大堡礁 2

The Great Barrier Reef is the largest coral reef on our planet. It's one of the seven recognised wonders of the natural world. The reef itself is the place that most people explore, but there's much, much more. The coral reef is actually a very small part of this underwater world, as little as 7%. The remaining 93% encompasses a variety of other habitats, each one remarkable in its own way. And there are environments on land that are also important to the reef. Some close to shore, others inland. Altogether, there are more than 100 different types of habitat in and around the Barrier Reef, each with its own distinctive plants and animals. There are creatures that you would expect to see on a reef and others that you would not. What links the varied marine and land locations together is a place that has barely been explored, a deepwater lagoon that lies between the coast of mainland Australia and the outer reef. These habitats interconnect, and all are vital to the wellbeing of the Great Barrier Reef. This is the little-known story of one of the most complex and spectacular ecosystems on Earth. The Great Barrier Reef is over 2,000 kilometres long, which means the lagoon that lies between the outer reef and the Australian mainland is vast, an area one and a half times the size of the British Isles. Pick a spot anywhere in the lagoon and you'll likely find sand. For about two-thirds of the seafloor here is a shifting underwater desert. It looks barren, but there is life here. It's just that you don't often see it. Garden eels, and an unlikely alliance, a fish and a shrimp that share a burrow. In a world where most food is out of sight, it takes a predator with special talents to find it. And this is that predator, the ray. The ray's special skill is to find living things under the sand, and the largest species to do this here is the stingray. It finds its prey by detecting the minute electrical fields produced by muscles when they contract, including heart muscles, something no animal can ever switch off. But they have to swim directly over a beating heart to know that it's there. By sucking and blowing, this stingray excavates its target deep under a heap of soft sand. Many rays feeding together produce a series of furrows on the seafloor, and all that puffing and blowing can attract unwelcome attention. The hunter can just as easily become the hunted. This is a stingray's worst nightmare, a great hammerhead shark. It has electroreceptors too. They're spread across the underside of its very broad head, which it sweeps back and forth searching for prey. The stingray has a formidable weapon, a venomous barb. But one hammerhead was found with 96 barbs in its body, and it seemed none the worse for wear. The stingray's first line of defence is to remain very still in the hope that the shark doesn't find it. The commotion cues the other rays to escape, but it attracts other sharks, like jackals it'd kill, black-tipped reef sharks. Dramas like this are played out every day on the floor of the lagoon, but few people are there to witness them. One reason few people visit the floor of the lagoon is that it is relatively deep. It can be up to 100 metres in places, so diving here is not easy. But it is well worth the effort. Dotted across this vast underwater desert are ghostly oases where amazing life forms have taken hold. They are strange, almost alien worlds. And if the place seems strange, many of the creatures living here are even stranger. These oases are created not by plants, but by animals. They're corals, but not the major reef-building corals. Unlike their hard coral cousins, they don't have a chalky skeleton, and they thrive at depths where the light is less intense, capturing food from the water currents with eight feathery tentacles. These soft corals are important because baby fish hide amongst them. Nearly half of all the adult fish on the reef proper grow up in nurseries in the lagoon. They arrive as larvae, swept in from the ocean by the tide. Then, as the tiny fish grow, they hop from one refuge to the next across the floor of the lagoon to reach their final destination back on the barrier reef itself. As long as they hide amongst the corals and seaweeds, they're relatively safe. The danger comes when they break cover. This baby Queensland grouper may be just a few centimetres long now, but one day he'll weigh half a tonne. That's if he lives that long. He can't stay hiding forever. It's dangers like this camouflaged stonefish that force all life down here to find somewhere to hide. A surprising refuge is a highly venomous Stokes sea snake. It's picked up some hitchhikers, baby trevally, and become a mobile nursery. And wherever it goes, they go. With hiding places at a premium, fish will go to incredible lengths to hide down here, and none more so than this. The pearlfish is vulnerable out in the open, so while not feeding, it must conceal itself. But it has a particular taste in hiding places. This is a sea cucumber, which feeds on the seafloor. Its body is basically a living tube. It sucks in sand at one end, extracts anything edible, and passes waste out the other. Just what the pearlfish has been looking for. But not the mouth end. It's drawn to the odour of the other end. Sea cucumbers are repulsive to most predators, so the pearlfish is safe