Academic Reading — Test 58

Reading passage

On the nutrient-poor soils of the tropical island of Borneo, a remarkable group of plants has overturned the usual relationship between flora and fauna. Rather than waiting to be eaten, these plants hunt. They belong to the genus Nepenthes, the tropical pitcher plants, and there are more species of them on Borneo than almost anywhere else on Earth. The reason for their unusual diet lies in the ground beneath them. The soils where they grow are typically acidic and severely deficient in nitrogen and other essential minerals. Ordinary plants struggle to survive in such conditions, but the pitcher plants have evolved a way to obtain these missing nutrients not from the earth but from the bodies of insects and other small creatures. The hunting apparatus that gives the genus its name is a modified leaf. As the leaf grows, its tip extends into a long tendril, and the end of this tendril swells and hollows out to form a vessel, or pitcher, that can hold liquid. The size and shape of these vessels vary enormously between species. Some are no larger than a thimble, while the giant pitchers of certain montane species are large enough to hold more than a litre of fluid and have, on rare occasions, been found to contain drowned frogs or even small rodents. Each pitcher is, in effect, a pitfall trap. An insect that ventures too close loses its footing, tumbles inside, and is unable to climb out again. Attracting prey is the first task, and the plants accomplish it through a combination of colour, scent and reward. Many pitchers are vividly marked in reds and purples, patterns that are thought to draw the attention of insects in the same way that flowers do. Around the rim of the vessel, a structure known as the peristome secretes nectar, offering a sweet reward to any visitor. This rim is the most treacherous part of the trap. When the peristome becomes wet with rain or dew, its surface turns extraordinarily slippery, and an insect feeding there finds that it can no longer grip the smooth, downward-sloping ledge. The waxy inner walls of the pitcher offer no purchase either, so once an insect has slipped past the rim, escape is almost impossible. At the bottom of each vessel lies a pool of liquid produced by the plant itself. This fluid is not merely water; it contains a cocktail of digestive enzymes, including proteases that break down proteins, capable of dissolving the soft tissues of trapped prey. In many species the liquid is also remarkably viscous, behaving rather like a weak glue. This stickiness ensures that struggling insects become further entangled and are dragged beneath the surface, where they drown and are slowly digested. The breakdown of the insects releases nitrogen and phosphorus, which the plant absorbs through the walls of the pitcher and uses to supplement what little it can extract from the soil. Not every species of Nepenthes relies solely on trapping insects, however, and some of the most surprising discoveries in recent biology concern those that do not. Several Bornean species have entered into mutually beneficial partnerships with mammals. One mountain species produces a large pitcher whose lid is positioned at exactly the right distance for a tree shrew to perch upon while it licks nectar from the underside. As it feeds, the shrew deposits its droppings into the vessel below, and the plant harvests nitrogen from this waste. Another species offers a roosting site to small bats, gaining nourishment from their droppings in return for shelter. In these cases the plant has, in a sense, exchanged the role of predator for that of host, obtaining its nutrients without having to kill anything at all. The interior of a pitcher is far from being a sterile death trap. A whole community of organisms, known collectively as inquilines, lives within the fluid and is apparently immune to its digestive effects. Mosquito larvae, tiny crustaceans and specialised insects make their homes there, feeding on the captured prey and, in doing so, accelerating its breakdown. The plant tolerates these lodgers because they hasten the release of nutrients it can absorb. This intricate web of relationships has made the pitcher plants of Borneo a subject of intense scientific interest. Yet many of the island's species are now threatened by the clearing of forests and by collectors who prize the more spectacular varieties, and conservationists warn that some may vanish before their biology is fully understood.