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The living things in an ecosystem1 affect each other in many ways. The consumers that kill other animals for food are called predators3.The word predator2 usually brings to mind pictures of lions and wolves,but such creatures as robins,frogs,and humans are also predators.Some predators,carnivores such as lions,depend entirely4 on animals they kill while many others,such as foxes and humans,eat plant food too.
Some people think of predators as "bad",though humans themselves are the greatest predators the world has known.Sometimes individual predators do prey5 upon farm animals,and these individuals have to be controlled.Too often,however,people try to wipe out entire populations of predators,with the mistaken idea that they are doing good.
People usually believe that predators have an easy time of it,killing6 defenseless prey.But studies of predators and their prey show that this isn't so .After ovserving tigers in Africa, Dr.George Schaller wrote:"The tiger's seemingly unbeatable array of weapons---its acute senses,great speed (but over short distances only),strength and size ,and formidable claws and teeth--have given many naturalists8 the impression that the tiger can kill at will...My experience shows quite the contrary--the tiger has to work quite hard for its meals...I estimate that ,for every wild prey killed,the tiger makes twenty to thirty unsuccessful attempts."
Another biologist made the same observation about wolves.After studying North American wolves for twelve years,Dr.L.David Mech concluded that are either young, old ,sick, weak, injured or diseased.Dr.Mech wrote:"As is true with most predators,the wolf is an opportunist...The predator takes whatever it can catch.If the wolf could capture prime ,healthy prey,it certainly would. But most of the time it cannot,It happens that all they prey species of the wolf are well equipped with superb detection, defense7, and escape systems.As long as these systems are in good working order,a prey animal is usually safe from wolf attack.
Predators are usually bigger and fewer in number than the animals they prey upon. The reverse is true of parasites10.These organisms live on or in other living things--their hosts,often spending an entire lifetime with them .In parasitism11 the parasite9 gets food and sometimes shelter, while the host gains nothing and may even suffer in some wya from the relationship.
Very few living things are free of parasites ,which are usually smaller and more numerous than their hosts. Indeed ,many parasites have parasites of their own.Jonathan Swift exaggerated only a little when he wrote:
And these have smaller stilll to bite 'em,
And so proceed ad infinitum.
Some biologists believed that most of the individual organisms now living are parasites ,since there are many parasitic14 fungi,bacteria,flatworms,insects,ticks,and mites15. Paresites are an important part of all communities, and like predators,often affect the number of other organisms in a community.Man has tried to use this ecological16 knowledge by deliberately17 bringing parasties or predators into an area where they might control the numbers of some pests. Sometimes this works well; often it does not.
In the 1870s ,sugar-cane18 planters in Jamaica were losing about a fifth of their crops to rats,and a planter brought mongooses from India in hopes that they would prey on the rats. Within a few years the number of rats had dropped dramatically. The rats became harder to find. Then the mongooses began eating native mammals,ground-nesting birds,snakes, lizards,land crabs,and anything else they could find .They even took to eating sugar cane. Some of the creatures they wiped out had been useful controls on insect numbers, and the insect damage to sugar cane increased. The mongooses themselbes became pest in need of control.
In aother instance, house sparrows were brought to the United States from England in hopes that they would help control elm spanworms in New York City's Central Park. The birds did not control the insects and habe spread across most of the nation,crowding out bluebirds and other native birds with which they compete for food and nesting sites.
People do learn from their mistakes,and experiences with mongooses,house sparrows, and other introduced organisms led to the passaga of strict laws controlling the importation of plants and animals to the United States. The idea o fusing parasites and predators to control pests has not been abandoned; it is just done with much greater care and advance study.This method of limiting the number of pests is called biological control, and there is hope that it will womeday eliminate the need for many of the insect posons used today.
The close association between parasite and host is an example of symbiosis19 which means "living together". There are a number of other examples of symbiosis in nature.In some relationships ,one organism benefits and the other is not affected20 at all.This is called commensalism . Fish called remores attach themselves to sharks. They get a free ride and eat fragments of the sharks' food . There are many other commensal relationships in the sea:practically every worm burrow,shellfish,and sponge contains animals that depend on the host for shelter or food scraps21. A biologist found 13,500animals living within shrimps22, but the total included ninteen species,among them a small fish.
