Long Distance Dispersal (LDD)

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.

Barringtonia asiatica (L.) Kurz

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 craveniae Baron & Baker

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.

Additional examples of imbalanced distributions include: Gluta (Anacardiaceae) - 1 sp. Madagascar, 28 spp. Malesia; Hibbertia (Dilleniaceae) - 1 (variable) sp. Madagascar, > 100 spp. Malesia, centered in Australia; Keraudrenia (Sterculiaceae) - 1 sp. Madagascar, 8 spp. Australia.


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.


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.


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