Heron Island on the Tropic of Capricorn
Great Barrier Reef of Australia:
Click on the images to see them full size.
|This is a place of deceptive calm. While tourists enjoy diving, walking and dining in the National Park, the wildlife (including the Pisonia trees!) conducts a savage predator-prey scenario. The trees in this, particularly the forest made up of a nearly pure stand of the sinister Pisonia grandis, qualify as a truly vindictive species (speaking anthropomorphically).|
The photos on this page are to help you orient to the vegetation patterns Island, which is a coral cay. It is the stage on which the marine and terrestrial habitats act out the ancient moves of predators and prey, of hapless dispersal agent and dominant life form.
A reef platform makes up the bulk of the site. The coral cay that is now the Resort and National Park began to form on the northwest corner of the reef platform at about 8,500 years ago.
|The leeward side of the island, with Pisonia forest and a few fringe Casuarinas or beach pines.|
|The Pisonia forest on the leeward side is protected from salt-bearing winds. Wherever there is protection, Pisonias can outcompete the other vegetation by a strategy which produces diversity in the mainland Queensland Wet Tropics rainforest. How this happens to create a nearly pure stand of one species is HERE. Compare this with the conditions which allow one species to dominate a marine habitat at Shark Bay (Shell Beach) HERE.|
|A ship channel was cut to allow access of tourists to the island in the 1960s and it upset the delicate balance of water levels and the beach began to erode. The dark grey of the beach rock shows here. Attempts have been made to wall up the margins of the canal to restore the equilibrium levels of water which keep the cay intact. You can see the beach erosion on the aerial photos as well.|
|Close up of the pattern of channels and coral ridges in the reef front (bottom of picture) and the reef platform (top). These are the major routes of exchange of seawater and biota between the ocean and the reef flat.|
|Looking across the windward side of the National Park portion of Heron Island and its reef to the younger Wistari Reef.|
|The salt-tolerant woodland shows as grayer tones than the Pisonia forest. This island serves as a rookery for hundreds of thousands of birds, as well as being a permanent residence for others such as the reef egrets (give the island its name), the voracious rails, the arrogant silver gulls and the tiny ferocious silvereyes. Nesting usually starts in October and finishes when the birds leave at the end of the summer. Large umbrellas are provided in the rooms: during nesting season they protect from a lot more than rain!|
Wistari Reef at mid tide. The white sand is calcium carbonate particles for the most part derived from marine algae and corals making the reef platform. This is probably what Heron Island looked like about 8,500 years ago, sometime after the end of the last Ice Age. Because it is submerged at high tide, the seeds of land plants brought in by birds and marine currents have not taken hold.
Eventually, enough sand will accumulate so that salt-tolerant grasses start to stabilize the dunes. As the cay builds, larger species which tolerate salt-laden winds will become established, a lens of fresh water will "float" inside the porous structure of the cay on top of a seawater base. Coral cay adapted Pisonias arrive via their own special dispersal by black noddys. It is a relationship where life and death are intimately bound.
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Heron Island Pisonias