雅思阅读第016套P2-WATER HYACINTH: BEAUTIFUL YET DESTRUCTIVE

WATER HYACINTH: BEAUTIFUL YET DESTRUCTIVE

1. Despite possessing vibrant purple flowers and being attractive to the eye, the water hyacinth has often been referred to as the most problematic aquatic plant in the world’s waters. Due to its aesthetic appeal, water hyacinth, which is native to South America, has been distributed to many different regions and now thrives in the southern states of the USA and many subtropical and tropical locations. It has also been observed to be relatively tolerant of cooler climates and is routinely sold as an ornamental plant for domestic use in a number of horticulture centres.
2. Though the hyacinth species is distinctive in appearance, another aquatic floating plant – water lettuce – is sometimes mistakenly identified as water hyacinth. Water lettuce, however, does not have the same attractive flowers, has larger leaves and is less tolerant of cooler climates. Water hyacinth has rounded waxy, green leaves which grow up to around 6 inches in width and floating leaf stems which grow up to 12 inches in length. Flowers are typically between 2 to 3 inches in width and as many as 15 flowers, each purple on the outside and containing a yellow centre, may grow from each plant.
3. Many of the problems associated with the water hyacinth are due to its incredible growth and reproduction capabilities, which have made it difficult to control and allow it to quickly dominate the environment in which it grows and spreads. Its growth patterns are characterised by a rapid formation of an impenetrable vegetation mass; botanists say that one plant can produce around 5000 seeds and in one study two plants were observed to produce 1200 plants in as little as 4 months. Following nature’s usual pattern, water hyacinth seeds are distributed outside of the immediate area by birds, fauna, wind and water currents, facilitating growth in surrounding areas previously free of the plant.
4. Domination of environments by water hyacinth populations has a number of negative implications. For humans, difficulties may be faced in getting boats through areas of rivers and lakes where the plant is present and fishing and swimming opportunities may be limited. However, the implications for the ecosystem of the immediate environment may be of even greater concern. The density of the mass of water hyacinth populations can prevent adequate amounts of sunlight and oxygen reaching the water: as a result, significant numbers of fish may die, other species of plant growing below water level are compromised and the ecosystem of the immediate area can therefore become unbalanced. Furthermore, the conditions created by the presence of water hyacinth, while detrimental to most forms of life, are perfect for encouraging growth of deadly bacteria often found in poorly oxygenated areas of water.
5. In the southern states of the USA, in Florida in particular, water hyacinth is now under maintenance control. The plant population can be limited in a number of ways: including use of herbicides, clearance equipment and bio-control insects. However, efforts to minimise the population of water hyacinth need to be continual and consistent; experts warning that unless control methods are upheld, the problem can easily reoccur. Some say inattention for as little as a twelve month period would allow numbers to quickly return to infestation level; hardly surprising given that the species is known to be able to double in as little as 12 days.
6. Water hyacinth is thought to have been introduced into Africa in the 1800s; its presence at Lake Kyoga was first identified in 1988 and at Lake Victoria in 1989. In the mid 1990s, water hyacinth was estimated to dominate 10% of the latter lake’s waters. However, by 1998, the plant was almost completely eliminated from East African waters; this being achieved predominantly by the use of bio-control insects, in this case snout beetles, a type of weevil which feeds only on the water hyacinth species of plant. Tens of thousands of the weevils were distributed throughout the lake areas of East Africa, their habit of feeding on the leaves and laying their eggs in the plants’ stalks eventually causing the plants to die and sink to the bottom of the lake. In addition, the plant population was removed using mechanical clearing equipment and by hand with the help of a machete.
7. Despite earlier success, however, negative repercussions of human activity have caused the return of water hyacinth to East African waters. Uganda’s Lake Kyoga, has recently once again experienced problems with infestation. Sewage and agricultural waste making their way into the waterways and thereby creating an excess of nutrients in the water have been the main contributing factors to the re-emergence of water hyacinth. In addition, high levels of nitrogen in rainfall, which enters the water cycle from the smoke created by wood burning cooking fires used in the region, also serves as nutrition to the increasing plant population. Restriction of human activity on lakes such as this, caused by the infestation of water hyacinth has enormous implications; villages such as Kayago, which is in close proximity to the lake, are often almost completely dependent on fishing activity for their economy and food source.
8. While the infestation of water hyacinth in Lake Victoria at the time of writing stands at 0.5%, far below the 10% level experienced in the middle of the 1990s, experts fear that growth could once again become out of control. The main concern is that, as a result of changing weather conditions, the activity of the snout beetle weevils may be less effective than in the past. The region around Lake Victoria has experienced an extended period of drought and while the water hyacinth is capable of living and reproducing both in lakes and surrounding dry land, its predator, the snout beetle can only survive on water. Plant populations growing in lakeside locations are therefore under limited threat from the insect brought in to control them and are consequently able to reproduce in relative freedom.

SECTION 2: QUESTIONS 14-26

Questions 14-18

Reading Passage 2 has eight sections A-H.

Which paragraph contains the following information?

Write the correct answer A-H in boxes 14-18 on your answer sheet.

14 ______________ Why the use of bio-control insects may be less successful than the first time.

15 ______________ The implications of reducing commitment to control of plant populations.

16 ______________ The problems water hyacinth causes other species of plants and animals.

17 ______________ How human activity has contributed to the problem

18 ______________ Reasons water hyacinth is found in many parts of the world

Questions 19-23

Classify the following features as characteristics of

Write the correct letter A, B, C or D in boxes 19-23 on your answer sheet.

19 ____________ has aesthetically pleasing purple flowers.

20 ____________ does not thrive well in colder temperatures.

21 ____________ has leaves which are on average 2-3 inches in diameter.

22 ____________ is commonly found in water.

23 ____________ produces flowers made up of more than one colour.

Questions 24-26

Do the following statements agree with the information given in reading passage 2?

In boxes 24 – 26 on your answer sheet write:

24 ________________ Presence of dense water hyacinth populations can encourage the development of certain harmful forms of life.

25 ________________ The current problem of dominance of water hyacinth on Lake Kyoga is less serious than in the 1980s and early 1990s.

26 ________________ Sewage and waste created by farming have had more of an impact on the return of the water hyacinth population in Uganda than nitrogen- rich air.

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