In some symbiotic23 relationships, both organisms benefit. The most common and widespread example of this mutualism is a team of plants called lichen24. You can find lichens25 clinging to rocks and tree trunks almost anywhere. Part of the lichen is a fungus26. Within it are colonies of green algae27 cells. The fungus provides support and traps water which is used by the algae. The algae make food which is consumed by teh fuges. Thus both kinds of plant benefit.
The organisms that make up a lichen couldn't survive long apart. In other cases of mutualism, the two organisms may be together only part of the time. Bires called egrets often perch28 on the backs of African mammals such as rhinoceroses30. The birds feed on lice and ticks in the mammal's skin. This benefits both organisms. Also , the rhinoceros29 may be warned of danger when an egret flies in fright from ites back. But neither species is so dependent on teh other that it can't survive by itself.
By Laurence Pringle
The planned mass slaughter of five thousand elephants in Kenya's Tsavo Game Park cannot be contemplated32 without emotion.Some hunters will refuse to shoot an elephant . It is an understandable attitude . There is something about these great beasts that fills one with fascination33 and awe34, and something akin35 to a deep respect. They are like bisitors from another world, another age. They are the most magnificent of animals. In the animal kingdom the lion is king, yet where the elephant walks the lion fears to tread.
Anyone who has shot an elephant will never forget it . A professional big game hunter in Africa will tell you that the bigger the animal the more terrible seem its death throes. Ther hunter is overcome with the sheer horror of his destruction-- until hours later the hunting instinct exerts itself once more and dulls the dinder thought.
This is what is gong to happen in the Tsavo Game Park--five thousand times. Unfortunately there is sufficient justification36 for this massacre37 . And Kenya's is not a unique example. In Zambia the game department is busily engaged in a similar operation--the killing or "cropping" as it is called of 1,500 elephants in teh Luangwa Valley of the Eastern Province. Buffalo38 and hippo are also being thinned out and their meat sold in the butchers'shops of Lusaka and Ndola(elephant meat 1s. 9d. a pound, buffalo 2s. 3d. a pound).
In Zambia game officials are using spotter aircraft with air-to -ground radio to plot the position of herds39. Hunters are following up and killing the beasets with dart40 guns, using a fatal drug that kills painlessly and almost instantaneously. It is to be hoped the Kenyans will use an equally humane41 method.
Cropping is necessary became wild animals retreating before expanding human populations in Africa no longer have the wide open spaces to roam that they had half a century ago . They are gathering42 with greater density43 in the areas still left to them. More and more attention is being given to conservation of wild life in allocated44 game reserves but as herbs increase in these reserves the food content diminishes.
As the herds cannot be permitted to leave their reserves and encroach on farmland to search for food, the only solution for their survival is to limit their numbers to the grazing potential. Elephants pose the biggest problem. They are a tougher breed,live longer than most animals, and are able to adapt themselves to all climates and altitudes. They also have greater appetites and destroy the areas where they feed, pushing over trees to strip off the leaves. The average 6-ton bull elephant eats about 1,000 poungs of food (grass ,leaves, branches ,bushes) every 24 hours and needs about two square miles of grazing land. Tsavo's 8,000 square miles can comfortably accommodate only 4,ooo elephants.
The rhino,which competes with the elephant for choice grazing grounds, is being starved out of areas inhabited by elephants.
In Tsavo Game Park a year ago Poyal Navy helicopters were used to spray elephants with pink paint from the air in an operation to count their number. I t was learned that there are 15,000 elephants in the pard. That is too many. Several thousand must die.
The destruction of a wild beast may seem of little consequence compared with the affairs of men. Yer it has its own poignancy45.Stuart Cloete describes in his book, The Curve and the Tusk,such a scene as two elephants move towards the hunter's gun:" Like battleships, but silent as butterflies, they drifted slowly,eating their way ,resting trunks hand in hand... always silent and unhurried, always with the spoor of the tusked46 elephant in that if his tuskless brother,they came on ,drifting giants as silent as thistledown. Each step brought them nearer to death..."
