Academic Reading Practice Test 1

For most of the twentieth century the beaver was a footnote in European natural history. Trapped relentlessly for its fur and for castoreum, a secretion once prized by perfumers and apothecaries, the animal had been squeezed into a handful of protected pockets in Norway, France and the Russian interior. By 1900 the entire continental population may have numbered no more than twelve hundred. A century of legal protection and deliberate reintroduction later, the figure is comfortably above a million. What has surprised biologists is not the recovery itself, which was planned and paid for, but the addresses the animals have chosen.

Beavers are now regularly recorded inside city limits. There are established colonies on the Danube in Vienna, on drainage channels in the Dutch polders, and on a stretch of river in Munich that runs between a tram depot and a supermarket car park. In 2019 a pair built a lodge within sight of a motorway junction outside Glasgow. The conventional expectation was that a shy, largely nocturnal rodent would retreat from human activity as it recovered. Instead it has done the opposite, and the reason appears to be straightforward: cities are full of the things beavers want.

The first is water that does not disappear. Urban rivers are heavily engineered, and one consequence of that engineering is unusual reliability. Storm drains, treated outflows and impermeable surfaces keep water moving through a city channel in dry months when a rural stream of the same size would be reduced to a trickle. The second is food. Municipal planting favours fast-growing softwoods — willow, poplar, alder — precisely the species a beaver prefers, and these are planted along exactly the riverbanks the animal wants to occupy. The third is the absence of predators. Wolves and lynx have not returned to European cities, and the risks that remain are traffic and dogs, both of which a nocturnal animal can largely avoid.

The results are not uniformly welcome. A beaver's defining behaviour is the construction of dams, and a dam is an obstruction. Where a rural landowner may lose a corner of a field to flooding, an urban authority can lose a culvert, a footpath or a section of railway embankment. Prague has removed dams from a tributary of the Vltava on several occasions; each time the animals rebuilt within a fortnight. The cost of repeated removal has led several municipalities to conclude that it is cheaper to redesign the infrastructure than to fight the animal, and a small industry of mitigation devices has grown up around the problem. The best known of these is the flow device, a pipe laid through a dam that maintains a set water level while leaving the structure standing. Because beavers locate breaches by the sound of running water, and a submerged intake makes no sound, the animals typically do not repair the leak.

Set against these costs is a growing body of evidence that beaver activity delivers benefits a city would otherwise have to buy. A dammed channel slows the passage of water through a catchment, which reduces the peak of a flood downstream. Studies on several small English catchments have recorded reductions in peak flow of between thirty and sixty per cent on reaches occupied by beavers. The ponds that form behind dams trap sediment, and with it the nitrates and phosphates that wash off roads and gardens, so water leaving a beaver-modified reach is measurably cleaner than water entering it. The wetland mosaic that results — open water, dead standing timber, silted margins — is habitat for amphibians, dragonflies and wetland birds that a straightened concrete channel cannot support.

Quantifying this is harder than it sounds. Almost all the flood-attenuation figures come from small, well-instrumented rural catchments, and it is not obvious that they scale. A city catchment is dominated by hard surfaces and pipes, and the fraction of the total flow a beaver dam intercepts may be small. There is also a timing problem: dams reduce peak flow reliably in moderate rainfall, but in an extreme event a saturated pond has no storage left to give, and a dam that fails at the wrong moment can make matters briefly worse rather than better.

What is not in dispute is that the decision has already been made for us. The animals are present, they are breeding, and eradication is neither legally available nor publicly acceptable in most of the countries concerned. The practical question facing urban river managers is no longer whether to have beavers, but which of their works to tolerate, which to modify, and which to price into the maintenance budget.