IELTS Academic Reading Practice Test 478

Stretching for thousands of kilometres across three Australian states, the Dingo Fence is among the longest structures that people have ever built. It runs from the Darling Downs in southern Queensland, through western New South Wales, to the Great Australian Bight in South Australia, and its purpose is simple: to keep dingoes out of the sheep country to its south. Australians usually call it the Dog Fence, because it is not one barrier but a chain of connected fences maintained by different authorities.

The dingo is the largest land predator in Australia. It has often been classified as a subspecies of the grey wolf, though its status is disputed, descended from animals carried to the continent by seafarers from Asia several thousand years ago, and it is equally at home in desert, tropical forest and alpine country. Dingoes hunt kangaroos and wallabies, but they also take livestock, and a pack can kill a number of sheep in a single night. For a flock owner, the losses are not only the animals killed: sheep that are repeatedly disturbed by predators breed less successfully and produce fewer lambs.

Sheep arrived with the first European settlers, and by the middle of the nineteenth century the industry had spread far inland. As flocks moved into drier country, attacks by dingoes became a serious economic problem. Colonial governments offered bounties for scalps and pastoralists organised hunts, but neither method produced lasting control, because dingoes breed quickly and can travel long distances in search of food. Fencing seemed the only reliable answer, and in the 1880s several colonies began to build long barriers.

The first of these barriers were intended mainly to stop rabbits. Rabbits had spread across the continent with devastating speed, and wire-netting fences were erected to protect pastures. Pastoralists soon noticed that the same fences also excluded dingoes, and the netting was modified accordingly. Long stretches were in place by the early twentieth century, and from the 1940s the fences of the three states were gradually linked until they formed a continuous line. The present route, however, has changed more than once, as sections were moved to follow water, roads or new grazing land.

In its standard form the fence is about 1.8 metres high. Wire netting is stretched between timber or steel posts, and the mesh continues below ground level as a skirt, which discourages animals from digging underneath. Gates allow vehicles and stock to pass, and where the fence meets a road or a railway it is carried over or under the obstacle. Its total length is usually given as more than 5,500 kilometres, although no single figure is exact, since sections are added, abandoned or rerouted from year to year.

Maintenance is the great difficulty. The fence crosses some of the driest and most sparsely populated land in the country, where annual rainfall may be under 200 millimetres. In Queensland and New South Wales the work is shared between landholders, who are responsible for the sections on their own properties, and government-appointed inspectors who patrol the line and check that it is sound. Repairs are needed constantly: floodwater washes away posts, sand buries netting, kangaroos collide with wire, and rust attacks joints. Each state manages its own portion, so the condition of the fence varies from one region to another.

Where the fence is well maintained, it works. Dingoes are largely absent from the sheep country of South Australia and western New South Wales, and lambing rates there are much higher than they would otherwise be. Yet the barrier has never been completely effective. Dingoes can dig, climb and squeeze through small gaps, and floods occasionally open long stretches. Moreover, the animals south of the line are often not pure dingoes but wild dogs of mixed ancestry, descended from domestic dogs that escaped or were released; these hybrids breed more freely and are harder to control than dingoes.

In recent decades ecologists have become interested in the fence for a different reason. The dingo is an apex predator, and there is a growing body of evidence that it suppresses smaller predators such as the red fox and the feral cat. If that is so, the fence may be doing more than protecting sheep: by excluding dingoes from the south, it may be allowing foxes and cats to flourish there, with consequences for small native mammals and ground-nesting birds. Studies comparing land on either side of the barrier have reported striking differences in the numbers of some species, although rainfall and grazing pressure make firm conclusions difficult.

The fence also interrupts the movement of animals that are not its target. Kangaroos and emus gather in large numbers where it blocks a familiar route, and they can damage the netting as they try to cross. Some birds and reptiles become entangled in the mesh, and populations of less mobile species on either side have diverged. For farmers the fence is a tool; for conservation biologists it is an unplanned experiment on a continental scale.

