而且土生土长的植物比外来物种更适合在花园中栽培，外来的物种很可能成为本地物种的天敌或竞争者。
Native plants are also better for the environment than plants from other regions. Plants transported from other regions may become a noxious weed in desert climates and choke out native plants.
19世纪末，狗舌草被偶然引入新西兰，像许多入侵的外来物种一样，它很快就变成了一种有害生物。
Ragwort was accidentally introduced to New Zealand in the late nineteenth century and, like so many invading foreign species, quickly became a pest.
“只有一小部分外来物种会在它们的新栖息地引发问题。”田纳西大学生态学和进化生物学教授唐·史密斯说。
"Only a small percentage of alien species cause problems in their new habitats,'' said Don Smith, professor of ecology and evolutionary biology at the University of Tennessee.
在美洲大陆上，外来物种是在第一次接触欧洲后引进的。
Across the American continents, exotic species are those introduced after the first European contact.
生态学家警告说，外表可能具有欺骗性，而且这些外来物种中可能有许多是为人接受的，只是因为没有人记载过其有害影响。
Appearances can deceive, ecologists caution, and many of these exotics may be considered acceptable only because no one has documented their harmful effects.
外来物种和本地物种的区分是人为的。
Distinctions between exotic and native species are artificial.
然而，生态学家警告说，这些表象可能并不是真的；而且，这些外来物种中有许多可能被认为是可接受的，只是因为没有人记载过它们的有害影响。
Yet appearances can deceive, ecologists caution, and many of these exotics may be considered acceptable only because no one has documented their harmful effects.
对非外来物种的研究没有得到类似的结果，这使得研究人员推测鸟类的社会学习能力可能与哺乳动物的社会学习能力有根本性的区别。
Studies on nonalien species have not produced similar results, leading researchers to speculate that avian social learning may be fundamentally different from that of mammals.
生态学家通常把外来物种定义为人们偶然或故意带到新地方的物种。
Ecologists generally define an alien species as one that people, accidentally or deliberately, carried to its new location.
我们正在做的另一件事是除掉生长在花园里的一些外来物种，并种回一些本地植物和树木。
The other thing we're doing is getting rid of some of the foreign species that are growing in the gardens and putting back some native plants and trees.
现在，外来食肉植物的黑市交易非常活跃，以至于植物学家一直对某些稀有物种的位置三缄其口。
The black market trade in exotic carnivorous plants is so vigorous now that botanists are keeping the location of some rare species a secret.
一些科学家质疑这样的假设，即在自然生态系统中，外来物种的出现是绝对不会被接受的。
A number of scientists question the assumption that the presence of alien species can never be acceptable in a natural ecosystem.
默认情况下，没有外来物种的生态系统被认为是最自然的生态系统。
Ecosystems free of species defined as exotic are, by default, considered the most natural.
在美国总共15万的物种中，有7000种外来物种，当中只有10%是入侵物种。
Of the 7,000 alien species in the United States—out of a total of 150,000 species—only about 10% are invasive.
在美国总共15万的物种中，有7000种外来物种，当中只有10%是入侵物种。
Of the 7,000 alien species in the United States—out of a total of 150,000 species—only about 10 percent are invasive.
令人惊讶的是，本土物种的灭绝很少归因于外来物种的竞争。
Surprisingly few extinctions of indigenous species can be attributed to competition from introduced species.
他们说把外来物种描绘成天生的坏物种是一种不科学的做法。
They say that portraying introduced species as inherently bad is an unscientific approach.
如紫色花的亚洲牡丹和草莓番石榴等外来物种，威胁着如无刺玫瑰和树蕨等脆弱的夏威夷本土物种。
Invaders such as purple-flowered Asian melastome and strawberry guava threaten delicate Hawaiian natives such as thornless roses and tree ferns.
当地的生态系统并不能应付这个外来物种。它正在摧毁我们的环境。
The ecosystem down there can't handle this non-native species. It's destroying the environment.
怎样处理外来入侵的有害物种？
麦克·金南博士说，在某些情况下，向一个国家引进外来物种的人的动机是好的，但导致了灾难性的后果。
Dr MacKinnon says in some cases, alien species have been introduced to a country by people with good motives in mind, but they lead to disastrous results.
这方面的一个经典案例就是金苹果蜗牛，即人们目前所称最具破坏力的侵入性外来物种之一。
One classic example is the Golden Apple snail - now cited as one of the most devastating invasive alien species.
大西洋货轮还在带入外来物种——目前的数量已经达到185种。
Atlantic freighters still bring in foreign species—there are now 185.
可怕的“长头梦想家”琵琶鱼看上去好似《异形》电影中的生物，是最近才在格陵兰岛水域发现的外来物种。
Looking like a creature from the Alien movies, this nightmarish "longhead dreamer" anglerfish (Chaenophryne longiceps) was until recently an alien species to Greenland waters (map).
由于事实上不可能将这些外来物种彻底根除，因此重要的是南非应寻找控制它们滋生的办法。
Since it's virtually impossible to eradicate them, it's vital for the country to seek ways to bring the alien species under control.
但是本地土著物种在以大约每年两个的速度消失，而新的外来物种每年却不止一个。
But native species disappeared at a rate of roughly two per year, while more than one newcomer arrived.
相比之下，入侵性较弱的外来物种所具有的“武器库”可能至少同样存在于某些土产物种。
The more benign aliens, by contrast, were predicted to have arsenals also found in at least some native species.
事实上，这个外来入侵物种问题的解决方案可能就在于饭桌上。
In fact, the answer to this invasive species problem may lie on the dinner table.
岛上的环境污染和人口的迅速增长超过了他们处理废物的能力，往往加强外来物种的影响。
The impact of invasive species is often compounded by pollution and burgeoning human populations on the islands, which have outstripped their capacity to deal with waste.
岛上的环境污染和人口的迅速增长超过了他们处理废物的能力，往往加强外来物种的影响。
The impact of invasive species is often compounded by pollution and burgeoning human populations on the islands, which have outstripped their capacity to deal with waste.
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