As New York City parents debate how to give all students fair access to good public schools, some advocates in Brooklyn want to abolish middle schools' use of academic criteria to select students.
Some supporters of the proposal for District 15 from its Diversity Plan Working Group are optimistic they will win, partly because New York City Schools Chancellor Richard Carranza has questioned the principle for accepting students by ability. Other parents are cautious, saying they want their hard-working children to have the chance to earn their way into desirable schools.
District 15, which includes low-income immigrant neighborhoods such as Sunset Park and more-affluent(富裕的)areas like Park Slope, is one of the first in the city trying to change its admission system to better integrate(整合)all of its middle schools. Its Diversity Group, which includes parents and city Department of Education officials, has tried to drum up support by hosting workshops and seeking community input.
Michele Greenberg, a District 15 parent, calls the proposal more fair than the current selective system, which she said discriminates against students with few resources. "Children shouldn't be rejected because they don't somehow fit," she said.
Department officials said they will decide on the proposal this summer. If approved, the plan would mark a huge change from today's method. Now, students rank the schools they want to attend, and schools rank students they want to enroll, based on varying criteria such as course grades, test scores, behavior, attendance, punctuality and auditions. The department makes matches. Many parents complain this complex process brings massive anxiety.
Alina Rodriguez, a special-education teacher who works and lives in District 15, feels torn. As a mother, she believes her daughter would get into a strong middle school through selective admissions. But as a teacher, she worries many of her students aren't prepared for more severe choices. "I want them to be pushed but don't want them to fail," she said.
Screened(筛选的)admission has led to enrollment disparities(差异)by income and race. About 70% of the district's white students clustered at three top-performing, low-poverty middle schools last year, including M.S.51, Math & Science Exploratory School and New Voices, according to state data. At two lower-performing schools, by contrast, nearly all students were poor and Hispanic. Andrew Robertson, a District 15 parent, said the proposal would equalize the playing field. "The people so frightened by the concept are the modern-day version of people worried about civil rights," he said.
Any foreigner who has tried to learn Chinese can tell how hard it is to master the tones required to speak and understand. And anyone who has tried to learn to play the violin or other instruments can report similar challenges.
Now researchers have found that people with musical training have an easier time learning Chinese. Writing in the online edition of Nature Neuroscience, researchers from Northwestern University say that both skills draw on the same parts of the brain that help people discover changes in pitch(音高).
One of the study's authors, Nina Kraus, said the findings suggested that studying music ''actually tunes our sensory system''. This means that schools that want children to do well in languages should hesitate before cutting music programs, Dr. Kraus said. She said music training might also help children with language problems.
Mandarin(普通话)speakers have been shown to have a more complex encoding(编码) of pitch patterns in their brains than English speakers do. This is because in Mandarin and other Asian languages, pitch plays a central role. A single-syllable word can have several meanings depending on how it is intoned.
For this study, the researchers looked at 20 non-Chinese speaking volunteers, half with no musical background and half who have studied an instrument for at least six years.
As they were shown a movie, the volunteers also heard an audio tape of the Mandarin word ''mi'' in three of its meanings: squint, bewilder and rice. The researchers recorded activities in their brain stems to see how well they were processing the sounds. Those with a music background showed much more brain activities in response to the Chinese sounds.
The lead author of the study, Patrick C. M. Wong, said it might work both ways. It appears that native speakers of tonal languages may do better at learning instruments.
Recently, a group of scientists decided to find out what the funniest joke in the world was. This was obviously a difficult task, as no two people really agree about what is funny and what is not—especially when they are from different countries.
Here is the joke which the experts decided was the funniest joke in the world：
Two hunters were out in the woods. One of them fell to the ground. He didn't seem to be breathing; his eyes were closed. The other hunter took out his mobile phone and called the emergency services.
"My friend is dead!" he cried to the operator. "What can I do？"The operator said，"Don't worry. First, make sure he's dead." There was a silence, and then a shot was heard. Bang! The hunter's voice came back on the line. He said, "OK, now what?"
