Driftwood: IELTS Academic Reading practice test

The floor of the deep ocean covers more of this planet than every continent combined, and almost nothing grows there. Sunlight fails a few hundred metres down, so no plant can live on the bottom, and the animals that do must wait for their food to arrive from above. It arrives as marine snow: a continuous drift of dead plankton, faecal pellets and fragments of jelly, falling for weeks through four kilometres of cold water and largely consumed on the way. What reaches the bottom is a fine dust. The communities that live on it are sparse, slow-growing and, by the standards of a coral reef, close to starving.

Against that background, consider what happens when a whale dies. A carcass of forty tonnes sinks within a few days, and lands on a patch of sea floor that would otherwise receive, in organic carbon, about as much in two thousand years as has just arrived in an afternoon. For a long time nobody had watched what followed. Then in 1987 a submersible working in a basin off southern California came upon the skeleton of a large whale lying on the mud, surrounded by animals that had no business being there, and the study of what are now called whale falls began.

What follows the landing happens in a sequence that has since been documented at dozens of sites. First come the mobile scavengers. Sleeper sharks, hagfish and dense swarms of shrimp-like amphipods strip the soft tissue at a rate that surprised the first observers, removing several tonnes of flesh in a matter of months and leaving a clean skeleton within a year or two. The second stage belongs to the sediment. Scraps of tissue and oily residue enrich the mud for several metres around the bones, and worms, snails and small crustaceans arrive in extraordinary densities to exploit it, often in numbers found nowhere else on the abyssal plain. The third stage is the longest and the strangest. The bones of a whale are up to two thirds fat by weight, and bacteria living inside them break that fat down in the absence of oxygen, releasing hydrogen sulphide. Other bacteria use the sulphide as an energy source, and around them assembles a community of clams, mussels and tube worms that lives, in effect, on chemistry rather than on sunlight. This stage can persist for several decades.

One inhabitant deserves separate mention. In 2002 researchers examining a skeleton in Monterey Bay found the bones covered in small red-plumed worms of a kind nobody had described, which turned out to be doing something no animal was thought able to do: dissolving bone and feeding on what came out. They were named for the habit, and they have neither a mouth nor a gut. Instead they grow root-like tissue into the bone, packed with bacteria that do the digesting on their behalf. The first specimens collected were all female, and the explanation, when it came, was peculiar even by the standards of the deep sea: the males are microscopic, never develop past a larval stage, and live in their dozens inside the tube of a single female.

The communities that gather at the sulphide stage look a good deal like those at hot vents on mid-ocean ridges, and this has produced an appealing hypothesis. Vents are scattered, short-lived and separated by thousands of kilometres of ordinary sea floor, and how their specialised animals get from one to the next is a genuine puzzle. If whale falls support similar creatures, they might serve as stepping stones, allowing a population to cross an ocean in a series of short hops rather than one impossible leap. Genetic work has given the idea partial support at best. A number of genera are shared, but the overlap is smaller than the hypothesis requires, and several of the whale-fall species turn out to be distinct lineages that have been separate for a very long time.

There is an uncomfortable corollary. Commercial whaling in the nineteenth and twentieth centuries removed the great majority of the large whales from the world's oceans, and whatever else that did, it reduced the supply of whale falls by a similar proportion. If a species of bone worm or sulphide-dependent clam existed only on such carcasses, and the carcasses became ten times rarer within a century, then some of those species have probably gone. We have no way of knowing which, because nobody had described them, and there is no method by which an extinction of this kind could ever be confirmed. It is an absence rather than a loss, and it does not appear in any register.

Research now proceeds partly by experiment. Teams acquire carcasses of stranded whales, tow them out and sink them at instrumented sites, then return year after year with cameras and sampling gear. The work is slow and its results arrive on a timescale that funding cycles do not respect. Meanwhile a new pressure is arriving: the nodule fields of the abyssal plain have become commercially interesting, and the machinery proposed to collect them would disturb exactly the sort of sea floor on which these episodic communities depend, at a point where the inventory of what lives there remains substantially incomplete.

