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.

A food allergy (过敏) is a medical condition that occurs when the body’s protection system perceives certain foods as a threat and responds to them with an allergic reaction.

Symptoms (症状) of a food allergy

【小题1】 However, a severe reaction can take up to a few hours following exposure. Symptoms can range from mild to severe. And they may differ each time a specific allergen appears. Symptoms can include: a skin rash or hives, swelling of the face or lips, or feeling lightheaded or dizzy. 【小题2】 That can send the body into shock or make it difficult to breathe.

Judging a food allergy

Doctors consider a number of factors when investigating a food allergy. Those include both a patient’s symptoms and family history of allergies. A physical examination may be performed to rule out any other conditions. 【小题3】

Treating a food allergy

It may not be possible for a patient to completely avoid foods that induce (导致) an allergic reaction. For minor reactions, you can take some medicine. For severe reactions, however, turn to doctors for help.

【小题4】

The only sure way to prevent an allergic reaction to food is to carefully avoid the allergens. This can be a difficult process. That requires patients to carefully study food labels to ensure the food is safe for them. 【小题5】 So it’s necessary to inform family or friends of the food that can cause a food allergy.

Black taxis have been a common sight in London for many years. Now these taxis and their drivers have become the focus of a new study into Alzheimer’s (阿尔兹海默症). For those on the outside, it may seem that behind the wheel of these black taxis are just common people who help move us to our places. But hidden within their brains is a map of London’s streets that has put GPS technology to shame for many years.

“The knowledge”, the test for London’s taxi drivers, stands among the hardest tests one could ever experience. It includes remembering information repeatedly from the memory of minute details about between 25, 000 and 56, 000 streets in London, depending on who’s taking the test, from the Trafalgar Square to the tiniest residential lanes (居民巷).

Usually, the hippocampus (马体) feels the influences of Alzheimer’s most. The hippocampus controls the brain’s short-term memory and spatial memory (空间记忆) systems. University College London and Alzheimer’s Research UK are coming together to study these taxi drivers’ brains. And the taxi drivers’ hippocampi continue to grow as they go on doing the job for more years. This suggests that perhaps there’s something we can do to reproduce the influence on the general population.

Lead researcher Hugo Spiers was part of the team which 20 years ago found that, like birds’, the taxi drivers’ hippocampi slowly got bigger. In fact, research has found for years that any animal that requires detailed spatial knowledge of their land experiences growth in the hippocampus.

Spiers’ team hopes to deal with Alzheimer’s by studying the taxi drivers “brains, since the hippocampus becomes smaller with the development of Alzheimer’s. To collect more information, Spiers has asked thirty London’s taxi drivers connected to an MRI machine to drive around. The machine will allow the researchers to keep a real-time watch on the workings of the hippocampus. “It’s been a joy to help scientists fight the disease,” said taxi driver Robert Lordan.