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MALAGASY/INDO-AUSTRALO-MALESIAN PHYTOGEOGRAPHIC CONNECTIONS


Home | Introduction | Gondwanan Relicts | "Lemurian Stepping Stones"
Long Distance Dispersal | Conclusion | References

LONG DISTANCE DISPERSAL (LDD)
  • prevailing easterly winds and Indian Ocean currents
  • Indo-pacific strand flora
  • "recent" dispersal to the Mascarenes
  • often stochastic in nature
  • 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.

    Distribution map B. asiatica (L.) Kurz
    Barringtonia asiatica
    (L.) Kurz

    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.

    Distribution map S. craveniae Baron & Baker
    Strongylodon craveniae
    Baron & Baker

    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.

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