Australia鈥檚 70 000-kilometre coastline is a scientist鈥檚 paradise. It offers immense coral reefs, craggy cliffs battered by Antarctic gales, penguin rookeries and man-eating crocodiles. And although five of every six Australians live near the shore, much of this vast coastline remains uninhabited and virtually unknown. There are two Australias, say marine scientists: the one where everyone lives, and the one where nobody lives, the four points on Australia鈥檚 compass.
THE EAST
MANY 鈥 perhaps most 鈥 Australians have never stood in a mangrove forest. It鈥檚 not hard to see why, even though mangroves line thousands of miles of the continent鈥檚 northern and eastern coasts. Mangroves are not the sort of place most people would choose for a casual stroll.
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鈥淢angrove鈥 is a generic term for what are essentially intertidal forests 鈥 trees and shrubs that have adapted to life with their roots awash in mud and salty or brackish water. A well-developed mangrove forest is not a simple stand of straight-trunked trees. Trunks and branches spider about in all directions, and a tangle of prop roots reaches as high as six metres above the mud. Even so, clambering about on the roots and swinging from branches often turns out to be safer than stepping on the bare mud below, which can be soft enough to swallow someone up to the waist. The tidal creeks where mangroves grow are also home to crocodiles, which helps make a forest trip less than perfectly relaxing.
Not surprisingly, then, the researchers who study mangroves 鈥 the 鈥渕angrovellists鈥, some call them 鈥 are a rather unusual breed. Most are compact and wiry, with weather-beaten faces that show the effects of many weeks in the field. These, after all, are people whose idea of fun is three weeks in a swamp, knee-deep in smelly mud.
Much of the research on Australian mangroves has centred on Hinchinbrook Island on the Queensland coast, not far from the Australian Institute of Marine Science in Townsville. Mangroves grow on all the world鈥檚 tropical coasts, however, functioning as important nurseries for fish and other marine life. But in many populous countries, particularly in Asia, mangrove forests are falling to human development just as rapidly as the more highly publicised rainforests.
Like rainforests, too, relatively little is known about mangroves 鈥 but the mangrovellists find plenty to keep themseIves entranced. 鈥淭he thing that intrigues me most about mangroves is their adaptability. The same species can be a little bonsai form or a huge tree 30 metres high,鈥 says Norman Duke, one of the world鈥檚 few freelance mangrove experts, now living in Townsville. The key to mangroves鈥 many forms, Duke says, has to do with salinity. The uppermost reaches of the intertidal are submerged only at the highest tides. During the rest of the tidal cycle, evaporation concentrates the salt in the water that remains in the soil. In regions where a pronounced dry season reduces rinsing by rainwater, soil salinity can rise to more than five times that of seawater, and nothing grows 鈥 a condition Duke calls 鈥渨et desert鈥. Where tidal waters reach a little more often, a dwarf forest of the most salt- adapted tree species develops; less tolerant species join the mix still lower down the tidal range. The result is distinct zones of diversity.
Besides the tangle of roots and branches, another strange feature of mangroves immediately stands out. 鈥淭his is a highly productive forest,鈥 says Duke. 鈥淭here should be heaps of litter down here, yet the forest floor is almost spotless.鈥 For years, scientists thought the leaf litter was simply swept out to sea by the ebbing tide. But thanks to a simple but ingenious experiment, researchers now know that the true explanation is quite different, and lurks at the bottom of the many golf-ball-sized crab burrows that riddle the forest floor.
Several years ago, Alistar Robertson, a mangrove researcher, fastened string to mangrove leaves, tied the free end of the strings to tree roots, and dropped the tethered leaves on the forest floor for a few hours. 鈥淪ure enough, 90 per cent of the time or more, the leaf would be gone, and you鈥檇 find the line straight down a hole,鈥 recalls Duke. Small crabs 鈥 mostly Sesarma and related genera 鈥 pick the forest floor clean, hauling the detritus down their burrows to eat. The crabs also decimate germinating mangrove seedlings, keeping the forest nearly free of young trees as well.
Both in numbers and in importance, these crabs dominate the fauna of Hinchinbrook鈥檚 mangrove forest. But other animals also live in the mangrove forest. Heard but rarely seen, pistol shrimp make their distinctive cork- popping sounds 鈥 produced by snapping a peg-and-socket device on their claws 鈥 to warn off intruders. Open patches of forest floor are littered with Telescopium snails, looking like slightly undersized ice-cream cones covered with a greenish-brown scum of mud and algae.
