAEOTROPICAL KINGDOM

MADAGASCAN

REGION

S.E. ASIA-MALAYAN

REGION

DECCAN-MALAYSIAN

SUB-KINGDOM

Seychelles

Agalega

Amirantes

Coetivy

Comoros Tromelin

Cargados

Carajos

Mauritius

ReunionRodrigues

Addu

Farquhar

Maldive

Islands

Laccadive

Islands

Aldabra

Group Christmas

Cocos orKeeling Island

Island

Archipelago

Chagos

Andaman

Islands

Nicobar

AFRO-ORIENTALDOMAIN Islands

DECCAN

REGION

Figure 3.7Phytogeographical classification of the Indian Ocean islands. (Redrawn from Renvoize 1979, Fig. 1.)