6 Broken Chains-Mid

Broken Chains FOCUS Question How does a disrupted food chain affect an ecosystem?

断裂的食物链如何影响生态系统?

Everything's Connected In the brief Alaskan summer, tiny plants use energy from the Sun to quickly sprout and bloom. Busy little rodents called lemmings gather up and eat these plants. Arctic foxes hunt and eat the lemmings. This is an example of a food chain—the movement of energy from one living organism to another. Arctic plants support more than just lemmings. These plants also provide food for insects, hares, and caribou. In addition to nourishing foxes, lemmings are food for snowy owls, weasels, and wolves. These connected food chains make up a food web. If lemmings disappear, what happens to the foxes, wolves, plants, hares, and other organisms in the food web? If a food chain breaks, the effects can reach far and wide.

万物相连在阿拉斯加短暂的夏季，微小的植物利用来自太阳的能量迅速发芽开花。 忙碌的小啮齿动物旅鼠聚集起来吃这些植物。 北极狐捕食旅鼠。 这是食物链的一个例子——能量从一个生物体到另一个生物体的运动。 北极植物不仅养活旅鼠。 这些植物也为昆虫、野兔和驯鹿提供食物。 除了给狐狸提供营养，旅鼠还是雪鸮、黄鼠狼和狼的食物。 这些相连的食物链组成了一个食物网。 如果旅鼠消失了，那么狐狸、狼、植物、野兔和食物网中的其他生物会发生什么? 如果食物链断裂，其影响将会广泛而深远。

Natural Cycles Lemmings in the Alaskan tundra make up a population— all the members of a species living in one place. Populations rise and fall naturally. For example, during an especially warm Alaskan summer, more plants grow. With this extra food, the lemmings have more offspring. Now Arctic foxes, which prey on the lemmings, are able to have more young and their population rises. Before long, with too many predators and not enough food to go around, the lemming population crashes. Then many foxes starve or move to another location to find food. Without hungry lemmings, the plants soon recover With more plants and fewer predators, the lemming population explodes again. While these populations go up and down, the food chain stays in balance over the long term. However, if a species disappears entirely, the food chain breaks.

阿拉斯加苔原上的旅鼠组成了一个种群——一个物种的所有成员生活在一个地方。 人口数量自然地上升和下降。 例如，在阿拉斯加特别温暖的夏天，会有更多的植物生长。 有了这些额外的食物，旅鼠会有更多的后代。 现在，捕食旅鼠的北极狐能够生育更多的后代，它们的数量也增加了。 不久之后，由于捕食者太多，食物不足，旅鼠的数量急剧下降。 然后，许多狐狸要么挨饿，要么搬到另一个地方去觅食。 没有饥饿的旅鼠，植物很快就恢复了随着植物的增加和捕食者的减少，旅鼠的数量再次激增。 虽然这些物种的数量上下波动，但食物链长期保持平衡。 然而，如果一个物种完全消失，食物链就会断裂。

Missing Links Species sometimes disappear for natural reasons. Diseases and natural disasters can leave missing links in food chains. But more often, people are the cause of broken food chains. In the 1800s, people hunted sea otters for their warm, waterproof fur. The animals came close to extinction. Otters eat sea urchins, and sea urchins eat kelp—a large seaweed. Without otters, the sea urchin population exploded. Urchins devoured entire kelp "forests." These forests provided food and shelter to hundreds of other species in the food web. These animals now had nothing to eat and nowhere to go. Today, sea otters are protected. Their numbers have rebounded enough to keep the sea urchin population in check. Many kelp forests have recovered, too.

物种有时会因为自然原因消失。 疾病和自然灾害会在食物链中留下缺失的环节。 但更多时候，人类才是食物链断裂的原因。 在19世纪，人们为了海獭温暖防水的皮毛而猎杀海獭。 这些动物濒临灭绝。 水獭吃海胆，海胆吃海带——一种很大的海藻。 没有水獭，海胆数量暴增。 海胆吞噬了整个海带“森林”。 这些森林为食物网中的数百种其他物种提供了食物和住所。 这些动物现在没有东西吃，也无处可去。 如今，海獭受到保护。 它们的数量已经反弹到足以控制海胆的数量。 许多海带森林也恢复了。

Invasive Links The sea otter example shows what happens when a species disappears. But food chains also break when new species come in. Invasive species are organisms that move into a new area and disrupt the natural food chain. These new links in the chain show up in ecosystems all over the world. But their effect may be greatest in Australia. Australia is a continent surrounded by water. For millions of years, food chains there had a healthy balance of native species—organisms that evolved naturally in the area. When people from Europe arrived in the 1700s, they brought many invasive, or nonnative species. These new plants and animals quickly disrupted many food chains.

海獭的例子说明了当一个物种消失时会发生什么。 但当新物种进入时，食物链也会断裂。 入侵物种是指进入一个新地区并破坏自然食物链的生物。 这些新的链条出现在世界各地的生态系统中。 但它们对澳大利亚的影响可能最大。 澳大利亚是一个四面环水的大陆。 数百万年来，那里的食物链一直保持着当地物种——在该地区自然进化的生物——的健康平衡。 当欧洲人在18世纪抵达时，他们带来了许多入侵或非本土物种。 这些新的植物和动物很快破坏了许多食物链。

When people from Europe started moving to Australia in the late 1700s, pet cats began running wild. Native prey animals had never seen cats. They had no defenses against them. Even today, cats continue to overhunt Australian birds and mammals. In 1859 a hunter released several rabbits into the Australian countryside. With no predators, the rabbit population exploded. They devoured native plants, turning many areas into barren desert. In 1935, people brought cane toads to Australia to eat insects that damaged sugar-cane crops. Big mistake! Instead of eating the insects, the toads ate native rodents, snakes, and birds. Do You Know? In 1890, a man brought European starlings to New York City. He wanted Central Park to have all the birds mentioned in plays written by Shakespeare. European starlings now live across the United States.

18世纪末，当欧洲人开始移居澳大利亚时，宠物猫开始四处流浪。 当地的猎物从未见过猫。 他们没有防御。 即使在今天，猫仍然在过度捕食澳大利亚的鸟类和哺乳动物。 1859年，一位猎人在澳大利亚乡下放了几只兔子。 由于没有天敌，兔子的数量暴增。 它们吞噬了当地的植物，把许多地区变成了贫瘠的沙漠。 1935年，人们把蔗蟾蜍带到澳大利亚，吃破坏甘蔗作物的昆虫。 大错误!蟾蜍不吃昆虫，而是吃当地的啮齿动物、蛇和鸟类。 你知道吗?1890年，一个人把欧洲椋鸟带到了纽约市。 他想让中央公园拥有莎士比亚戏剧中提到的所有鸟类。 欧洲椋鸟现在生活在美国各地。