Regional Influences in Australasia
Mature propagule of
The Australasian subregion includes: New Guinea, New Zealand, western Pacific islands, and Australia. Mangroves occur in a more-or-less continuous distribution along most coastal areas extending to temperate latitudes around the North Island of New Zealand and southern Australia. There are no obvious barriers to propagule dispersal. Nevertheless, there are notable patterns in diversity gradients shown in the map below. The diversity of mangroves in Australasia is maximal (>30 species, and up to 45) in the Indonesian-New Guinea archipelago extending to north eastern Australia. Beyond this, numbers drop-off in two distinct ways: numbers decline toward higher latitudes reflecting a temperature gradient where species have different limiting lower temperature tolerances; and, declining species numbers eastward where each is limited differently by dispersal range and niche availability.
Mangroves in Australia and Its Territories
Australia’s mangroves occur in all mainland States and Territories that have coastal boundaries, including Western Australia, the Northern Territory, Queensland, New South Wales, Jervis Bay Territory, Victoria, and South Australia. As seen in the Table (opposite), most species occur along the northern coast but they have differing ranges west and east. Some distributions are continuous and some are widespread, but others are restricted to particular local areas. Mangroves occur also in several Australian external territories offshore, including:- in the Tasman Sea - Lord Howe Island (31° 33’ S; 159° 05’ E) with a few scattered shrubs of Avicennia marina and Aegiceras corniculatum, and Norfolk Island (29° 02’ S; 168° 01’ E) with Excoecaria agallocha; in the Indian Ocean - the Cocos Islands (12° 10’ S; 96° 52’ E) with Rhizophora apiculata (apparently introduced) and Pemphis acidula, and Christmas Island (10° 30’ S; 105° 39’ E) with Barringtonia racemosa, Bruguiera gymnorhiza, Bruguiera sexangula, Cynometra iripa, Heritiera littoralis and Pemphis acidula. Christmas island has an unusually elevated inland mangrove forest that appears relict of a time when sea levels were much higher. Hosnie’s Spring supports mangroves dominated by Bruguiera gymnorhiza and B. sexangula, some 120 metres inland of a seaward cliff, and approximately 24-37 m above sea-level. At Hosnie’s Spring, they attain a height of 40 m. Conditions favourable for normal mangrove establishment do not appear to have existed since the last interglacial period, implying this stand has persisted in its present location for up to 120,000 years.