How does an ecosystem(生态系统) work? What makes the populations of different species the way they are? Why are there so many flies and so few wolves? To find an answer, scientists have built mathematical models of food webs, noting who eats whom and how much each one eats.

With such models, scientists have found out some key principles operating in food webs. Most food webs, for instance, consist of many weak links rather than a few strong ones. When a predator(掠食动物) always eats huge numbers of a single prey(猎物), the two species are strongly linked; when a predator lives on various species, they are weakly linked. Food webs may be dominated by many weak links because that arrangement is more stable over the long term. If a predator can eat several species, it can survive the extinction(灭绝) of one of them. And if a predator can move on to another species that is easier to find when a prey species becomes rare, the switch allows the original prey to recover. The weak links may thus keep species from driving one another to extinction.

Mathematical models have also revealed that food webs may be unstable, where small changes of top predators can lead to big effects throughout entire ecosystems. In the 1960s, scientists proposed that predators at the top of a food web had a surprising amount of control over the size of populations of other species—including species they did not directly attack.

And unplanned human activities have proved the idea of top-down control by top predators to be true. In the ocean, we fished for top predators such as cod on an industrial scale, while on land, we killed off large predators such as wolves. These actions have greatly affected the ecological balance.

Scientists have built an early-warning system based on mathematical models. Ideally, the system would tell us when to adapt human activities that are pushing an ecosystem toward a breakdown or would even allow us to pull an ecosystem back from the borderline. Prevention is key, scientists say, because once ecosystems pass their tipping point(临界点), it is remarkably difficult for them to return.

If you visit the new Mummies(木乃伊) show at the American Museum of Natural History(AMNH), in New York City, don’t miss the Gilded Lady. Scientists say she probably died of lung disease when she was in her 40s---about 2,000 years ago---in Egypt. They even have a model of her skull(头骨). Based on that, an artist was sure how she may have looked when she was alive. And yet, the mummy’s coffin has never been opened. Instead, scientists used a machine called a CT scanner(扫描仪) to look inside.

CT scanners were developed to help doctors examine patients. But the machines turned out to be perfect for studying mummies, too.

“A hundred years ago, scientists would usually open mummies’ coffins. This did a great deal of harm to those mummies,” say AMNH president Ellen V. Futter. “We just don’t do that anymore. We can do so much better. By using CT scanners, scientists can know about an ancient person’s diet from some of a mummy’s hair. Pieces of bone can show who was related to whom.”

Some mummies in Peru were buried with a skull hanging from their neck. Scientists thought these mummies wore the skull of an enemy. But DNA evidence showed that the mummies were buried with the skull of an ancestor(祖先).

The 19 mummies in the show are from Egypt and Peru. They had completely different reasons for mummifying the dead. The ancient Egyptians believed that mummification allowed an ancestor to live on in the next world. The people who prepared the Gilded Lady thought she would continue to see, hear, and smell. In ancient Peru, people practiced mummification to stay connected with their ancestors. Some families kept mummies in their home.

Futter calls the mummies in the show “messengers from another time.” With the help of technology, scientists will continue to uncover mummies’ secrets so we can understand it.

Many parents don’t allow or help their children to learn to swim. 【小题1】 A child who has learned to swim has the confidence to attend activities about water, like parties. More importantly, the child is safer in and around water. Here are some tips for you to help your child learn to swim.

Introduce your child to water early in life. Children who become familiar with water in the early days are less likely to be afraid of water later. 【小题2】 You can build a private pool for your child. Or allow your child to use a public pool. Most public pools have special pools for the youngest children.

For older children who want to learn to swim but have a fear of water, introduce them to water gradually. Be sure not to avoid the pool when the water is cold and uncomfortable because you want the experience to be pleasant. What if your child only wants to hang his feet in water at first? 【小题3】

Once your child has be cone comfortable with water, he will be able to begin the basics. 【小题4】 They’ll be followed by learning how to float (浮起来), kick and so on. If you find it challenging to teach your child to swim, consider asking a private instructor to work with him.

【小题5】 Never allow a child who can’t swim into the water alone and never leave a child unobserved in the water. Follow all pool and water safety recommendations at all times. And before swimming, check the possible dangers in the pools.