问答题
5. The mountain yellow-legged frog was once common in the Sierra Nevada Mountains in Southern California, yet its numbers are in such steep decline today that it has been placed on the endangered species list. The reason for its decline has been attributed to three different factors, all of which have combined to devastate the mountain yellow-legged frog population.
The first problem has to do with the introduction of trout into the lakes and streams in the Sierra Nevada Mountains in the late nineteenth century, which disturbed the balance in the ecosystem there. Trout were a nonnative species in that region, and they quickly grew in numbers, primarily by feeding on mountain yellow-legged frog tadpoles and juveniles. Prior to the introduction of the trout, the frogs had no aquatic predators, so it was not instinctive for them to flee from the trout.
A second problem involves the various pesticides that have been utilized to kill pests both on trees and farmland for many years. During periods of rain or snow, the pesticides combine with the water and enter the local water system. They have therefore contaminated the water and wound up killing large numbers of frogs. Examinations of dead frogs have shown that many of them contained excessive amounts of pesticides in their bodies when they died.
A third problem that has affected the environment in which the frog lives is the growth of a deadly fungus in the region. This fungus attacks adult frogs and destroys the keratin in their bodies. Keratin comprises a signification portion of the skeletons and teeth of frogs, so their entire bodies become weakened and result in the frogs dying. Some frogs die as quickly as two weeks after becoming infected with the fungus.
The first problem has to do with the introduction of trout into the lakes and streams in the Sierra Nevada Mountains in the late nineteenth century, which disturbed the balance in the ecosystem there. Trout were a nonnative species in that region, and they quickly grew in numbers, primarily by feeding on mountain yellow-legged frog tadpoles and juveniles. Prior to the introduction of the trout, the frogs had no aquatic predators, so it was not instinctive for them to flee from the trout.
A second problem involves the various pesticides that have been utilized to kill pests both on trees and farmland for many years. During periods of rain or snow, the pesticides combine with the water and enter the local water system. They have therefore contaminated the water and wound up killing large numbers of frogs. Examinations of dead frogs have shown that many of them contained excessive amounts of pesticides in their bodies when they died.
A third problem that has affected the environment in which the frog lives is the growth of a deadly fungus in the region. This fungus attacks adult frogs and destroys the keratin in their bodies. Keratin comprises a signification portion of the skeletons and teeth of frogs, so their entire bodies become weakened and result in the frogs dying. Some frogs die as quickly as two weeks after becoming infected with the fungus.
【正确答案】Sample Note-READING
three reasons mountain yellow-legged frog population has declined
1 trout
-were introduced into frogs' ecosystem→upset balance
-trout feed on tadpoles and juveniles
-frogs lacked instinct to flee
2 pesticides
-enter water system→contaminate water
-kill many frogs
3 deadly fungus
-attacks adult frogs→destroys keratin in bodies
-weakens frogs→can die up to two weeks after getting infected
Sample Note-LISTENING
not sure why frog population is declining
1 trout may not be responsible
-were introduced 100+ years ago
-frogs only started declining 40 years ago
2 pesticides may not be responsible
-strongest pesticide concentrations at lowest levels
-but frogs dying at all levels of mountains
3 frog has protection from fungus
-can cover self with skin secretions
-some frogs die, but many can protect themselves
Sample Essay
The topic of the lecture and the reading passage is the numerous deaths of the mountain yellow-legged frog, which have caused it to be placed on the endangered species list. In the reading passage, three possible causes for the deaths of the frog are proposed, but the professor disregards each hypothesis.
The professor agrees with the author of the reading passage that trout, which are a nonnative species in the Sierra Nevada Mountains, eat lots of frogs. However, she tells her students that the trout and frogs have lived together for a century but the frog population only began declining forty years ago. She therefore disagrees with the reading passage that the trout upset the balance of the ecosystem in which the frogs reside.
Next, the professor again agrees that pesticides which get into the water kill lots of frogs. But she mentions that frogs are dying everywhere on the mountains, not just in their lower levels, which have the highest concentrations of pesticides.
