滚动题组23阅读理解+阅读表达+书面表达(时间:60分钟总分:80分分数:)滚动题组第28页.阅读理解A(2019天津卷)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 speciesincluding 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.【语篇解读】本文是一篇说明文。主要讲述了科学家们利用建立的食物网数学模型,发现了食物网运行的基本原则以及人类活动对于生态系统的影响。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.答案:B解析:细节理解题。根据文章第二段第一句中的“.scientistshavefoundout.infoodwebs.”得知科学家们发现了食物网运行的关键法则,即B项。2.A strong link is found between two species when a predator .A.has a wide food choiceB.can easily find new preyC.sticks to one prey speciesD.can quickly move to another place答案:C解析:细节理解题。根据第二段第三句中的“Whenapredator.arestronglylinked.”得出结论,只吃一种猎物是“stronglink”。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.答案:D解析:推理判断题。根据第三段第一句“Mathematicalmodels.entireecosystems.”可知“食物链顶端掠食动物的变化可能会给整个生态系统带来大的影响”。故选D项。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.答案:A解析:推理判断题。根据文章第四段的例子以及本段的最后一句话可知“人类的这些行为极大地影响了生态的平衡”。故选A项。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.答案:D解析:推理判断题。根据文章最后一段第二句可知这个系统会告诉我们何时对把生态系统推向瘫痪的人类活动作出调整以及最后一句话中的“Preventioniskey”可知D项正确。BThere is a very special wolf that lives in Africa.They are actually called,the African wild dogs,or painted dogs.These animals live in open woodlands,or in the plains of Africa.Today,however,most of the animals can be found in eastern or southern Africa.These animals are beautiful.They have rounded ears instead of pointed like most dogs,and they have four toes on each of their four feet,also a unique characteristic because other dogs have five.Their fur is always different from each other.The fur is usually a combination of black,brown,red and white,and sometimes even yellow.No wonder theyre called the painted dogs.The African wild dog is a very social animal.They live in groups and have numbers from five to about twenty.The members of a group are always very close to one another.They communicate in their own language or body language.When someone is very old or sick,the othe