美国之音英语听力 VOA慢速（2013.4.23）

　　深圳中考网 美国之音英语听力（VOA慢速），4月23日更新：

Earth Day

　　Hi, I’m Caty Weaver. Today is Earth Day. Millions of people around the world will pick up trash, learn about recycling, plant trees or take some other action to observe the day.

　　On AS IT IS we take a look at two recent environmental studies that improve our understanding of the natural world.

　　A new study disputes a widely held belief about how evolutionary changes in organisms take place. Scientists have long believed that such changes take place slowly --- over hundreds, thousands or millions of years.

　　But now, new evidence suggests species can evolve much more quickly when they have to. Avi Arditti has the story.

　　Tim Benton is a professor of biology at the University of Leeds, in Britain. He studies how living organisms react to changes in their environment. He and other researchers just published a paper on the subject in the journal Ecology. The paper examines why ocean species have dropped so sharply in size and number over the past 50 years.

　　“Is this a response that is due to them having less food or the temperature of the water changing from climate change or is it a response that is due to natural selection working and evolutionary biology happening?”

　　To find the answer, Professor Benton’s team of researchers carried out a series of laboratory experiments with soil mites. Soil mites are very small spider-like creatures that reproduce quickly. The researchers captured mites in the wild and put them in test tubes. Each tube had about one thousand mites.

　　“Every day, we just put in a little bit of food. And in some of the populations we took out the juveniles and in other populations we harvested the adults. And then we just left them to it over about 100 weeks.”

　　That is normally enough time for about 20 generations of soil mites. However, in the controlled environment, the soil mites competed for food, sex and survival in different ways than they would have in the wild.

　　The researchers measured the insects’ growth rates, genetics and reproduction over this relatively brief period. They observed that natural selection produced meaningful evolutionary changes. As Tim Benton observed, the length of time it took the mites to reach adulthood doubled during the experiment.

　　“And because it is taking them so much longer to grow up, then that means that the population responds to changes in a different way. Population growth rate is slower, which means that there are very large changes in population dynamics, the way the population size responds to environmental change in itself.”

　　Professor Benton says the mite study suggests there is a powerful relationship between environmental and evolutionary change. He says more studies like this need to be done by different groups. If those result in similar findings, he says, people will more easily accept the force of evolutionary change in ecological time.

　　“So over a single life time, one hundred years, there are likely to be very large changes and if we don’t start thinking about the evolutionary changes as well as the changes in the environment, then the things we put in place to protect the species we want to manage won’t actually work.”

　　The researcher says fisheries management is one place where the findings might have a critical effect.

　　“Given that we’re harvesting large animals all the time --- that’s what we do when we go out fishing --- the phenotypic response that we see in the reduction of size is likely to be an evolutionary response. And that’s what we found in out laboratory study. So what that means is if you stop fishing because your stock is getting depleted and the animals are increasingly smaller, there is no necessity that they will be able to recover because you’ve had a hard-wired evolutionary change. So they won’t just be able to spring back.”

　　And there is no guarantee they will again grow larger and larger.

　　Professor Benton adds that environmentally-led evolutionary changes could have serious effects on other wildlife protection efforts, and on disease and insect control programs.

　　As the team at Leeds studies interactions between evolution and the environment, another group of scientists is following pollutants by examining trees. Jim Tedder reports on the investigation.

　　Much clothing, furniture and other products are covered with chemicals meant to slow the spread of fire. These fire retardants, however, can become poisons in nature. And they are in the environment. The chemicals are found in water, plants and the air. In the past, scientists had difficulty measuring the levels of these pollutants. But that recently changed with a technique developed by researchers at Indiana University.

　　Researcher Amina Salamova studies polybrominated diphenyl ethers and other chemical pollutants. PBDEs are widely used as fire retardants. But they have also been linked to human health problems.

　　Ms. Salamova explains.

　　“They can have an effect on neurological development, on reproductive system and they can affect your thyroid endocrine system.”

　　Concerns about those effects have led some government agencies and manufacturers to end the use of many chemical products. But, PBDEs can exist for years in the environment. Scientists do not know exactly where or how they spread.

　　That is why Amina Salamova and researcher Ronald Hites developed a new way to measure the presence and amount of PBDEs in the air. They do so be testing the bark covering trees.

　　“The tree’s ideal because it’s sitting there passively soaking up these compounds out of the atmosphere.”

　　A tree’s bark provides a large surface area that takes in the chemicals as both gasses and particles. A layer of bark remains on a tree for about five years. So it provides a record of the environment over time.

　　The current more common method of tracking PBDEs is complex. It involves forcing air through costly equipment. This also requires a lot of electricity and workers.

　　Amina Salamova says the tree bark method is a clear improvement.

　　“So what I see in future for tree bark is the ability to use this method in developing countries which don’t have a lot of funding for elaborate atmospheric studies. Also we can use this method in remote sites where there is no power.”

　　The two scientists have received bark from twelve areas, including Norway, the Czech Republic, South Africa and Indonesia.

　　In the laboratory researchers found evidence of fire retardants in the atmosphere at all twelve areas. The highest levels were found in the United States and Canada. What was surprising, however, was the high level found in some rural parts of Indonesia and Tasmania.

　　“There’s hardly anybody there. It’s really out of the winds of possible industrial pollutants.”

　　The researchers’ findings show the compounds are travelling surprisingly long distances. Amina Salamova says they hope to continue collecting tree bark to build a worldwide information base. That way scientists and governments can understand the extensiveness of fire retardants and find ways to remove them from the environment.

　　And that’s AS IT IS for today. I’m Caty Weaver.

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