At intervals, a mud turret rises from the forest floor, surrounding the burrow of a small goby known as a mudskipper. Unlike most fish, mudskippers are willing to spend some time out of water, and now and then one will flee a visitor鈥檚 approach by flapping away across the mud flat.
To a large extent, however, the fauna of mangrove forests remains unknown. 鈥淵ou and I could go wading along the shores of the British Isles and the majority of the polychaetes we collect would have a name,鈥 says Russell Hanley, a zoologist at the Northern Territory Museum in Darwin who specialises in marine worms. 鈥淗ere, if we spend a day collecting intertidally, we might be able to identify 10 per cent of what we collect. If we went subtidally, we鈥檇 be lucky to identify 1 per cent 鈥 and we might collect 130 to 150 species.鈥 But Hanley, a man with a passion for things that live in mud, doesn鈥檛 get out much any more. A lifetime in the sun has left his scalp ravaged by melanoma, and on doctor鈥檚 orders he must limit his time outdoors.
Overall, Hanley estimates, the northern mangrove forests contain some 500 species of invertebrates, with perhaps 300 of those species still unnamed. Most of them congregate in mounds of sediment churned up by the shrimplike crustacean Thalassina. To Hanley鈥檚 amazement, the mounds harbour not just the usual intertidal inhabit ants such as polychaetes and crustaceans, but also insects 鈥 beetles, moth larvae, fly larvae, and more. 鈥淚ntertidal crickets. Heaps of them. You鈥檒l be pulling apart a mound and you might find a polychaete worm, then the next chamber might have two or three larvae of a native bee. Then you might find a crab. So [insects] are intimately a part of this native system. What鈥檚 really interesting is that the literature doesn鈥檛 mention insects at all. One wonders whether the things were actually absent or whether researchers left them out because they didn鈥檛 expect to find them,鈥 he says.
At the other end of the scale, tidal creeks lined by mangroves are favourite haunts of the estuarine crocodile, Crocodylus porosus, Australia鈥檚 most feared predator. The biggest bull crocodiles stake out breeding territories in the creeks, where they may make a meal of an unwary person.
Mangrove researchers see crocodiles frequently and say they always assume one may be nearby. The researchers take care to avoid unnecessary risks, but few bother to carry weapons to defend themselves. To have any hope of shooting an attacking crocodile in time, a potential victim would have to carry a loaded gun at all times, says Hanley. But ever since he shot a hole in his boat a few years ago when his gun went off accidentally, Hanley has left the rifle at home (see Map).FIG-mg19493701.jpg
THE WEST
AT Hamelin Pool, halfway up the coast of Western Australia, the world looks much as it did over three billion years ago. To one side, the ochre earth stretches away to the horizon below a cloudless, deep blue sky. To the other, where the land meets the sea, wavelets lap gently on barren sand. The water is crystal clear and empty-even of plankton. But there is life hidden here. On close inspection, the black boulders that dot the shoreline prove not to be rocks at all but stromatolites 鈥 colonies of cyanobacteria. These are the descendants of the first primitive organisms to emerge on this planet. It was in places like Hamelin pool, scientists think, that these blue-green algae first emerged three and a half billion years ago and began generating oxygen through photosynthesis, priming the early atmosphere for the explosion of life that followed.
Each stromatolite consists of a living skin of tiny cyanobacteria intermingled with trapped grains of sand and sediment and growing on top of the remains of thousands of past generations. Modern algae are often found mixed in with them.
These living fossils are nature鈥檚 big survivors, says Phillip Playford, former director of the Geological Survey of Western Australia. He discovered Hamelin pool鈥檚 stromatolites in the 1950s as a young geologist mapping the area for petroleum exploration. A decade later, his interest in Western Australia鈥檚 treasure-trove of fossil stromatolites 鈥 the richest anywhere in the world 鈥 led him back to learn more about the biology of their living forms.
鈥淭he reason that Hamelin Pool is the place for stromatolites today is that its salinity is about double that of seawater, and most of the organisms that would eat cyanobacteria and algae are absent,鈥 says Playford. 鈥淎s a result, you鈥檙e getting back to conditions of the Precambrian when you didn鈥檛 have much animal life.鈥
Hamelin Pool stands at the head of Shark Bay, a shallow expanse of water more than 200 kilometres long trapped by a narrow peninsula and a string of islands. A sand bar halfway down the bay prevents tidal currents from flushing its upper reaches, including Hamelin Pool. The area鈥檚 hot, dry climate turns this stagnant region into an enormous evaporating pan where the salt-tolerant stromatolites flourish, safe from the grazing of herbivores and overgrowth by other algae. Although stromatolites also occur in the Bahamas and a few other places around the world, Shark Bay鈥檚 beds are so important that the UN has declared the area a World Heritage Site.
