IELTS Academic Reading Practice Test 454

Approximately 1,600 kilometres from any ocean, surrounded by the Siberian interior, lies a body of water containing roughly one-fifth of the planet's unfrozen freshwater. Lake Baikal descends to more than 1,600 metres at its deepest point and is frequently characterised as the most ancient lake on Earth, having originated in a continental rift valley approximately 25 million years ago. Throughout several months annually its surface solidifies into ice exceeding a metre in thickness, while snow persists on adjacent hillsides for much of the year. At first consideration, therefore, it appears an improbable habitat for a seal.

Nevertheless, the lake sustains a substantial population of Pusa sibirica, the Baikal seal, known locally as the nerpa. It remains the sole seal species occupying fresh water exclusively; the Saimaa and Ladoga ringed seals of northern Europe, the only other seals restricted to lakes, are generally classified as subspecies of an otherwise marine animal. Mature adults measure approximately 1.3 metres in length and weigh between 50 and 70 kilograms, with males marginally larger and darker. Beneath the epidermis lies a thick stratum of blubber, insulating the animal against water close to freezing point.

The ancestry of the nerpa has never been conclusively established. Seals are marine mammals, whereas Baikal lies deep within a continent. The explanation most frequently advanced is that the ancestors of the Baikal seal entered the lake from the Arctic Ocean during one of the colder episodes of the Pleistocene, when the geography of northern Asia differed considerably from that of today. A route along the Yenisei and its tributary the Angara, which flows out of the lake, is considered plausible, and the Lena has also been proposed. Some investigators suppose that the seals were subsequently trapped as the climate ameliorated; others maintain that the lake may have been colonised on more than one occasion. Genetic analyses indicate that the population has remained isolated for a considerable period, but they have not resolved the route.

Open water remains available for much of the year, but from approximately January until May the lake is covered by ice, and this seasonal constraint shapes the seals' existence. Each animal maintains several breathing holes by scraping at the ice with the claws of its fore flippers, and the margins of these apertures become worn and smooth through repeated use. In late winter, females haul themselves onto the ice and give birth inside a lair hollowed out of the snow overlying it. The single pup is born with white fur, which conserves warmth and makes it difficult to distinguish against the snow. The pups remain in the lair until the ice begins to break up.

The diet is dominated by fish, above all by the golomyanka, an unusual species found nowhere else. Two forms inhabit the lake, both with bodies containing an exceptionally high proportion of oil and very little bone, and both spending daylight hours at great depth before ascending towards the surface at night. Seals feed upon them and upon several varieties of sculpin, another group restricted to Baikal. Hunting is accomplished by diving, often for several minutes at a time, and the animals have been recorded at depths of hundreds of metres.

People have hunted the nerpa for centuries, utilising its meat for food, its fat for lamps and its skin for clothing. Hunting was organised on an industrial scale during the twentieth century, and in the middle of that century apprehension concerning declining numbers resulted in restrictions. Quotas were subsequently raised and lowered again, and the scale of the catch remains contentious: proposals to permit more hunting have provoked protests from conservationists and objections from scientists who emphasise that the population cannot be counted with confidence.

The greatest long-term danger is probably the loss of ice. Several investigations have reported that the ice season on Baikal is becoming shorter and the ice thinner, and that the trend is expected to persist as the region warms. A shorter season affords pups less time to grow before the ice disintegrates, and it may also affect the fish upon which the seals depend. The consequences for the wider food web remain uncertain.

Snow matters as much as ice. A lair provides insulation and conceals the pup from predators, and a female will abandon it if she is disturbed. If rain falls on the snow before the pup is capable of swimming, the roof of the lair may collapse, leaving the young animal exposed to cold and to foxes and birds of prey. Years of poor snow cover are thought to be followed by lower survival among pups.

Counting the animals is not straightforward. The principal method is to count them from aircraft in spring. At that season seals haul out on the ice to moult and gather in groups that can be photographed. The figures must then be corrected for the animals in the water, which cannot be seen, and nobody knows what proportion that is. Estimates published in recent decades have ranged from approximately 80,000 to 120,000, a range wide enough to make the setting of hunting quotas difficult.

