The difference between stepping-stone dispersal and long
distance dispersal (LDD) is only a matter of distance, which is
just one of the factors contributing to probability of dispersal;
the former might also be described as short to medium distance
dispersal. Any dispersal across a fragmented space is rendered
more probable when a taxon possesses an added degree of vagility
(small, resistant, aerodynamic, or buoyant disseminules), and/or
time. That dispersal over especially long distances has occurred
is undeniably proven by the many Asian taxa that have reached the
Hawaiian Islands in recent history, a distance of 8,000 km. As
Raven (1979) noted, the distance from Australia to Madagascar is
only about 2/3 (5,400 km) of that from Asia to Hawaii, and both
source and target have existed for an extremely long time.
Prevailing easterly winds and ocean currents in the Indian Ocean
further increase the probability for LDD from Malesia to the
western Indian Ocean (Renvoise 1979). Accepting the fact that
LDD is potentially continuously occurring, implies that it has
undoubtedly occurred recently, as is witnessed by dispersal to
the young volcanic Mascarenes (e.g., a phyllodic Acacia of
certain Australian affinity (Bell & Evans 1978, who nevertheless
conclude a former land connection between Australia and
Mauritius)).
Despite extrinsic directional forces, LDD is often
stochastic in nature, and more often that not results in a highly
imbalanced distribution.
Two widespread species of Barringtonia
(Lecythidaceae) are among a long list of mangrove and littoral
species (the Indo-Pacific strand flora) to have reached
Madagascar by ocean dispersal. Nevertheless, the distributions
of B. asiatica and B. racemosa in the western Indian Ocean
contain a random component: B. asiatica is present on Mauritius,
but not on Reunion, and has failed to reach the E. African coast;
B. racemosa has reached the E. African coast, but is not present
in the Mascarenes; both are present in the Seychelles (Payens
1967). To some extent the distributions reflect differing
ecological preferences (B. asiatica on sand just above the tide
line; B. racemosa in estuarine habitat), but there is also a
stochastic element.
Strongylodon (Fabaceae) includes a
widespread species, S. lucidus, that has reached Hawaii and
Tahiti (Huang 1991). In the western Indian Ocean, S. lucidus is
present only on Reunion, although Madagascar also harbors a
distinct section Craveniae with two species, indicative of an
earlier dispersal event.
As disjunct outliers of the current Indo-australo-malesian
flora, those Malagasy humid forest species whose affinities lie
far to the east possess especially high information content, and
therefore increased conservation value (Vane-Wright et al. 1991).
Barring the discovery of fossil evidence of former distribution
(Coetzee & Muller 1984), extant taxa provide our only means of
constructing historical biogeographic hypotheses. From a
phylogenetic standpoint, relict taxa constitute long basal
branches critical for understanding both ingroup, and broader,
higher level outgroup relationships. Extinctions of outlying
taxa have greater biogeographic (and possibly evolutionary)
consequences than extinctions of core-area taxa, although, of
course, neither is desirable. Just as the Malagasy language and
customs continue to reveal the S.E Asian origin of the Malagasy
people, let us hope that the Malagasy flora will also retain its
far eastern character.
ACKNOWLEDGEMENTS
I wish to dedicate this paper to Professor Hugh H. Iltis,
who fostered my interest in phytogeography. I thank
the people of Madagascar for welcoming vazaha
scientists with such open arms. This research was conducted
while holding NSF grant DEB-9024749.
Barringtonia asiatica (L.) Kurz
Strongylodon craveniae Baron & Baker