Shark Bay definitely belongs to the second Australia. A long day鈥檚 drive north of Perth 鈥 which is perhaps the most isolated major city anywhere on Earth 鈥 the human population lives in a few scattered sheep stations and a smattering of tiny roadside tourist enclaves. Even the scientists have no permanent base camp on the bay.
Yet researchers come, a few at a time, and not just for the stromatolites. The metre-high ridges of gleaming white 鈥渟and鈥 that line the beach at Hamelin Pool, for example, turn out on closer inspection to consist of tiny clam shells, barely the size of a child鈥檚 fingernail. These clams, Afrocardium erugatum, live in the sediments farther down the bay and wash ashore in drifts during cyclones. Each storm tosses up a random jumble of shells several centimetres deep, but over the weeks that follow, the wind gradually flips all the shells on the surface so that their convex sides face upward. These flipped shells separate the deposits of successive cyclones, and geologists may be able to use them to measure the frequency of cyclones over the past few thousand years, Playford says.
Shark Bay supports a unique set of plants and animals because its geography isolates it, providing a small, unique environment. Most of the coastline鈥檚 biology, however, is set by one factor 鈥 the Leeuwin Current, a stream of warm water that flows southward along the coast in autumn and winter. One of the best places to see its impact is at Rottnest Island, a popular resort and former military stronghold that sits in the current just 16 kilometres offshore from Perth.
The Leeuwin is special because, compared with other currents, it flows backwards. Almost everywhere else, the west coasts of continents are swept by cold currents flowing towards the equator. These cold currents are part of the general ocean circulation, which runs clockwise in the northern hemisphere and anticlockwise in the southern hemisphere. This pattern drives ocean waters westward along the equator, where they pile up against the east coasts of the continents and flow away towards the poles.
Around Australia, however, some of this water also leaks past the islands of Indonesia and trickles southward along the coast of Western Australia as the Leeuwin Current. The current has an enormous impact on marine life all down the coast. Its most dramatic effect is to quash the upwelling of nutrient-rich deep waters, which fertilise rich fisheries on most west coasts. 鈥淚f you look around the world, all the west coasts have a big fishery except Australia,鈥 says Hugh Kirkman, a marine ecologist at the Perth base of CSIRO. 鈥淭here鈥檚 no upwelling here, so the waters have bugger-all nutrients.鈥
The Leeuwin鈥檚 warm water also carries tropical marine life much farther south than usual. At Rottnest Island, for example 鈥 well into what should be temperate waters 鈥 divers can swim through a bed of sea grasses typical of cool waters and suddenly find themselves in the middle of a lush coral reef, complete with brightly coloured moon wrasses and butterfly fish. Just a few kilometres away from the Leeuwin鈥檚 warming influence, coral is scarce and the biota is more typical of the island鈥檚 temperate latitude.
The coral at Rottnest is the Leeuwin Current鈥檚 gift to high latitudes. To see coral reefs at their full flowering on the west coast, however, means a three-hour plane flight from Perth due north over Shark Bay to Exmouth, where the coast begins to bend to the northeast. An hour鈥檚 drive back to the south brings you to Ningaloo Reef, 280 kilometres long and one of the largest fringing reefs in the world. Unlike the Great Barrier Reef off the Queensland coast, which can take a couple of hours to reach by boat, Ningaloo is so close to the shore that at places you can easily reach the reef by swimming out from the beach. Coral reefs don鈥檛 usually cling so close to continental shores, because river runoff clouds the coastal waters with sediment and nutrients that choke the corals. Australia鈥檚 arid western coast has few rivers, however, and inshore waters have almost no nutrients.
Every autumn, Ningaloo is host to a biological spectacular when dozens of whale sharks congregate on the reef. As large as houses, these gentle plankton-feeders allow divers and snorkellers to swim among them freely. Nowhere else in the world do whale sharks gather in such numbers, along with hundreds of manta rays also move in on the reefs off the west coast. No one knows why these giants descend on the reef, but most scientists think they come to feed, taking advantage of an explosion of planktonic life that follows the autumn spawning season.