Lastly, the professor discusses a fungus which is killing many frogs. Although she agrees that some frogs are killed by the fungus, she remarks that they have a defense against it since they can cover their skin in a secretion which keeps them safe from the fungus. So she believes something else must be causing the frogs to die.
three reasons mountain yellow-legged frog population has declined
1 trout
-were introduced into frogs' ecosystem→upset balance
-trout feed on tadpoles and juveniles
-frogs lacked instinct to flee
2 pesticides
-enter water system→contaminate water
-kill many frogs
3 deadly fungus
-attacks adult frogs→destroys keratin in bodies
-weakens frogs→can die up to two weeks after getting infected
Sample Note-LISTENING
not sure why frog population is declining
1 trout may not be responsible
-were introduced 100+ years ago
-frogs only started declining 40 years ago
2 pesticides may not be responsible
-strongest pesticide concentrations at lowest levels
-but frogs dying at all levels of mountains
3 frog has protection from fungus
-can cover self with skin secretions
-some frogs die, but many can protect themselves
Sample Essay
The topic of the lecture and the reading passage is the numerous deaths of the mountain yellow-legged frog, which have caused it to be placed on the endangered species list. In the reading passage, three possible causes for the deaths of the frog are proposed, but the professor disregards each hypothesis.
The professor agrees with the author of the reading passage that trout, which are a nonnative species in the Sierra Nevada Mountains, eat lots of frogs. However, she tells her students that the trout and frogs have lived together for a century but the frog population only began declining forty years ago. She therefore disagrees with the reading passage that the trout upset the balance of the ecosystem in which the frogs reside.
Next, the professor again agrees that pesticides which get into the water kill lots of frogs. But she mentions that frogs are dying everywhere on the mountains, not just in their lower levels, which have the highest concentrations of pesticides.
Lastly, the professor discusses a fungus which is killing many frogs. Although she agrees that some frogs are killed by the fungus, she remarks that they have a defense against it since they can cover their skin in a secretion which keeps them safe from the fungus. So she believes something else must be causing the frogs to die.
【答案解析】 [听力原文]
Now listen to part of a lecture on the topic you just read about.
W Professor: Attempts to add the mountain yellow-legged frog to the endangered species list finally ended in success in 2013, but it may be too late for us to prevent the frog from going extinct. One reason is that we're not quite sure what has caused its numbers to decline so precipitously. Without that knowledge, it'll be hard to determine how to go about saving the animal.
There are many who blame the trout living in the waterways of the Sierra Nevada Mountains for the decline of the frog population. While it's true that the trout catch and eat lots of frogs, consider this fact...Trout were introduced to that area more than 100 years ago, but it wasn't until around, oh, forty years ago that the frog population entered into decline. So for more than sixty years, the trout and frogs lived together in a state of equilibrium.
Pesticides, which enter water systems along with rain and snow runoff, are similarly blamed for killing the frogs. However, the strongest concentrations of pesticides are always at the lower levels of the mountains since, uh, water flows downhill. But...frogs are dying at the upper, middle, and lower levels of the mountains, so I don't believe pesticides are solely to blame either.
Now, urn, in recent decades, a deadly fungus has been killing lots of species of frogs in the region. The mountain yellow-legged frog has a defense against the fungus though. What defense...? Well, it can cover its skin in secretions which protect it from the fungus. Of course, uh, the fungus still manages to kill some frogs, but most of the frogs are able to protect themselves from it. So, um, if you ask me, there must be something else we haven't discovered yet that's killing so many frogs.
Now listen to part of a lecture on the topic you just read about.
W Professor: Attempts to add the mountain yellow-legged frog to the endangered species list finally ended in success in 2013, but it may be too late for us to prevent the frog from going extinct. One reason is that we're not quite sure what has caused its numbers to decline so precipitously. Without that knowledge, it'll be hard to determine how to go about saving the animal.
There are many who blame the trout living in the waterways of the Sierra Nevada Mountains for the decline of the frog population. While it's true that the trout catch and eat lots of frogs, consider this fact...Trout were introduced to that area more than 100 years ago, but it wasn't until around, oh, forty years ago that the frog population entered into decline. So for more than sixty years, the trout and frogs lived together in a state of equilibrium.
Pesticides, which enter water systems along with rain and snow runoff, are similarly blamed for killing the frogs. However, the strongest concentrations of pesticides are always at the lower levels of the mountains since, uh, water flows downhill. But...frogs are dying at the upper, middle, and lower levels of the mountains, so I don't believe pesticides are solely to blame either.
Now, urn, in recent decades, a deadly fungus has been killing lots of species of frogs in the region. The mountain yellow-legged frog has a defense against the fungus though. What defense...? Well, it can cover its skin in secretions which protect it from the fungus. Of course, uh, the fungus still manages to kill some frogs, but most of the frogs are able to protect themselves from it. So, um, if you ask me, there must be something else we haven't discovered yet that's killing so many frogs.