Origins of Mangroves
Distinctive pollen grains of Rhizophora stylosa (lips), Nypa fruticans (spiky hamburger) and Sonneratia caseolaris (bipolar).
Map of Eocene position of continental fragments and locations of mangrove fossils surrounding the ancient Tethys Sea.
Mangroves, as we know them, have been around for at least 50 million years, and possibly a lot longer. For instance, the origins of Avicennia have been linked to the opening of the Atlantic Ocean extending back almost 100 million years. In any case, species like Rhizophora and Avicennia were widely distributed by 50-55 million years ago. At that time the ancient Tethys Sea, a shallow equatorial sea full of coral reefs and islands, was fast closing as the massive continental fragments continued to shift and jostle across the globe. This process of continental drift appears to have played a pivotal role in determining both the diversity and type of mangroves found today, as well as their current global distributions. Mangroves have not evolved and dispersed uniformly, and there was no common centre of origin. However, mangrove plants share common adaptations, individual species have evolved differently and developed at different rates. Today’s distributional patterns are useful fingerprints from which we might trace the origin of each species. Current distributions of individual taxa show numerous instances of unusual occurrences and absences that demonstrate finite dispersal limitations, especially across open water. Furthermore, there are some genetic discontinuities that occur despite the lack of any current dispersal barriers. In each case, such unusual distribution patterns provide important clues and evidence of past geological and climatic conditions that tell us something about the origin and evolution of mangroves.
Right - fossilised leaves of Nypa from the Eocene period found near Strahan, in southern Tasmania. photo: Mike Pole
Spreading Seed to Distant Shores
Mangrove species are dispersed by water-buoyant propagules (fruits, seeds and hypocotyls), allowing them to take advantage of estuarine, coastal and ocean currents both to replenish existing stands and to establish new ones. The ability to accomplish this differs for each species. Furthermore, dispersal ability varies significantly among species with comparable distributional ranges. A good example is shown with the widely distributed global genera Rhizophora and Avicennia. Propagules of Rhizophora have considerable longevity at sea surviving over 3-4 months, while those of Avicennia last only up to 3-4 weeks. Such differences profoundly affect dispersal range, and this is shown by the much greater eastward extent of IWP Rhizophora than Avicennia eastward across the Western Pacific. Each mangrove species differs in its establishment success and growth development rates, and each has unique tolerance limits and growth responses. In practice, species are influenced by the interplay of different factors along key environmental gradients that are conveniently considered at four geographic scales — global, regional, estuarine and intertidal. Global distributions are affected largely by temperature while regional distributions are affected also by rainfall. By comparison, local distributions upstream in estuaries and across the tidal profile are influenced by a much greater number of factors including: species presence, rainfall, salinity, catchment area and tidal range. Mangroves respond to all these factors and their presence at any location demonstrates the effectiveness of past and current dispersal and establishment strategies of respective species.
Right - Rhizophora seedlings colonise accreting banks often deposited from runoff into downstream estuarine reaches.
- Floating keeled capsule of Heritiera littoralis.
- Exploded capsule fragments and seeds of Excoecaria agallocha.