This spawning season displays a striking feature unique to Australian coral reefs. Eight nights after the March or April full moon, all the corals 鈥 some 200 species 鈥 on Ningaloo and other west coast reefs release their eggs and sperm in a single orgiastic spasm. 鈥淭he night of the coral spawning, the water is red with eggs,鈥 says Ray Masini, a marine ecologist at Western Australia鈥檚 Department of Environmental Protection in Perth. 鈥淭he sea has gone from crystal to milk with sperm. But the next day, it鈥檚 all gone. Unless you were there on the night, you wouldn鈥檛 know. You wouldn鈥檛 see it.鈥 As the eggs develop into embryos, they draw so much oxygen from the water that if wind or tide keeps the sea from flushing new water onto the reef, millions of the reef鈥檚 inhabitants can asphyxiate.
The event puzzles scientists. Corals on the Great Barrier Reef also spawn in a single short period, but in spring instead of autumn. Masini鈥檚 colleague Chris Simpson thinks the odd seasonal pattern is a legacy of the migration of coral in the distant past down the coasts of Australia from their evolutionary origins in the Indo-Pacific. Ancestral corals presumably spawned throughout the summer, as corals in that region still do today. But the autumnal Leeuwin Current would have swept larvae from only the latest-spawning individuals down the west coast of Australia; in contrast, the Eastern Australian Current, the corals鈥 source of transport down the east coast, is strongest in spring and so would carry only the earliest spawners (see Map).FIG-mg19493702.jpg
THE SOUTH
鈥淭HE north is glamorous with coral reefs and mangroves, but the south is truly unique,鈥 says Leon Zann, whose survey of Australia鈥檚 marine environments for the Great Barrier Reef Marine Park in Townsville has given him one of the nation鈥檚 broadest overviews of the coast. Most of the species found in Australia鈥檚 tropics are common to the Indo-Pacific region. In contrast, Australia鈥檚 temperate coasts have been isolated from similar habitats in South Africa and South America ever since the break-up of Gondwanaland about 60 million years ago.
The result is a wonderland for marine biologists. The southern coasts have developed their own flora and fauna, the marine equivalent of the marsupials found on land. In most groups, 80 to 90 per cent of the species are found nowhere else, and even those whose ranges do extend beyond Australia are often shared only with New Zealand. The south coast boasts by far the richest marine flora in the world 鈥 more than 1100 species of macroscopic algae and more than half the world鈥檚 seagrass species. 鈥淔or me it鈥檚 certainly the most interesting part of Australia,鈥 says Graham Edgar, a zoologist at the University of Tasmania who shyly confesses a fondness for a group of mostly flea-sized crustaceans called tanaids. 鈥淚f the water temperatures were a bit higher, the general public would appreciate it more, too.鈥
Despite its biological riches, much of the south coast is unexplored. Ever since Europeans first came to Australia, sailors have shunned the Great Australian Bight 鈥 the broad bay that stretches like a bite out of a biscuit across most of the south coast west of Adelaide 鈥 because its shoreline offers no natural harbours or shelter from Antarctic gales. Indeed, earlier this year a government expedition conducted the first surveys of parts of that coast since the 19th century.
On shore, the bight is bounded by Australia鈥檚 vast Nullarbor Plain, a desert so bleak that continental-scale maps show the location of individual motels along the single highway that traverses it. Any visitor determined enough to reach the head of the bight, however, will be rewarded with a spectacular sight from the clifftops: dozens of southern right whales, clearly visible as they drift along at the water鈥檚 surface, use the area as a calving ground.
The south coast, particularly around Tasmania, is also home to Australia鈥檚 largest colonies of seabirds. The most abundant of these birds, the short- tailed shearwater, numbers in the tens of millions. These shearwaters are long-distance migrants that spend the northern summer in the Arctic, then return to the small islands off the Australian mainland and Tasmania to breed in the southern summer. Unlike most seabirds, they nest underground in burrows excavated from the sandy soil. A million or more birds crammed onto a small islet create a pocked and pitted landscape that looks like it has been worked over by a mob of frenzied prospectors.
Shearwaters are enthusiastic parents 鈥 so much so that by the end of the summer, each pair鈥檚 single chick weighs almost as much as both its parents together. Abandoned when the parents start their northward migration, the pudgy fledglings spend the autumn running, exercising their wings, growing flight feathers, and losing weight for their own long migration.
Although Tasmania is rich in seabirds, its waters have fewer marine species than the rest of Australia鈥檚 south coast, perhaps as a result of climate changes in the geological past, says Edgar. The island projects farther into the cold Southern Ocean than any other part of Australia, which means that its biota includes those species least able to cope with warmer water. When ocean temperatures rise, as they have at least once in the past few thousand years, species adapted to cold water risk extinction because they have nowhere further south to retreat.