This is perhaps amusing. Culturally, it depends on us knowing that often hunters are not considered to be very intelligent people, and that often they are quite violent. But perhaps this is not so all over the world. It's also quite a "black" joke—a joke about something which isn't really a funny subject. The experts also found the second funniest joke in the world. Here it is：
Sherlock Holmes and Dr. Watson went on a camping trip. After dinner, they went to sleep. Some hours later, Holmes woke up. "Watson, look up at the sky and tell me what you see." "I see millions of stars, Holmes," replied Watson.
"And what do you infer from that?"
"Well, there are billions of stars ... we are a small part of the universe ..."
"Watson, you idiot!" he said. "Someone has stolen our tent!" I personally think this is better.
Can scientists in the end decide what is funny? Some things are much too complicated, even for scientists.
Your colleague's sharp comment keeps replaying in your mind. Two of your students are trapped in a "he said/she said" battle. When you reflect on your emotional reactions, you sometimes get caught up in cycles of negative feelings, which can make you feel even worse. If so, the answer may lie in a skill called "self-distancing", the ability to take a step back and view yourself more objectively. According to a research, when people adopt self-distancing while discussing a difficult event, they make better sense of their reactions, experience less emotional suffering, and display fewer signs of stress.
But what might self-distancing look like in action? Consider a typical "he said/she said" student conflict where they are each focusing on their own feelings. One is thinking," I can't believe he did that to me." And another insists, "She really hurt my feelings." However, if you ask them to take the self-distancing, they might step outside of themselves and ask broader questions: "Why was he so hurt in this situation?" or "How did her anger affect him?"
Although this approach may sound too simple to be effective, studies indicate that a change in point of view can have a powerful effect on the way people think, feel, and behave. Here are several different techniques you can try.
First, consider how a thoughtful friend might respond after quietly observing their situation. Besides, avoid using the pronoun "I". Focus on using third-person pronouns, he, she, they, and they were able to see the stressful event as challenging rather than threatening. Finally, ask yourself, "How would I feel about this one week from now or ten years from now?" This form of mental time travel may be effective because our attention is directed away from our immediate, concrete circumstances.
Since the beginning of history, people have played games. Children in one country may play games that are quite different from those played anywhere else in the world. Here are some interesting games that you and your friends can try.
The Mouse Game (The Philippines)
In this game, a piece of string is attached to the player's hand. One part of the string represents the "cat" while the other part is the "mouse". The objective of the game is for the cat to catch the mouse and for the mouse to escape just in time. It teaches you to focus and move your fingers fast.
Piñata (Mexico)
A piñata is a colorful container made from papers and filled with sweets and toys. Children are blindfolded and take turns at trying to hit the piñata, which is usually suspended(悬挂) on a tree or pole. When someone breaks open the piñata, the children run to get the treats. This game improves your sense of direction.
Help (China)
Chasing games are popular all around the world. In China, one player is chosen to be the chaser and runs after the other players until someone is caught. No equipment is required to play this game, but it helps to run fast and have a loud voice for shouting.
Doorkeeper (Afghanistan)
In the villages of Afghanistan, there is no limit to the number of players who can take part in the game. A small rubber ball is all you need to play the game. It helps to have quick reactions and focus your eyes on the ball because you need to prevent the ball getting into the net.
My dear friend has invited me to speak at a women's meeting she was holding in Honduras. The night before I was to leave, I decided a warm bath would help me sleep. I turned on the hot water handle on the tub (浴缸) and waited. I complained, "Let me have some warm water!"" took forever for the hot water to work its way through the pipes to the back of the house. Needless to say, the bath wasn't relaxing.
The next afternoon, I arrived in Honduras. After the meeting, my friend took me to meet some of her family members. They lived in a stick-and-mud house and slept on dirt floors. The women cooked outside on the stove. Still, even in these terrible conditions, everyone I met had a smile on his or her face. They were so generous and almost always suggested we stay for coffee.
On our way to one home we passed a pond which was black and dirty. One woman pushed the cover back with a stick while the other put the water into a bucket. I prayed (折持)," Please don't let them offer coffee."
After we were invited in, a little girl ran inside. She held a mango in each hand. The one in her left hand was half eaten. This little beauty held out her right hand and offered me the other mango, which I gladly accepted. Her eyes danced. I remember thinking, "She has no idea that she is poor."
When it was time to leave, we walked outside and I noticed a muddy stream that ran beside their home. My friend said, "That is where they bathe. It is also where the animals drink and they push out waste."