inside. It'll stay there until it's time to feed again. It doesn't harm its host, but the bad news for sea cucumbers is that pearlfish are happy to share their temporary home with others. It seems there's plenty of room for all. The barrier reef we know now is comparatively young. During the last ice age, sea levels were 120 metres lower than they are today, and this whole area was high and dry, and covered in forest. Then, about 10,000 years ago, ice at the poles began to melt, and the seas rose. The great barrier reef we see today grew up, creating the sheltered waters of the lagoon behind it. And during that one great event, another important habitat was formed. As the sea levels rose, it cut off areas of high ground, creating islands. There are 600 of them scattered about the lagoon. Any land animals cut off from the mainland had to adapt or perish. Captain Cook named Lizard Island after an animal that thrived here. A monitor lizard, or goanna if you're a local. When hunting, the goanna is alert to any movement. Only if the grasshopper remains still does it have a chance of escaping the goanna's attention. Balancing on its back legs and tail like a tripod is a trick few other lizards can perform, but it's effective to get to those just out of reach places. The goanna's sense of smell is as important as its sight. Its forked tongue helps it to detect food from a distance. It compares the strength of a smell reaching each of the two prongs and can pinpoint where it comes from. A rotting fish is irresistible. Goannas are usually solitary, but here on Lizard Island they'll tolerate others as long as there's plenty to go around. Small goannas give way to larger ones. Foraging a variety of foods has helped the goanna build a large population on Lizard Island. Adaptability has also helped another larger reptile thrive in these waters. The saltwater crocodile, better known locally as the salty. It's the world's largest reptile and common on the lagoon's islands because it's at home in saltwater and fresh, sometimes swimming far out at sea. So it's not unusual to find a three-metre crocodile hauled out on an island of beach. Salties, though, start life with more modest dimensions. This one-year-old is no more than 30 centimetres long. Hunting at the junction between air and water means there's a good choice of food. Like mudskippers. He'll have to improve as a hunter if he's going to grow into a six-metre giant. And he'd better watch his back. There's a bird about that could easily take a baby crocodile. The white-bellied sea eagle, the second largest eagle in Australia. They can be seen just about anywhere around the lagoon. You're as likely to spot one amongst the trees as you are over the outer reef. So an island is a perfect base. They may be specialised to catch fish, but like the goannas and crocodiles, they're adaptable too. They'll even have a crack at these. Spectacled fruit bats are big. They have a wingspan of about a metre and they roost in island forests as well as those on the mainland. Babies have to cling on tight, but that's the least of a mother's worries. Diving into the tangle of branches and grabbing a bat hanging at its roost site is not an option for a large bird. For the eagles to have any chance, the bats need to be airborne. These are little red fruit bats. During the day, they gather in roosts of up to a million, so they're not hard to find. An eagle's appearance creates panic. Bats are agile flyers, but the eagle's powerful claws gave it the edge. The continental islands are magnets for wildlife, providing shelter, lookout points and hunting grounds. They're oases for life, but they're not the only ones. Below the surface of the lagoon is an island of a very different kind, a shipwreck. This is the SS Yongala, a 100-metre passenger ship. She sank during a cyclone in 1911, and now she sits on the seabed 30 metres deep. 122 people lost their lives, but out of this human tragedy has come an opportunity. 100 years underwater has created something very, very special. The Yongala is regarded by many as the greatest wildlife wreck on Earth. The wreck provides shelter on the featureless plain for more than 120 fish species, this concentration of life feeding on food swept in by the exceptionally strong currents. The living here is so good, many of the young fish stay here for their entire lives rather than move to the outer reef when they grow up. There are sea turtles down here too. This one's a loggerhead. And the wreck is a favourite hangout for another species, a hawksbill turtle, here to feed on the soft corals that coat the wreck. The soft corals are one of the main reasons that all this life is attracted to the Yongala. Almost every available space on its once smooth hull is covered with them. The soft corals are a refuge for millions of small fish, but they must dart out from time to time to feed on the plankton in the current. And wherever small fish gather, it's not long before something bigger turns up to eat them. Queensland gropers are real giants, the largest bony fish on the reef. This Yongala resident is as big as a small car, which is why he's known locally as the VW. His mouth's so big he could swallow sharks and rays whole. But another resident has an even deadlier bite. Sea snakes have more potent venom than many of their