新编大学英语教程第四册Unit 06

 The living things in an ecosystem 1 affect each other in many ways. The consumers that kill other animals for food are called predators 3.The word predator 2 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 entirely 4 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 prey 5 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,killing 6 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 naturalists 8 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, defense 7, 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 parasites 10.These organisms live on or in other living things--their hosts,often spending an entire lifetime with them .In parasitism 11 the parasite 9 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:
 
So ,naturalists observe,a flea 12
 
Hath smaller fleas 13 than he on him prey;
 
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 parasitic 14 fungi,bacteria,flatworms,insects,ticks,and mites 15. 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 ecological 16 knowledge by deliberately 17 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-cane 18 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 symbiosis 19 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 affected 20 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 scraps 21. A biologist found 13,500animals living within shrimps 22, but the total included ninteen species,among them a small fish.
 
In some symbiotic 23 relationships, both organisms benefit. The most common and widespread example of this mutualism is a team of plants called lichen 24. You can find lichens 25 clinging to rocks and tree trunks almost anywhere. Part of the lichen is a fungus 26. Within it are colonies of green algae 27 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 perch 28 on the backs of African mammals such as rhinoceroses 30. The birds feed on lice and ticks in the mammal's skin. This benefits both organisms. Also , the rhinoceros 29 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
 
Slaughter 31 of the Elephants 
 
The planned mass slaughter of five thousand elephants in Kenya's Tsavo Game Park cannot be contemplated 32 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 fascination 33 and awe 34, and something akin 35 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 justification 36 for this massacre 37 . 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. Buffalo 38 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 herds 39. Hunters are following up and killing the beasets with dart 40 guns, using a fatal drug that kills painlessly and almost instantaneously. It is to be hoped the Kenyans will use an equally humane 41 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 gathering 42 with greater density 43 in the areas still left to them. More and more attention is being given to conservation of wild life in allocated 44 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 poignancy 45.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 tusked 46 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