The evening I returned home I went into my perfect bathroom and twisted the hot water handle. While bathing, I thought of this simple pleasure. Clean water wasn't my right; it was luck.
Genetic testing offers people insight into the types of diseases they are most likely to develop — but a new study suggests most people do not alter their lifestyles based on this information. These tests — known as genome sequencing — analyze a person's DNA, telling patients about their known risk for diseases like cancer or diabetes. But being told you're at a higher risk for lung cancer doesn't seem to motivate anyone to quit smoking or alcohol, this study suggests. Because of this, the scholars argue that genetic testing should be banned as a tool for improving people's health.
Today's finding came from pulling data from 18 other studies that followed people after they received the results of genetic tests. Receiving information about genetic risks didn't inspire people to eat differently, exercise more, or stop smoking, "Expectations have been high that giving people information about their genetic risk will empower them to change their behavior, but we have found no evidence that this is the case," study author Theresa Marteau, director of behavior and health research said in a press release.
Genetic testing, which the National Institutes of Health says costs anywhere from $100 to $2,000, has become much more accessible as commercial testing companies such as 23andMe and Sure Genomics have sprung up. These companies are not allowed to share disease risk estimates with consumers thanks to the Food and Drug Administration. However, today's study didn't specify whether the genetic testing were purely from academic sequencing, or if any of these companies had had a role in supplying the data.
Genetic testing doesn't get people to change their behavior for the better, but it doesn't have any known negative effects either. Knowing the results of these tests didn't change people's depression or anxiety levels. And there's no indication that testing inspires people to pick up risky or dangerous health habits either, the study found.
Actually a genetic predisposition to a certain disease is common among people. Some people are born weak in heart. Some are innately vulnerable in digestive system. But these most common risk factors usually don't raise a person's chances of getting the disease by a significant amount. It's possible that some of the patients in the study had substantially high disease risks based on their DNA profile, but those patients tend to be rather rare. "It's still likely that communicating this type of information is very valuable to some people, but it's just that there aren't that many of those people," Zikmund-Fisher from the University of Michigan said. "The idea that providing genetic risk information is going to be transformative to everyone seems unlikely."
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A doctor entered the hospital hurriedly after being called in for an important surgery. He found the boy's father in the hall waiting worriedly.
On seeing him, the father cried out, "Why did you take all this time to come? Don't you know that my son is in danger? Don't you have any sense of duty?"
The doctor smiled and said, "I am sorry. I wasn't in the hospital and I came as fast as I could after receiving the call and now, I wish you'd calm down so that I can do my work."
"Calm down?! What if your son was in this room right now? Would you calm down? If your own son dies while waiting for a doctor, then what will you do?" said the father angrily. The doctor smiled again and replied, "We will do our best and you should also pray for your son's healthy life."
The surgery took some hours after which the doctor went out happy, "Thank goodness! Your son is saved!" And without waiting for the father's reply, he carried on his way running by saying, "If you have any questions, ask the nurse."
"Couldn't he wait some minutes so that I can ask about my son's state?" shouted the father when seeing the nurse minutes after the doctor left.
The nurse answered, tears coming down her face. "His son died yesterday in a road accident. He was at the burial when we called him for your son's surgery. And now that he saved your son's life, he left running to finish his son's burial."
Your driving record is something that follows you from the day you get your license until the day you hang up your keys. And it's something that can have an impact on different aspects of your life.
It's a good idea to aim to have a clean driving record. It signals to car insurance companies that you're a responsible and safe driver. This pays off for you since it leads to getting better car insurance rates.
But what's considered a clean driving record? If you get any kind of ticket, do you automatically not have a clean driving record anymore?
Here's what you need to know about having a clean driving record:
Your driving record is a public record of your driving history. Remember, from the moment you get your license, you have a driving record.
On your driving record you can find things like:
·Driving license status
·License classifications
·License points
·Traffic accidents
·Moving violations (违反) and fines
·Defensive driving classes taken
So you have a driving record. But what's the distinction between that and a clean driving record?
A clean driving record means your driving history is free of any accidents, moving violations, or points. But don't let this definition scare you off. All insurance companies have different rules and definitions of a clean driving record. Some insurance companies overlook minor moving violations. So if you have a single speeding ticket or some other relatively minor offense that usually doesn't prevent you from having a clean driving record.
