How Invasive Species Impact Native Ecosystems入侵物种对本地生态系统的影响

Invasive species are the second largest threat to biodiversity after habitat loss. An invasive species is a species that is not native to a particular area， but arrives （usually with human help）， establishes a population， and spreads on its own. Invasive species have much larger impacts on an ecosystem than other species. They have a disproportionate effect， which is what makes them so harmful.

入侵物种是生物多样性面临的第二大威胁，仅次于栖息地丧失。入侵物种并非某个特定地域的本土物种，而是从别处来到该地（通常由人类协助），继而建立种群，最终自行蔓延的物种。相较于其他物种，入侵物种对生态系统的冲击要猛烈得多。它们拥有巨大的影响力，也因此极具危害性。

Not all species that arrive in a new location become invasive—in fact， most do not. Lots of garden plants are imported from other places， and even if they sometimes grow wild， they do not achieve big populations and do not make a huge splash on native species survival. These are just called non-native or introduced species. The introduced species that do become invasive are the ones that cause big problems.

并非所有到达新地域的物种都会变得有侵略性。实际上，大部分新来的物种不具备这一特性。许多园林植物是从其他地方引入的，尽管有时它们肆意生长，但不会形成大规模的种群，也不会对本土物种的生存造成重大影响。人们将这类物种称为“外来物种”或“引入物种”。只有造成严重后果的引入物种才是具有侵略性的入侵物种。

Some places are especially vulnerable to invasive species. Islands usually have lots of endemic species and few large grazers or predators; this makes island species more at risk when non-native species are introduced.

有的地域特别容易受到入侵物种的伤害。岛屿通常拥有众多特有物种，又少见大型食草动物或捕食性动物。因此，引入外来物种时，岛屿上的物种会面临更大的风险。

Other places have very low rates of species invasion， usually because the climate is especially harsh， like in the Arctic， or organisms need special adaptations to live in that particular habitat， like mangrove trees that can tolerate living in saltwater. As humans move around the globe， more non-native species are turning up in these harsh habitats， some from similar habitats in other parts of the world. These species are more likely to become invasive.

还有的地域，物种入侵率非常低，通常因为气候条件极端恶劣，比如北极地区；或者因为生物需要特殊的适应能力才能在那种特别的栖息地存活，例如红树林能够忍受在咸水中生长。随着人类在全球范围内四处迁移，越来越多的外来物种现身环境恶劣的栖息地。其中部分物种来自世界其他地区类似的栖息地，它们成为入侵物种的可能性更大。

Here are some ways invasive species impact native ecosystems.

以下是入侵物种影响本地生态系统的几种方式。

Habitat modification

改变栖息地

European beachgrass （Ammophila arenaria） was introduced to sand dunes on the coast of California， Oregon and Washington in the 1800s. Beachgrass was planted to stabilize the ever-shifting dunes， and it worked well. Too well. European beachgrass forms huge stands of tall， itchy grass and is usually the only species living on dunes that once had a large variety of native grasses and wildflowers. The beachgrass provides more cover for predators that eat seabirds and native plants.

19世纪，人们将欧洲滨草引入美国加利福尼亚州、俄勒冈州和华盛顿州海滨的沙丘。种植滨草的目的是稳固不断移动的沙丘，而这一方法取得了显著成效。不过，影响过度了。如今，高大无比、令人瘙痒的欧洲滨草大片生长，通常是沙丘上唯一存活的物种，而那里曾经遍布各种各样的本土野生花草。滨草也为捕食海鸟，食用本土植物的掠食者提供了更多掩护。

In another instance of habitat modification， North American beavers （Castor canadensis） became invasive in Tierra del Fuego， Argentina. In the 1940s， 50 beavers were introduced to start a fur industry. Now， there are between 50，000 and 100，000 beavers wreaking havoc on the native trees. Unlike many North American trees that grow back from their roots when beavers chew them down， South American trees such as the southern beech die when a beaver gnaws down their trunk. Beavers have turned 40 million acres of forest into meadows and converted streams into bogs.

入侵物种改变栖息地的另一个例子是北美海狸入侵阿根廷火地岛。20世纪40年代，火地岛引入50只海狸，自此开创毛皮工业。如今，火地岛上有5万至10万只海狸，它们对当地的树木造成了严重破坏。不少北美树种被海狸啃断后可以从根部重新生长。与之相反，南美树种（如南方山毛榉）被海狸咬断树干后往往会死亡。海狸已将4000万英亩森林变成草地，把多条溪流变成沼泽。

Competition for resources

资源竞争

Invasive species can compete with natives for food and space. Zebra mussels （Dreissena polymorpha） have spread throughout the Great Lakes region very quickly since the 1980s. Mussels need hard surfaces to live on， and zebra mussels take up space that native mussels could use， or sometimes just settle on top of the native mussels， killing them in the process.

入侵物种可能会与本土物种争夺食物与空间。自20世纪80年代以来，斑马贻贝在五大湖地区迅速蔓延。贻贝需要生存在坚硬的表面上，而斑马贻贝占据了可供本土贻贝使用的空间，有时甚至直接生长在本土贻贝身上，导致本土贻贝在这个过程中死亡。

Predation

捕食

Guam， an island in the Pacific Ocean， has lost most of its birds to an invasive species. The brown tree snake was introduced to the island and wiped out three-quarters of the native bird species and two of the eleven native lizard species. Brown tree snakes have been introduced to other Pacific islands and had similar impacts in those places by eating eggs， young and adult birds， and reptiles.

关岛是太平洋上的一个岛屿，岛上大部分鸟类因一个入侵物种而灭绝。引入关岛的棕树蛇导致3/4的本土鸟类物种和11种本土蜥蜴中的2种灭绝。另有若干太平洋岛屿也引入了棕树蛇，它们在那些岛屿上偷食鸟蛋，捕食幼鸟、成年鸟以及爬行动物，对当地生态系统造成的破坏与关岛的类似。

Herbivory

食草性

On the eastern side of the United States， two insects have been wiping out dominant trees1. The Asian balsam woolly adelgid （Adelges piceae） has killed 90%–99% of Fraser fir trees in some areas in the Appalachians by sucking the sap out of the trees. This adelgid also affects fir trees in Washington state and Oregon. Its relative， the hemlock woolly adelgid （Adelges tsugae）， has killed most hemlock trees in New England.