By Peter Younghusband
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1 ecosystem | |
n.生态系统 | |
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2 predator | |
n.捕食其它动物的动物;捕食者 | |
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3 predators | |
n.食肉动物( predator的名词复数 );奴役他人者(尤指在财务或性关系方面) | |
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4 entirely | |
ad.全部地,完整地;完全地,彻底地 | |
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5 prey | |
n.被掠食者,牺牲者,掠食;v.捕食,掠夺,折磨 | |
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6 killing | |
n.巨额利润;突然赚大钱,发大财 | |
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7 defense | |
n.防御,保卫;[pl.]防务工事;辩护,答辩 | |
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8 naturalists | |
n.博物学家( naturalist的名词复数 );(文学艺术的)自然主义者 | |
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9 parasite | |
n.寄生虫;寄生菌;食客 | |
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10 parasites | |
寄生物( parasite的名词复数 ); 靠他人为生的人; 诸虫 | |
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11 parasitism | |
n.寄生状态,寄生病;寄生性 | |
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12 flea | |
n.跳蚤 | |
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13 fleas | |
n.跳蚤( flea的名词复数 );爱财如命;没好气地(拒绝某人的要求) | |
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14 parasitic | |
adj.寄生的 | |
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15 mites | |
n.(尤指令人怜悯的)小孩( mite的名词复数 );一点点;一文钱;螨 | |
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16 ecological | |
adj.生态的,生态学的 | |
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17 deliberately | |
adv.审慎地;蓄意地;故意地 | |
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18 cane | |
n.手杖,细长的茎,藤条;v.以杖击,以藤编制的 | |
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19 symbiosis | |
n.共生(关系),共栖 | |
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20 affected | |
adj.不自然的,假装的 | |
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21 scraps | |
油渣 | |
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22 shrimps | |
n.虾,小虾( shrimp的名词复数 );矮小的人 | |
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23 symbiotic | |
adj.共栖的,共生的 | |
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24 lichen | |
n.地衣, 青苔 | |
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25 lichens | |
n.地衣( lichen的名词复数 ) | |
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26 fungus | |
n.真菌,真菌类植物 | |
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27 algae | |
n.水藻,海藻 | |
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28 perch | |
n.栖木,高位,杆;v.栖息,就位,位于 | |
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29 rhinoceros | |
n.犀牛 | |
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30 rhinoceroses | |
n.钱,钞票( rhino的名词复数 );犀牛(=rhinoceros);犀牛( rhinoceros的名词复数 );脸皮和犀牛皮一样厚 | |
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31 slaughter | |
n.屠杀,屠宰;vt.屠杀,宰杀 | |
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32 contemplated | |
adj. 预期的 动词contemplate的过去分词形式 | |
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33 fascination | |
n.令人着迷的事物,魅力,迷恋 | |
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34 awe | |
n.敬畏,惊惧;vt.使敬畏,使惊惧 | |
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35 akin | |
adj.同族的,类似的 | |
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36 justification | |
n.正当的理由;辩解的理由 | |
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37 massacre | |
n.残杀,大屠杀;v.残杀,集体屠杀 | |
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38 buffalo | |
n.(北美)野牛;(亚洲)水牛 | |
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39 herds | |
兽群( herd的名词复数 ); 牧群; 人群; 群众 | |
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40 dart | |
v.猛冲,投掷;n.飞镖,猛冲 | |
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41 humane | |
adj.人道的,富有同情心的 | |
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42 gathering | |
n.集会,聚会,聚集 | |
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43 density | |
n.密集,密度,浓度 | |
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44 allocated | |
adj. 分配的 动词allocate的过去式和过去分词 | |
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45 poignancy | |
n.辛酸事,尖锐 | |
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46 tusked | |
adj.有獠牙的,有长牙的 | |
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