Seals are curious and display little fear of people. They frequently follow boats, raising their heads to observe the passengers, and in summer they can often be seen lying on rocks along the shore. Why they should be so inquisitive is not known.

In April and May, across much of Europe, the call of the common cuckoo is one of the sounds of spring. The familiar two-note call, borrowed by poets and clockmakers, is made by the male. The bird itself is seldom seen: it is a shy, grey, hawk-shaped creature that slips between hedges and trees. What makes it remarkable is not its appearance but its domestic arrangements. The female common cuckoo will never build a nest, incubate an egg or feed a chick. Instead she lays her eggs, one at a time, in the nests of other birds, and leaves the entire business of parenthood to them.

The common cuckoo is one of roughly a hundred bird species, about one per cent of the world total, that are obligate brood parasites, meaning that they cannot raise their own young. Brood parasitism has evolved separately in several unrelated families, among them the honeyguides of Africa and the cowbirds of the Americas. The arrangement has obvious advantages.

A female is freed from the weeks of incubation and the months of feeding that occupy most small birds, and she can put her energy into producing more eggs, spread across many different nests. The costs fall on the hosts, which spend weeks of effort on a chick that is not theirs and, in the process, often lose their own brood.

Success depends on timing. A female may spend days watching the comings and goings at a potential host's nest, learning when the owners leave and what stage their clutch has reached. She needs to lay while the nest is unguarded and while the host is still laying, so that her egg will be incubated alongside the others. The visit itself is brief: the egg is laid in a matter of seconds. In many cases she has also removed one of the host's eggs, which she may eat or carry away, so that the clutch does not contain more eggs than the host expects.

Different females specialise in different hosts, and each female's eggs resemble those of the species she targets in colour, size and the pattern of spots. A cuckoo that parasitises reed warblers lays a greenish, speckled egg, while one that uses meadow pipits lays an egg closer to the pipit's own. These lineages, known as gentes, are inherited through the female line, since it is the female who chooses the nest, and a daughter inherits her mother's host preference along with the appearance of her eggs. Hosts, in turn, reject eggs that look unlike their own, so the pressure on the parasite to match is continuous.

Hatching is timed to the cuckoo's advantage. Its incubation period is short, around twelve days, so the young parasite usually emerges before the host's own chicks, while they are still eggs. The newly hatched bird is blind and naked, yet within hours it begins to work a hollow in its back under the eggs and chicks in the nest and to push them over the rim, one by one. Chicks reared in isolation perform this as readily as any other, so the behaviour requires no instruction and no example.

Left alone, the cuckoo chick then turns to persuasion. Its begging call is extraordinarily rapid and loud, and recordings have shown that it resembles the calls of a whole brood of host chicks rather than a single one. The hosts respond as if they were feeding several mouths, working harder than they would for their own young, and the chick grows far beyond the size its foster parents could ever have produced. After it leaves the nest it continues to be fed, for as long as several weeks, by birds a fraction of its size.

Hosts are not helpless. Many species inspect their clutches and eject an egg that does not match, and some abandon a parasitised nest altogether. Others attack any cuckoo that comes near, gathering around it and calling until it leaves, and there is evidence that experienced birds respond faster than young ones, which suggests that they learn. Some researchers have argued that the female cuckoo's barred grey plumage, reminiscent of a sparrowhawk, delays the attack for the few seconds she needs.

The struggle has produced some strange tactics. In Spain, great spotted cuckoos parasitise magpies, and researchers noticed that magpies whose nests had been cleared of the cuckoo's egg sometimes found their own eggs destroyed afterwards. The cuckoos, it seemed, were returning to punish rejection, so that a magpie which accepted the foreign egg did better than one that resisted. This 'mafia' hypothesis is still argued over, and the behaviour has not been demonstrated in the common cuckoo.

Much remains uncertain. Egg mimicry is often good but rarely perfect, and hosts vary in how strictly they check; it is not clear what keeps the two sides in balance. Nor is it understood why some host populations accept cuckoo eggs far more readily than others a few hundred kilometres away. The answer may differ from species to species: a host that raises a single brood a year may lose more by rejecting an egg than one that can try again. What is certain is that the relationship is not static. Each improvement in the parasite's disguise is met by a sharper eye in the host, and the contest, in evolutionary terms, never ends.

