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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 says because once ecosystems pass their tipping point（临界点），it is remarkably difficult for them to return.