The wider lesson concerns how the deep sea works at all. Our ecological intuitions were formed on land and in shallow water, where energy arrives steadily and the question is how it is divided. On the abyssal plain energy arrives almost not at all, and then in enormous, unpredictable parcels, and the organisms that matter are the ones specialised in finding such a parcel, exploiting it quickly and dispersing before it is exhausted. It is an economy of windfalls, and a whale is the largest windfall there is.

Whenever a scientific result is shown to be wrong, a familiar reassurance follows: this is science working as intended, because science is self-correcting. The phrase does a great deal of work. It is the reason a reasonable person is asked to trust a body of knowledge produced by fallible people with careers to build, and it converts the discovery of an error from an embarrassment into a demonstration of health. What is rarely examined is whether the claim is true, and if so, on what timescale, in which fields, and through whose labour.

The question stopped being abstract about a decade ago. A collaboration of several hundred researchers took a hundred studies published in leading psychology journals and repeated them as faithfully as the original reports allowed, with larger samples and analysis plans fixed in advance. Rather more than a third produced a statistically significant result in the same direction as the original, and the average size of the effects was roughly half. Comparable exercises in preclinical cancer biology and in experimental economics returned figures that were better in one case and worse in the other, but nobody has yet run such an exercise and found that most of the literature held up.

The causes are structural rather than personal, and this matters, because a problem produced by incentives is not solved by exhorting people to be honest. Journals have historically published findings and not their absence, so a study that finds nothing tends to remain in a drawer, and the published literature is therefore a biased sample of the research actually done. Samples have often been too small to detect the effects being looked for, which guarantees that whatever does reach significance overstates the size of the effect. And the analysis of a data set involves dozens of defensible choices, each of which can be made after looking at the result, so that a determined and entirely sincere researcher can arrive at a publishable finding from data that contain nothing.

A good deal has been done about this. Preregistration requires a researcher to state the hypothesis and the analysis before collecting data, which removes the flexibility that made the problem possible. Registered reports go further: the journal reviews the design and commits to publishing whatever the study finds, which removes the incentive to find something. The effect is measurable. Where conventional literatures report null results in a small minority of papers, registered reports in the same fields return them around sixty per cent of the time, which is a change in what gets published rather than in what is true.

Not everyone is convinced that the reform movement has diagnosed the illness correctly. Teodor Vasiliev, who works on the methodology of the social sciences, argues that a failed replication is far less informative than it is treated as being. Human behaviour is sensitive to context, and a study conducted in a different country, a decade later, with a different population, is not the same study; where the original effect depended on a condition nobody thought to record, its absence the second time is expected rather than damning. He adds, pointedly, that the reformers now have careers and funding of their own, and that a movement whose product is criticism has its own reasons to find a great deal to criticise.

The first half of this is a serious argument and the second is not. That a critic benefits from criticising says nothing about whether the criticism is correct, and the same logic would dispose of the original findings just as efficiently. The point about hidden conditions, though, deserves an answer, and the answer is that it is only an argument at all when the condition is specified in advance. A moderator proposed after a replication fails, to explain why it failed, can be invented for any result whatever, and a theory protected by such moves has stopped being a theory. The remedy is exactly what preregistration asks for: say beforehand what the effect depends on, and let the replication test that.

Where I think the reassurance genuinely misleads is in its timescale. Over decades and across large questions, science plainly does correct itself: the wrong theories of heat, of disease, of continental fixity all gave way, and none of them gave way because their proponents changed their minds. But the individual finding is a different matter. A clinician deciding next month, a teacher choosing a method for September, a minister drafting a policy this year, all need to know whether a particular result is reliable now, and for that purpose the correction mechanism is far too slow. Retractions take years. A discredited finding continues to be cited, often approvingly, long after the retraction, and the citation rate barely moves.

My own conclusion is that the adjective is wrong. Science is not self-correcting; it is correctable, which is a different and less comfortable property. Correction does not happen automatically, as a consequence of the scientific method operating in the background. It happens when a particular person spends two years and a grant repeating somebody else's experiment, and it does not happen when nobody is willing to fund that, when no journal will print the result and when the work counts for nothing at the next promotion round. The reforms that matter, therefore, are the boring institutional ones, because they decide whether checking is a career or a hobby.