Urban agriculture is an awkward category. It includes allotments and community gardens, vegetables grown on rooftops and in window boxes, herbs produced in glasshouses at the edge of a city, and salad leaves grown indoors under lamps in racks several metres high. What these activities have in common is only that food is produced within or close to a built-up area. Whether such growing deserves public money has become a surprisingly bitter question, and it turns, in the end, on what the money is meant to achieve.

Supporters make a wide range of claims. Food grown locally is fresher and travels less far: a lettuce harvested a few kilometres from the shop that sells it does not need to be cooled and flown across a continent. Urban growing, they add, puts green space into streets that have little, absorbs rainwater that would otherwise overload drains, and can cool the air on hot days. Community gardens, in this account, are places where neighbours meet, where children learn that carrots grow in soil, and where people who have moved to a city from the countryside can keep a skill that matters to them.

Sceptics reply that the arithmetic of feeding a city does not support these claims. A city of several million people consumes thousands of tonnes of grain, meat and dairy produce every day. Even generous estimates of what could be grown on its roofs, parks and vacant land suggest that such production would supply only a small percentage of residents' calories, and that it would be dominated by high-value crops such as salad leaves and herbs rather than staples. From this perspective, money spent on urban plots would do more good if it protected farmland on the city's edge, where yields are higher and costs lower.

The most conspicuous form of urban farming is the vertical farm, in which crops grow in stacked layers, usually without soil, under artificial light in carefully controlled temperature and humidity. Such installations produce herbs and salad leaves reliably, all year round and without pesticides, using very little water. But they consume large amounts of electricity, mainly for lighting, and the crops that can be grown profitably in them are limited to those that are light, quick-growing and sold at a high price. Several prominent vertical-farming businesses have failed in recent years, and the failures have been blamed on energy costs and on the difficulty of competing with produce grown in fields.

Economists who have examined the question tend to begin with a simple test: public money should go where it produces the greatest benefit, including benefits for which markets do not pay. If the aim is a more secure food supply, they argue, the money would achieve more if it were spent on storage, transport links and agricultural research than on rooftop plots. If the aim is a greener and more sociable city, then the money should be judged as spending on parks and community facilities rather than as agricultural investment. What troubles them about much urban-farming advocacy is that it mixes the two arguments, claiming the social benefits of a garden and the food-security benefits of a farm, without evidence for either.

The social case is nonetheless stronger than the sceptics allow. In Britain, waiting lists for allotments in some cities run to years, which is at least a sign of demand. Community gardens have been created on derelict land in poor neighbourhoods and are run by volunteers, and there is evidence that gardening improves diet and mental health among participants, though such studies are small and often depend on self-reported data. A school that keeps a garden can teach biology, nutrition and cooking in ways that a worksheet cannot. None of this justifies subsidising commercial salad production, but it does justify supporting particular kinds of urban growing with modest sums.

Singapore is the example most often cited of a government taking urban farming seriously. The city-state, which imports more than 90 per cent of its food, has set a target of producing 30 per cent of its nutritional needs locally by 2030, and has funded research and rooftop and high-rise farms. Yet even there the strategy accepts that most food will continue to be imported, and the local production that has expanded fastest is of vegetables and fish rather than of the staple foods that occupy most of a diet. Singapore's circumstances, moreover, are unusual: it is a wealthy island with almost no farmland.

A further difficulty is the condition of urban soil. Land that has been built on, used for industry or covered with rubble may be contaminated with lead and other metals, and vegetables grown in it can be unsafe, particularly for children. Raised beds filled with imported soil solve the problem but add to the cost. Planners must also weigh food production against housing, and in cities where rents are high the case for using scarce land to grow lettuce is weak.

My own view is that public money for urban farming is justified, but only if it is spent on the right things and defended for the right reasons. It should go to community gardens and allotments, to school gardens, and to the advice and training that keep them running, and it should be concentrated in neighbourhoods where green space is scarce and incomes are low. It should not go to energy-intensive indoor farms selling salad to supermarkets, however impressive their technology. The argument for the money should be honest: these are public spaces that produce food, not a substitute for the countryside. Judged as social infrastructure, urban growing is good value; judged as a food-security strategy, it is mostly a distraction.