(2019·天津高考·C)
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.
【文章大意】本文主要向我们说明了生态系统以及保护生态系统的重要性。
1. What have scientists discovered with the help of mathematical models of food webs?
A. The living habits of species in food webs.
B. The rules governing food webs of the ecosystems.
C. The approaches to studying the species in the ecosystems.
D. The differences between weak and strong links in food webs.
【解析】选B。细节理解题。根据第二段的With such models, scientists have found out some key principles operating in food webs. 可知, 利用这种模式, 科学家们已经发现了在食物网中的一些关键的规则。故选B。
2. A strong link is found between two species when a predator .
A. has a wide food choice
B. can easily find new prey
C. sticks to one prey species
D. can quickly move to another place
【解析】选C。细节理解题。根据第二段的When a predator always eats huge numbers of a single prey, the two species are strongly linked可知, 如果一种掠食动物一直大量吃单一的动物种类, 它们就是紧密相连的。故选C。
3. What will happen if the populations of top predators in a food web greatly decline?
A. The prey species they directly attack will die out.
B. The species they indirectly attack will turn into top predators.
C. The living environment of other species will remain unchanged.
D. The populations of other species will experience unexpected changes.
【解析】选D。推理判断题。根据第三段的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. 可知, 当顶层的掠食动物稍微有变化时, 整个生态系统就会有很大的影响。由此可知, 如果顶层掠食动物的种群大幅减少, 其他物种的种群也会有出乎意料的变化。故选D。
4. What conclusion can be drawn from the examples in Paragraph 4?
A. Uncontrolled human activities greatly upset ecosystems.
B. Rapid economic development threatens animal habitats.
C. Species of commercial value dominate other species.
D. Industrial activities help keep food webs stable.
【解析】选A。推理判断题。第四段例子旨在阐明人类的没有计划的活动会在很大程度上影响生态系统的平衡。故选A。
5. How does an early-warning system help us maintain the ecological balance?
A. By getting illegal practices under control.
B. By stopping us from killing large predators.
C. By bringing the broken-down ecosystems back to normal.
D. By signaling the urgent need for taking preventive action.
【解析】选D。推理判断题。根据最后一段的Prevention is key可知, 预防是关键, 一旦生态系统超出了临界点, 恢复是很难的。故选D。