Do you continue to be healthy in social isolation (隔离)? NASA is looking for people to spend 8 months locked in a Russian lab for a new experiment.
When humans go to the moon and travel to Mars, they will need to be prepared for long-term space travel and even longer stays on these far-off destinations. Currently, NASA's Artemis program aims to land humans on the moon for the first time since NASA's Apollo 17 mission landed in 1972.
While the moon is the main goal of NASA's Artemis program, the agency's larger goal is to send people to Mars. But long term space travel and habitation (居住) won't be easy. Such missions will present both physical and mental challenges as astronauts work to not only survive, but perform important scientific research in uniquely difficult environments.
In the upcoming NASA-Russia experiment, a group of people will live in a closed facility at Russia's Institute for Biomedical Problems, which is in Moscow. This habitat facility was used in 2011 for a notable series of Russian mock (模拟的) Mars missions known as Mars500. During these missions, groups of people spent 520 and 105 days on two separate missions in the facility.
The facility, according to the statement, will have "environmental factors similar to those astronauts are expected to experience on future missions to Mars". The group of people will spend 8 months living together in isolation and working on scientific research.
By living and working in this Mars-like environment, participants in this study will help NASA researchers understand the psychological and physiological (生理的) effects of isolation. These missions help researchers not only perform relevant science experiments but also see how people might get on under strange conditions for long periods of time.
So, if you're finding that living in social isolation works for you, you might be particularly well-suited for living in a Mars-like environment and this could be for you. But first, you have to qualify. NASA is looking for "highly motivated" Americans between the age of 30-55, who must speak both Russian and English fluently and have a Master of Science, Doctor of Philosophy, Doctor o[ Medicine, or military officer training. Applicants with other related experience may also be considered. Those who are chosen to be a part of this group will be paid for their work.
A wife's level of education positively influences both her own and her husband's chances of having a long life, according to a new Swedish study.
In the study, researchers from the Swedish Institute for Social Research in Stockholm found that a woman's level of education had a stronger connection to the likelihood of her husband dying over education. What's more, they discovered that a husband's social class, based on his occupation, had a greater influence on his wife's longevity(长寿) than her own class.
"Women traditionally take more responsibility for the home than men do, and, as a result, women's levels of education might be more important for determining lifestyles－for example, in terms of food choices－than those of men," say Srs. Robert Erikson and Jenny Torssander of the Swedish Institute for Social Research in Stockholm.
The results show that a husband's level of education does not influence his longevity, but that men with partners who had quit studying after school were 25 per cent more likely to die early than men living with women holding university degrees. In turn, those married to women with university degrees were 13 percent more likely to die early than those whose wives had post-graduate qualifications.
According to the researchers, a woman with a good education may not marry a man who drinks and smokes too much or who drives carelessly, and men with such habits may not prefer highly educated woman. Drs. Erikson and Torssander also suggest that better-educated women may be more aware of what healthy eating and good health care consist of.
The findings suggest that education has a huge impact on how long and how well people live. It also reflects social factors, since educated individuals usually have better jobs, which allow them to afford healthier diets and lifestyles, as well as better health care.
Training for Saving First Aid at Work 3 days £279
Suitable for people aged 16+ who might need to provide first aid by responding to a wide range of accidents, injuries and illnesses that they could encounter in their workplace. Ideal for organizations whose needs assessment has identified a requirement for additional first aid training, especially, having employees with a disability or a medical condition.
Lunch break. Classroom setting. Certificate to issue.
AED with Life Support 2 days £107
Right choice for people, over 16，who want to learn how to use an automated external defibrillator (AED). First aid for unresponsive adults in addition. Currently available at our London, Edinburgh and Reading venues; however, you can organize a training session for a group of up to 15 people at your workplace by requesting a group booking.
Certificate valid for three years.
First Aid for Teachers Training 1 day £50
Specially designed for those working in a school environment and wishing to have the first aid skills to act in an emergency. Ideal for teachers and other school staff over 16. First aid skills to help a child who is unresponsive and breathing or not breathing, choking, having an allergic reaction or a head injury.
Not qualify the learner to act as a first aider.
