How does an ecosystem 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.
生态系统如何运作？是什么让不同物种的种群变成现在这样？为什么苍蝇这么多，狼这么少？为了找到答案，科学家们建立了食物网的数学模型，记录了谁吃谁以及吃了多少。
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 always eats huge numbers of a single prey, 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 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.
通过这些模型，科学家们已经发现了一些在食物网中运作的关键原理。例如，大多数食物网由许多弱连接组成，而不是一些强连接。当掠食动物总是吃大量的单一猎物，那么这两个物种就是强连接；当掠食动物依赖于多个物种时，它们之间就是弱连接。食物网可能由很多弱连接主导，因为这种安排在长期内更为稳定。如果掠食动物可以吃多个物种，它就能在灭绝中幸存下来。如果掠食动物可以在猎物变得稀少时转移至另一物种，那么就更容易发现，这个转换允许原来的猎物恢复。因此，这些弱连接可能会阻止物种之间相互毁灭。
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.
数学模型还显示，食物网可能不稳定，顶级捕食者的微小变化可能会对整个生态系统产生重大影响。20世纪60年代，科学家提出，处于食物网顶端的掠食动物对其他物种的种群数量有着惊人的控制力，包括它们没有直接攻击的物种。
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.
无计划的人类活动证明了由顶级掠食动物自上而下控制的想法是正确的。在海洋中，我们以工业规模捕捞鳕鱼等顶级食肉动物，而在陆地上，我们杀死了狼等大型食肉动物。这些行动极大地影响了生态平衡。
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, it is remarkably difficult for them to return.
科学家们已经建立了一个基于数学模型的预警系统。理想情况下，该系统将告诉我们何时适应人类活动，这些活动将生态系统推向崩溃，或者允许我们将生态系统从边界线拉回来。科学家说，预防是关键，因为一旦生态系统超过临界点，很难恢复。
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