Today the Dingo Fence remains a working structure rather than a monument. Sections are still patrolled, repaired and, where necessary, replaced, and the argument about what it should protect — sheep, native wildlife, or both — is far from settled. Few human constructions have shaped the distribution of an animal so directly over so large an area, and few are so little known outside the country that built them.

Most coastal cities expect the sea to come to them. In several of the world's largest urban areas, however, the land is moving in the opposite direction, and often much faster. This gradual lowering of the ground is known as subsidence, and in a few places it is measured in metres rather than centimetres. Mexico City, built on the bed of a drained lake, has sunk by several metres since 1900, and parts of Jakarta now lie below the level of the sea outside their walls. Subsidence is rarely dramatic in itself, but it multiplies the effects of everything else.

The most common cause is the removal of water from beneath the ground. An aquifer is not an underground lake but a layer of sand, gravel or cracked rock whose pores are filled with water, and that water helps to hold the grains apart. Pump it out faster than rain can replace it and the pressure inside the pores falls; the weight above is then carried increasingly by the grains themselves, which rearrange into a tighter packing. The surface drops. The extraction of oil and gas does the same, and so can the weight of new buildings on soft ground.

What happens next depends on what the aquifer is made of. Clean sand behaves a little like a sponge: if pumping stops and the aquifer refills, much of the compaction is reversed and the ground rises slightly. Clay is different. Its particles are flat and its pores are tiny, so water escapes slowly, and once the structure has collapsed it does not spring back. In a city built on thick clay, therefore, pumping produces a fall that is effectively permanent. Engineers therefore distinguish between elastic and inelastic compaction, and it is the second kind that worries them.

Mexico City is the classic example. The Aztec capital stood on an island in a shallow lake, and the Spanish built on the same ground; modern drainage has emptied the lake almost entirely. The clay beneath the city is soft, waterlogged and hundreds of metres thick. The cathedral in the central square has settled unevenly and had to be stabilised, the sewers no longer run at the gradients for which they were designed, and in some districts the ground has fallen by nine metres or more.

Jakarta's difficulties are more recent and more urgent. The city stands on a delta of soft sediments crossed by canals, and for decades its factories and households drew water from shallow wells because the piped supply was unreliable. In the north, where the ground is lowest, subsidence of as much as 25 centimetres a year has been recorded, and flooding has become routine. The government has responded on two fronts, legislating to move the seat of government to a new city on Borneo and building sea defences along the coast.

Other cities show that the process can be slowed. Tokyo sank by several metres in the decades after the Second World War as industry pumped groundwater, and then largely stopped sinking once restrictions on extraction were imposed in the 1960s. Subsidence in Bangkok also slowed after the authorities began charging for water drawn from beneath the city. The lesson is uncomfortable, because it means regulating users who have come to depend on cheap water.

Not all subsidence is caused by pumping. Many deltas are compacting naturally as their sediments settle, a process that rivers once offset by depositing fresh mud in floods; embankments and dams have ended that supply. In cities built on drained marshland, the peat itself is the problem. Once exposed to the air, peat oxidises, and the ground it supported shrinks and is carried away as carbon dioxide. Parts of the Netherlands, which has managed water for a thousand years, now lie lower than they did a generation ago for this reason.

Measuring all this accurately is a comparatively recent achievement. Satellite radar interferometry, usually called InSAR, compares images of the same area taken days or weeks apart and detects movements of a few millimetres, producing a map of a whole city rather than a handful of points. Ground stations using satellite navigation give long, precise records at particular locations, and borehole instruments measure the compaction of individual layers. Together these methods show that subsidence is rarely uniform: one district may be stable while the next drops by centimetres a year.

The consequences are cumulative. A city that has lost a metre of elevation is not merely closer to the sea; it drains less efficiently, so storms flood streets that were never designed to lie below the water table, and salt water pushes into aquifers and rivers. Buildings crack and tilt, pipes break, and roads and railway tracks subside unevenly. Because the damage accrues slowly, it rarely commands the attention that a single disaster would attract, and the cost of repairs is spread across many bodies over many years.

