North Stradbroke Island may be a sand dune island but it harbours an interesting, if not unique array of plant ecosystems. North Stradbroke Island is located 40 kilometres east of Brisbane, approximately 270 36’ 31. 32” S 1530 26’ 40. 12” E and enjoys a subtropical coastal climate.
The dune islands slowly formed over thousands of years, as layers of sand were gradually deposited by both changes in sea level and strong winds from the South East. Some of the corals that are found underneathe the fresh water lakes on North Stradbroke Island can be dated between 119 to 132 thousand years ago. It is second to Fraser Island as the largest sand island in the world, 38km long and 11 km wide. It is also the largest sand dune island in a chain that forms a protective barrier around Moreton Bay.
In the geological past, North Stradbroke Island was connected to the mainland via land bridges. These bridges enabled the spread of many plant species. Over time, sea level rise gradually isolated the island. The plants spread out and diversified, forced to adapt to the islands unique geomorphology and climate. Today, the island harbours approximately 450 plant species.
North Stradbroke Island represents a range of ecosystems. For example, there are about 6 sand systems, some dating back as far as 120 thousand years ago. Sand dunes form when wind blown sand is trapped by vegetation to form ridges. These ridges protect the small gullies below. Beach Spinifex (Spinifex sericeus), Pig Face (Carpobrotus glaucescens) and Beach Primrose (Oenothera drummondii) are species common to the outer edge of a dune community and play an important role in stabilising the sand.
Beach spinifex are perfectly suited to the dynamic environment of a dune, their structure and function have been shaped by its harsh conditions. But you might wonder how any plant could survive in such a nutrient poor environment like a sand dune. The reason? Its symbiotic relationship with a fungi species. These fungi grow all over the root system of the Spinifex. They fix the nutrients (except nitrogen) covering small grains of sand and pass these nutrients onto the Spinifex. This relationship is similar to coral species’ partnership with zooxanthellae (a microscopic algae that shares the products of photosynthesis, sugar, with its coral host).
Coastal Wattle (Acacia sophorae) and Coastal Sheoak (Casuarina equisetifolia) grow in dense communities and are protected by the sand ridges that form along the dunes. Coastal Wattle is an Australian native spreading shrub, considered to be a pioneer species that populates coastal sand dunes. Its success is based on its association with a nitrogen fixing soil bacteria called Rhizobium. The bacteria are present in root nodules, a formation on the roots initiated by the bacteria. The Rhizobium fix the atmospheric nitrogen present in sediment and make the nitrogen available to the shrub in a form the plant can absorb. In exchange, the Rhizobium receives the products of photosynthesis, sugar, from the plant. Coastal Wattle usually grows to a height of 3 metres and can spread out to between 10 to 15 metres.
Coastal Sheoak also fix nitrogen in the sediment through its symbiotic association with a species of fungi called Mycorrhizal. Sheoak grow quite densely in dune communities and probably owe their success to their ability to use leaf litter to their advantage quickly.
Sand dune ecosystems like the ones located on North Stradbroke Island are an amazing example of how the plants exposure to the elements has affected their adaptations to survive. The dune communities highlight their fragility and emphasise how important plants are to the health of marine and terrestrial ecosystems.
Written by Gabrielle Ahern
Salty Wave Blue – Into all things ecology.
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Stradbroke Island – Offical Website http://stradbrokeisland.com
Australian Plants – A Simple Botany of Wattles http://anpsa.org.au/APOL8/dec97-1.html
Australian National Herbarium – Australian Fungi http://www.cpbr.gov.au/fungi/mycorrhiza.html