Fire Marshal Training 4 days £364
Take this if you are over 16 and responsible for fire safety or appointed fire marshals in the workplace. Come to understand how a fire starts and spreads; how to assess and manage these risks; and how to use firefighting equipment. You must be physically able to carry out the practical elements of the course.
Role play- Classroom environment. Certificate.
It has been four years since Grandma Joy told her grandson that she had never seen the most beautiful views of America. Since then, they have gone on many cross-country trips.
Brad Ryan had been feeling tired of studying in a vet school when he visited his 85-year-old grandma in 2015. He hoped that spending some quality family time would help lift his spirits.
As he was telling his grandma stories of his travels, however, he was heartbroken to hear that she had never seen the ocean or mountains. Ryan then asked his grandmother if she would like to go hiking with him. Needless to say, she was more than happy to agree.
They have since visited 29 different national parks across America—from the Great Sand Dunes in Colorado to the Acadia National Park in Maine. Over the four years, Ryan and his grandma have driven 25,000 miles through 38 states. And it has greatly affected his quality of life. "Seeing it through my grandmother's eyes, who every morning wakes up and is thankful to be alive has taught me how to live," Ryan said. "With her, I've had to slow down and see life in a different way, which has made it a lot richer."
"I love my grandma so much, and it's just really giving me a lot of peace when she passed away. I can go back to these places, feel her spirit, and feel connected to her," he added.
Ryan posted all of their trips on Facebook. He hopes more young people will spare time to be with their elders. "I want young people to know that hanging out with your grandparents is cool. And there's so much we can learn from our elders," Ryan said.
A Chicago woman, Karen Autenrieth, who lost her wedding band nearly 50 years ago was reunited with the ring just in time for Valentine's Day.
The story began during the winter of 1973. "It was very cold and the snow piled up along the roads," Karen told the Chicago Tribune. "I had to help each kid climb over the large piles to get into my grandparents' yard. My hands were cold—I didn't have gloves on—and my ring just flew into the snow." Despite her best efforts, Karen was unable to find the ring.
In February, a Chicago man who lost his own wedding ring posted on Facebook asking if anyone had found it. In the comments section, Sarah Batka, a woman who lives in Autenrieth's grandparents' former home% said she had discovered a wedding ring eight years ago while gardening. It wasn't the man s ring» but it sparked a hunt for the rightful owner.
Two historians—Carol Flynn and Linda Lamberty from the Historical Society—set off to solve the mystery. They used every means to retrace the origin of the ring and finally drew a conclusion that a woman, Karen Autenrieth, whose grandparents had once lived in Batka's home, was the possible owner.
"The woman was on Facebook. So I asked whether this ring was hers. '' Flynn wrote in the post. "She replied—it was hers!"
After connecting, Flynn mailed the ring to Autenrieth, who received the package on Valentine's Day. 44Holy Mary, I can't believe it. Karen told the Chicago Tribune. "Just think I get the ring back after almost 50 years!
In a time when a dangerous number of people are overweight, many people seem to have forgotten the most important way to keep healthy and slim-exercise. And as a new study carried out on mice in the lab has shown, exercise early in life can reward you in your adult years.
A team of researchers at the University of California studied the effects of early exercise on adult physical activity, body mass and eating. They found that early-age exercise in mice had positive effects on adult levels of voluntary exercise in addition to reducing body mass.
"These results may have an effect on the importance of regular physical education in elementary and middle schools," said Theodore Garland, a professor of biology, who led the research project. "If kids exercise regularly through their school years, then they may be more likely to exercise as adults, which could have far-reaching positive effects on human health and well-being or happiness."
Although the positive effects of early-life exercise lasted for only one week, it is important to note that one week in the life of a mouse is the same as about nine months for humans, "Our results suggest that any positive effects of early-life exercise on adult exercise will need to be kept up if they are to be long-lasting."
His team of researchers found, too, that all mice having access to early exercise were lighter in weight than non-exercised mice.
Garland explained that, in general, exercise will stimulate appetite sooner or later. However, it is possible that certain types of exercise, done for certain periods of time or at certain light levels, might not stimulate appetite much, if at all, at least in some persons.
"If we could understand what sorts of exercise these might be, then we might be able to tailor（量身打造）exercise recommendations in a way that would bring the benefits of exercise without increases in appetite, leading to a better chance of weight loss," he said.