Edgar believes that another such sea-temperature rise is happening today. Water temperatures along Tasmania鈥檚 east coast have risen a whopping 1.5 掳C in the past twenty years as nutrient-poor warmer water moves south from the mainland. In that time, biologists have seen population explosions of large Balanus barnacles and the sea urchin Centrostephanus, says Edgar. At the same time, the large kelp Macrocystis 鈥 very sensitive to water temperature and its attendant nutrient levels 鈥 has dwindled. 鈥淭wenty years ago,鈥 says Edgar, 鈥渋t was difficult running a little runabout along the coast because of the kelp. Now most of that is gone.鈥
Even here, so close to Australia鈥檚 densely populated southeastern corner, scientists know too little about the coast to predict whether this change is a harbinger of global climate change or merely part of a natural cycle. And, because so much of Australia鈥檚 marine treasure remains unexplored, biologists do not know how broad an effect the warming has had. 鈥淭here鈥檚 no good distributional data [for most organisms],鈥 says Edgar. 鈥淪o no one can really say.鈥 (see Map)
THE NORTH
WITH a population of 70 000, Darwin is the northern coast鈥檚 only city and the jumping-off place for visitors to Arnhem Land, the remote wilderness at the top of the continent. Arnhem Land is Aboriginal territory, occupied and administered by the coastal people who make up almost all of its few thousand inhabitants. It is also the locus of a vigorous 鈥渟ea rights鈥 movement that aims to win comparable Aboriginal control over the adjoining seas.FIG-mg19493703.jpg
Much of the impetus for the sea rights movement comes from the settlement of Galiwinku, on Elcho Island off the north-eastern coast of Arnhem Land. Air North鈥檚 nine-seat Cessna flies from Darwin to Galiwinku daily. The two-hour journey takes you over some of Australia鈥檚 remotest country, from the mangrove-lined tidal creeks of Kakadu National Park to the eroded escarpments and open eucalyptus woodlands of Arnhem Land itself. The region is dry most of the year, but during the summer rains, lowlands turn to swamp and the whole countryside bursts into green. During the flight, you see barely any sign of human activity.
Galiwinku鈥檚 airport consists of a single paved runway, a metal-sided maintenance shed, a few fuel tanks, and the passenger terminal 鈥 a thatched roof set on poles over a single wooden bench. Ten minutes鈥 walk down the dirt road is the village, a few short streets with simple houses, mostly made of concrete, and often shared by three or four families.
In one such house, facing a stunning sandy beach that proves to be strewn with litter, lives Kenny Djekurr. Djekurr, a powerful, slightly greying man who looks to be in his late 30s, is one of the younger generation鈥檚 leaders in the sea rights movement.
Unlike Europeans, whose notion of habitable space stops at the water line, coastal Aborigines believe their home land includes both land and sea, Djekurr explains. Their songlines 鈥 a combination of creation myth and map 鈥 extend out into the ocean. Many of their sacred places, where events of note happened to their ancestors in the mythical past, are now under the sea (though some experts believe they may have been dry land thousands of years ago when the sea level was lower). Religious leaders would like to be able to keep fishing boats away from these sacred sites.
Animals such as sharks and sea turtles also play an important role in Aboriginal religion, Djekurr says. When these blunder into commercial fishermen鈥檚 nets, they are often simply shot and dumped over the side, where they later wash up on Aboriginal beaches. 鈥淭his hurts the people,鈥 says Djekurr.
To curb what they see as sacrilege, Aboriginal communities are asking the government in Canberra for the authority to manage the waters off Arnhem Land as Aboriginal territory, controlling who could enter the region, where they could go, and what they could do while there. This would give them sea rights comparable to the land rights they have held since 1977, when the federal government passed the Northern Territory Land Rights Act that gave Aborigines control over much of the Northern Territory, including Arnhem Land. Indeed, Djekurr and some other Aboriginal leaders want the government to create a marine park that would exclude commercial fishing altogether.
None of this will happen soon, however. Alter almost a year and a half of meetings along the coast of Arnhem Land, Aboriginal representatives will finally decide late this month exactly what they will ask from the government. A formal proposal will then be presented to the Australian and Northern Territory governments, where it is sure to prompt protests from a variety of groups. 鈥淭he old ones should have gotten sea rights at the same time as they got land rights,鈥 says Djekurr. 鈥淚t would have been easier.鈥 Djekurr expects negotiations to draw out for another five to ten years, and others say it may take a generation or more (see Map).FIG-mg19493704.jpg
