TOEFL Speaking Task 4 Practice 017Science

Narrator:Now listen to part of a lecture in an Environmental Studies class.

Professor:What happens biologically speaking when a major climate change occurs? Well, scientists generally agree that the extinction of various animal and plant species is one very likely effect. This has happened in the past. An example is the disappearance 35,000 years ago of a giant Australian bird called the thunderbird. Now these birds were so big they couldn't fly. But as I was saying, many scientists are convinced that it was a change in climate that caused this species to become extinct. Here's the evidence. Here is why scientists think it was a climate change. Researchers have discovered an enormous number of 35,000-year-old thunderbird bones all together in one spot. The bones were found near an ancient dried-up lake. Now it's really rare to find so many bones from the same species in one place, but there it is, you have all these 35,000-year-old thunderbird bones all together near this dried-up lake. So what's the explanation? Well, many scientists believe that a change in the Australian climate may be behind this. Their hypothesis is they think that during a very long dry period, when there was no rainfall, the birds may have flocked together at this lake. You see, during a drought, animals tend to gravitate towards the last few remaining water sources. But then if it still doesn't rain, if the drought lasts too long, the water source may dry up too. And if that happens, the animals that have gathered there, well, they die. Scientists think this ancient lake, where they found the bones, was one of the last remaining sources of fresh water during the drought. So that's where all these birds gathered. They survived there for a time. But eventually the lake dried up, and well, that was it. By the time the drought ended, the species was extinct.