Born in Philadelphia the youngest of five children, Ruiz grew up in a loving household in suburban South Jersey. But that is not her full story. Ruiz's father passed away when she was three, leaving her mother to raise the five children. The family moved more than she'd like to remember -three times when she was in 5th grade alone.
Though her mother was a working single parent trying to support her own family, she didn't hesitate to help others. Friends arrived needing a temporary place to crash and stayed indefinitely. Ruiz's aunt, diagnosed with cancer, moved in with the family after her graduation from high school and they cared for her through the end.
Ruiz became an EMT and worked nights in Philadelphia, eventually enrolling in Rowan College at Gloucester County, where she sought to develop her skills as a storyteller. Ruiz got such good grades at RCGC that she was offered a partial scholarship to study journalism at the University of Miami but realizing she could not get enough to cover everything the scholarship wouldn't, she didn't go to Miami. She applied to Rowan. She graduated with two degrees-Journalism and Radio, Television &. Film-from the College of Communication &. Creative Arts.
Last fall, Ruiz joined the staff of Rowan Radio 89. 7 WGLS-FM where she quickly became news director and this spring began an internship with the Elvis Duran and the Morning Show, a nationally syndicated radio program in New York City. The Duran program exposed her to a world of career possibilities, from behind the scenes production to on-air talent.
“I know what it's like to feel isolated, to feel like your story is not getting heard, " she said." Rowan provided me the tools to give a voice to the voiceless. My goal is to share powerful stories so that I can inspire as many people as possible to love themselves and to follow their dreams. "
We can make mistakes at any age. Some mistakes we make are about money. But most mistakes are about people. Did Jerry really care when I broke up with Helen? When I got that great job, did Jim really feel good about it, as a friend? Or did he envy my luck? And was Paul friendly just because I had a car? When we look back, doubts like these can make us feel bad. But when we look back, it's too late.
Why do we go wrong about our friends or our enemies? Sometimes what people say hides their real meaning. And if we don't really listen we will miss the feeling behind the words. Suppose someone tells you, you're a lucky dog. That's being friendly. But lucky dog? There's a bit of envy in those words. Maybe he doesn't see it himself. What he may be saying is that he doesn't think you deserve your luck.
"Just think of all the things you have to be thankful for" is another phrase that says one thing and means another. It could mean that the speaker is trying to get you to see your problem as part of your life as a whole. But is he? Wrapped up in this phrase is the thought that your problem isn't important. It's telling you to think of all the starving people in the world when you haven't got a date for Saturday night.
How can you tell the real meaning behind someone's words? One way is to take a good look at the person talking. Do his words fit the way he looks? Does what he says agree with the tone of voice? His posture(姿态)? The look in his eyes? Stop and think. The minute you spend thinking about the real meaning of what people to you may save another mistake.
Scientists in Israel have taught goldfish an unusual way to reach a target — by driving a small robotic car on dry land.
Six goldfish, regardless of their weight and size, took part in the experiment and were taught to drive in 30-minute sessions, conducted three times a week, every two days. The experiment was led by researchers at the Ben-Gurion University of the Negev. They wanted to see if goldfish's ability to navigate (导航) depends on their natural environment, or if they could navigate even under very unusual conditions.
To test the goldfish, the researchers created a special robotic car that could drive forward, backward, and from side to side. The scientists called the car a Fish Operated Vehicle (FOV).
Goldfish was placed inside a water tank on the FOV. Using a special system, the car tracked where the goldfish was and what direction it was swimming in. The FOV would then automatically move in the same direction. To prevent the goldfish from getting stuck in barriers such as walls, researchers equipped the car with lidar (激光雷达). The researchers tested them by placing the car in a small room with a pink target on one side — that was where the fish were supposed to arrive. The fish were rewarded with food when achieving the task.
The fish did indeed learn to drive the FOV, becoming more skillful in its operation over time both in terms of achieving tasks and in the time taken to complete that task. The researchers wanted to check that the fish weren't just memorizing a set of movements to earn a reward, so they repeated some tests with a new starting position or added other targets that were a different color. They found the goldfish were able to deal with those changes.
Though the experiment showed that the fish were able to navigate to a target, it's not likely that the fish knew they were controlling a car. The researchers say that in the future it may be possible to change the FOV to test these ideas.
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.