land-living relatives, and they put it to good use. The olive sea snake doesn't really look as if it's hunting, but when its small head disappears into a hole, it can trap and paralyse any fish hiding there. It's unusual for so many predators to be swimming so close to each other, but there's so much to eat here. Most of the action is on top of the wreck, where the strongest currents sweep in the most food. For the small fish that live here, to venture into the blue water is a huge gamble, but they've got to do it to feed. If they get it wrong and go too far, that can mean the difference between life and death. The amount of marine life to be found in the hundred metres of the wreck of the Yongala is truly staggering. Arguably, it's a greater concentration of life than on any spot on the barrier reef itself. In contrast to the middle, the landward edge of the lagoon is relatively shallow. Here, bright sunlight can reach all the way to the sandy bottom, where conditions are right for plants to grow. This is not seaweed, but a marine relative of the water lily, known as seagrass. It grows and flowers in vast meadows in clear water surrounding islands and along the shore of the mainland. And it supports a creature that ancient mariners once mistook for mermaids. Gathering in herds of a hundred or more are dugongs, or sea cows. They're relatives of elephants. Each one can weigh nearly half a tonne and munch through 40 kilos of seagrass a day. Dugongs were once abundant, but they've had an uneasy relationship with people. Nowadays, they're trapped accidentally in fishing nets and shark protection barriers, and traditionally Aboriginal hunters target them because they taste good, like prime beef. And recovery of a population is slow. A single baby is born every three to seven years. And this baby dugong may not be old enough to breed herself until she's 17 years old. A combination of all these factors means the population in the southern part of the reef has halved in the last decade. Now there are just 11,000 left in the entire Great Barrier Reef region. Yet there are still more dugongs in Australian waters than any other place on earth. Dugongs are not the only animals to feed on seagrass. Young green sea turtles like it too. He's after the most tender shoots. And this is his exclusive patch. He's very choosy about what he likes and what he doesn't. Like your lawn, seagrass needs to be constantly cut short for healthy growth. So he's actually farming his own little plot that he'll tend for several months before moving on. Seagrass meadows are also nurseries for baby fish, like these domino damselfish. They won't stay here forever. When they're bigger, they won't be able to hide amongst the slender stems, and they'll look for a better hiding place, maybe one even closer to the land. Where the sea meets the coast, saltwater meets the freshwater from rivers and streams. But one doesn't suddenly become the other. The water mixes slowly, creating a world unlike either, but connected to both. It's here that you find a special group of plants that have a profound impact on the ecosystem of the reef. Mangroves. They grow in brackish water, where no other trees can. Twice a day, the tide floods their roots with saltwater. At the same time, water from rivers flows through the mangroves and their lattice of roots acts like a giant tea strainer, slowing it down so that any sediments washed off the land can settle out. Bacteria break down the trapped sediment and other organic material, creating tonnes and tonnes of sticky mud. One teaspoon of mud contains more than 10 billion bacteria. And although there aren't many species that eat mud directly, those that do operate in large numbers. Legions of creatures perform the unglamorous but crucial task of breaking down the gloop. Mud whelks and fiddler crabs both eat the mud, and their waste is the vital product that spawns an entire food chain. Once it's in the water, clouds of shrimps devour it. The nutrients that started in the mud are now swimming around in small, easy-to-catch parcels. All this food makes the mangroves a great place for fish. Small residents, as well as youngsters that will one day move out to the barrier reef. Reef species like these rabbitfish may look relatively big, but they're still youngsters. These snapper also have some growing to do before they leave. And these young trevally will grow up one day and be major predators on the outer reef. The mangroves, seagrass meadows and the soft coral oases are vital nurseries for many fish on the reef, and they all provide two things, food and shelter. Predators, like these young blacktip reef sharks, hide here too. They hunt at the edge of the mangroves, but they can't get deep into the tangle of roots. So the young reef fish are safe, for now, until they make the big move out to the reef. But when they leave the safety of the mangroves, they must cross that sandy desert and the many hazards waiting for them in the lagoon. This is a mantis shrimp. From his neatly kept burrow, he surveys the world with the most complex visual system known to science. He's about the size of a man's forearm, and he has quite a reach. Even if a young fish avoids the dangers on the seabed, there are plenty more predators floating above. Patrolling these inshore waters is probably the most