Several questions remain open. It is not always easy to separate the human contribution from natural compaction. The behaviour of thick clay layers is hard to model, and estimates of how far a given amount of pumping will lower the surface vary between studies. Nor is it known how far an aquifer can be refilled before compaction becomes irreversible. What is clear is that stopping the pumping produces results within a decade, which makes subsidence one of the few problems in urban planning that can be substantially fixed.

In October 2019 the climbing route up Uluru, the great sandstone monolith in central Australia, was closed permanently. The decision was taken by the board that manages the national park, on which the local Aboriginal owners hold a majority, and it came after decades of requests from them. In the weeks before the closure, thousands of visitors queued for a final ascent, and photographs of that queue — a thin line of people filing up a rock that the owners regard as profoundly sacred — captured the difficulty of the question. Was this a long-overdue act of respect, or the loss of an experience that had become part of the country's idea of itself?

For the Aboriginal owners, the climb follows a route taken by ancestral beings, and the site's significance does not depend on whether a visitor shares their beliefs. This is the point that arguments about sacred mountains usually miss. A mountain may be holy to one community and merely spectacular to another, and the two descriptions are not rival accounts of the same fact: they answer different questions. Pilgrims have laboured up Adam's Peak in Sri Lanka for centuries, Sherpa communities regard Sagarmatha, the Nepali name for Everest, with reverence, and Mount Kailash in Tibet is walked around rather than climbed. In each case the objection is not to exercise but to treating a place of worship as a playground.

The economic case for keeping such routes open is not trivial. Mountain tourism employs guides, porters, drivers and hoteliers, often in regions where other work is scarce, and a ban can remove an income for which no replacement is obvious. When the Uluru climb closed, some operators predicted a fall in visitor numbers, though the pandemic that followed made the effect impossible to assess. The counter-argument is that a community should not have to tolerate an activity it finds offensive because outsiders profit from it, and much of the money earned on famous routes leaves the region with the companies that organise the trips.

A more principled objection comes from those who regard landscape as a common inheritance. On this view no group should be able to restrict access to a feature of the natural world on grounds that others cannot share, and religious conviction, however sincere, should not override the interests of the majority. The argument has force where a site is claimed exclusively and where access is used to exclude people. It is weaker where the community making the request has cared for the place for generations, and weaker still where the land was taken from that community by force and restored only recently. Critics add that the principle is applied to indigenous claims far more readily than to those of established churches.

Then there are the numbers. High mountains are fragile places. Ground above the treeline recovers slowly from heavy foot traffic. Litter and human waste persist for years in the cold. The rescue services that attend crowded routes are stretched, and Everest's spring season has at times produced queues on the summit ridge, with climbers waiting in the cold for their turn. Mount Fuji, at once a pilgrimage and one of the most popular walks in Japan, now limits the number of people allowed on its busiest trail each day and charges a small fee. Such rules are defended on grounds of safety and conservation, but they have an incidental benefit: a quieter mountain is precisely what many pilgrims say they have come for.

If outright prohibition and unrestricted access are both unsatisfactory, what lies between them? Several arrangements are already in use. Permits ration scarce routes; seasonal closures protect wildlife or the most fragile ground; fees pay for rangers, toilets and rescue teams; and compulsory guides ensure that visitors learn what a place means to the people who live beside it. The schemes that work best tend to be designed with those communities rather than imposed on them, and they usually include a path that allows ordinary visitors to approach the holiest ground without walking on it.

All of this raises the awkward question of authority. Who should decide? Park boards, tourism ministries, international committees and the courts have each claimed a say, and their priorities differ. My own view is that the decisive voice should belong to the community for whom the site is sacred, provided that its decisions can be challenged and that it does not use sanctity as a pretext for excluding people it dislikes. Custodianship is not ownership, and a custodian is answerable for what is kept.

The choice, then, is not between locking a mountain and leaving it open to all comers. It is between arrangements that treat a sacred place as a place of worship and arrangements that treat it as an attraction. Climbing is not the only way to approach a mountain, and the track around the base of Uluru is open to anyone willing to walk it. Respect, sensibly organised, is not the enemy of tourism; indeed it may be one of the things that visitors now come to find.