dangerous animal in the lagoon. The box jellyfish, whose stinging tentacles are quite capable of killing a person. This creature is much more than a passive drifter. It can move as fast as an Olympic swimmer, and it has 24 eyes, each complete with a lens that can form a detailed image. It uses its tentacles like a trawl net, and when a fish makes contact, thousands of microscopic stinging capsules explode into the prey's body, flooding it with paralyzing venom. The fish is killed quickly, and then hauled into the mouth on the underside of the bell. The venom that makes them such effective predators also make them lethal to humans. All of the habitats we have seen so far have an obvious connection to the lagoon, but there's one place that couldn't be more different from the underwater world that's critically important to the vigour of the Great Barrier Reef ecosystem. It's the tropical rainforest. These huge tracts of rainforest close to the coast are vital for the reef because they're regulating the flow of water and the sediments it contains. That's important because there's an awful lot of water here. Hot, humid tropical air gives rise to vast amounts of rainfall. It rains for at least 120 days a year, and as much as 60 centimetres can fall in a single day. It's one of the wettest places on Earth. Left unchecked by the forest, the water would wash out sediments and smother marine life. But the waterways flowing off the rainforest are relatively clear, and many are nothing short of spectacular. When sediments are released slowly, the nutrients in them help sustain life in the lagoon so everything flourishes. Where conditions are just right, corals manage to grow right next to the coast. Here, in one unique spot on the coast called Cape Tribulation, pristine tropical rainforest grows right down to the water's edge, a stone's throw from a coral reef. It's a place where two worlds meet, and animals from the reef and the rainforest can be found right next to each other. These footprints belong to one of the biggest land animals in Australia, the cassowary, a 1.5 metre tall flightless bird that's native to these forests. Its claws wouldn't look out of place on a dinosaur. And its kick is so violent that the cassowary has a reputation as the world's most dangerous bird. A parent bird is especially dangerous when rearing a chick. But don't worry, the cassowary is shy and rarely encountered in the wild. It's a key animal here. It feeds on the fruits of rainforest trees and shrubs and is one of the few animals that spread their seeds. The seeds of many forest trees can't germinate without animals like the cassowary. And as the forest is directly linked to the reef by regulating sediments entering the lagoon, the cassowary contributes to the health of the reef itself. But there are now less than 2,000 cassowaries living here, and they're becoming rarer all the time. Before European settlers arrived, much of the coast here was covered in forest. But today, things have changed significantly. Many coastal areas have been cleared for commercial crops, like sugarcane. And it's not only crops. Cattle ranches close to rivers, fish farms, six large coastal cities and many holiday resorts along the coast all have an impact on the reef system. Without the natural vegetation controlling the movement of water and the sediment it carries, heavy rain now washes millions of tonnes of silt, often laced with damaging fertilisers and pesticides, straight into the lagoon. Sediment plumes can be so extensive they sometimes spread to the outer reef. In the seagrass meadows, the fine silt shrouds the light-dependent plants and fertilisers feed algal blooms that choke the life from them. When the meadows die, the turtles, dugongs and baby fish that depend on them die too. Near the shore, the water can be so murky that less light reaches the corals. The rare inshore reefs have all but disappeared. All these habitats are interdependent. If you ruin one, it can have an impact on the others. And that includes the outer reef itself. To look after all these habitats, the Great Barrier Reef Marine Park sits at the heart of a network of protected areas, all interconnected and covering a vast area. After all, the reef's an inspiration to people all over the world. It attracts 1.5 million visitors each year who come to see the largest coral reef on Earth. Together with divers, yachtsmen and anglers, they inject $7 billion a year into the local economy. But the value to Australia is even greater than that. Towns and cities along much of the Queensland coast are not washed into the Pacific Ocean because the reef protects them. It means the barrier reef ecosystem is so much more than a coral reef. There are vast tracts of rainforests, mangrove swamps, seagrass meadows and soft coral oases in the deepwater lagoon. It is truly an extraordinary place. And the reef's influence goes far beyond Australian waters. The barrier reef is vital to wildlife from many other parts of the world. Wave after wave of voyages arrive here from across the ocean, from the islands of the South Pacific, from the Asian mainland and from as far away as the icy seas of Antarctica. These wildlife visitors create some of the most impressive natural spectacles on the Great Barrier Reef. And all this is the subject of the next program.