河马对生态系统意义重大?河马能为水生物制造大量的关键物质。体型庞大的河马是如何做到的，它们的数量又为什么在减少呢?研究者警告称河马的数量正在下降，这将有损生态系统。到底是怎样的?看看今天托福阅读材料的说明吧!
在进行托福阅读之前，一起学习一下文章中的疑难词!
tremendous adj. 极大的，巨大的；惊人的
Ingredient n. 原料；要素
Aquatic n. 水上运动adj. 水生的；水栖的
Mighty adj. 有力的；强有力的
Marine adj. 船舶的；海生的
Terrestrial adj. 地球的；陆地的
Vector n. 矢量；航线
crunch n. 咬碎vt. 压碎
appetite n. 食欲；嗜好
straw n. 稻草；吸管
fling n. 掷，抛vi. 掷，抛vi. 猛冲，急行
开始阅读吧!
A new study says the hippopotamus is very important to the health of Africa’s rivers and lakes. That’s because hippos produce a key ingredient for aquatic life -- and a lot of it. But researchers warn that hippo populations are on the decline and that could damage ecosystems.
The mighty hippopotamus is the third largest land mammal following the elephant and rhinoceros. Its name is derived from the ancient Greek for river horse. Hippos spend up to 16 hours a day in the water and venture on to land at night to feed. They eat tropical grasses, more than 200 kilograms per meal. And what goes in must come out. And there is the magic ingredient for Africa’s aquatic ecosystems – dung.
Douglas McCauley and his colleagues decided to study it.
“Well, we started looking at hippopotamus and realized that a big part of the story was their poop – was this vast amount of nutrients and energy that are moved across systems via their eating and their defecation,” he said.
McCauley is an assistant professor of ecology, evolution and marine biology at the University of California Santa Barbara. He said hippos cross the boundaries of ecosystems.
“Transiting between two different domains – between land, the terrestrial world, and then the aquatic world – means that there are also these important nutrient vectors. Because their eating lots and lots of stuff on land and then taking that all back to the place where they rest in the water -- in lakes and rivers -- and then basically dunging that all out. And it turns out when you start crunching the numbers to be a huge amount of material and energy and nutrients of this sort of natural fertilizer that is moved across these boundaries,” he said.
“It’s a big animal,” he said, “We’re talking about an animal that’s about 4,000 to 8,000 pounds. So, it’s got a big appetite. When you look across the entire continent and our estimate of how many hippos we now have in sub-Saharan Africa that comes out to millions of kilograms.”
So, by now you understand it’s a tremendous amount of dung. But did you know that hippo dung looks like straw?
“That’s right. So, they are mostly feeding on grass, sort of a straw-like grass. In fact, a lot of African grasses are used as straw and forage for cattle in places like America. So, they have these wonderfully well-developed lips that they use like a lawnmower to go across these short grass lawns that they actually manicure. And they keep them short because it seems to keep this resource accessible just to them,” said McCauley.
McCauley said it’s even used as a communication device.
“They actually use dung as a kind of signal amongst themselves. So, a male animal will dung at a sub-dominant male. They actually fling it back and forth with their paddle-like tail.”
But the really important thing about hippo dung is its place in the food chain.
“Well, there’s a lot of good stuff in dung. There’s a lot of nitrogen, carbon, even a bit of phosphorus. And some of these nutrients can be limiting in river systems. These are sort of the building blocks of life in some cases. So it really is injecting more of this food into these rivers. And a lot of the animals it seems in the rivers are quite happy to receive it. Some of them eat it up directly and some consumers, like fish, will eat insects that started their lives feeding on dung,” he said.
McCauley actually described it as a life force in Africa’s rivers and lakes. But he said if the water flow is too low, the dung could overwhelm the ecosystem and be a pollutant. Too much water and its value is diluted.
But as researchers learn more about the importance of hippos, there are fewer of them.
“Hippos are declining across sub-Saharan Africa. In the past decade or so, we’ve seen about a 10 to 20 percent decline in their numbers. And beyond how many there are they’re in a lot fewer places. There are entire countries that have completely lost hippopotamus -- Egypt, for example. Hippos used to be an iconic god. It was the god of birth in Egypt. You see the hippo god. It’s a goddess turning up on amulets and wands and swords,” he said.
McCauley said humans are responsible for most of the decline in the hippo population through hunting and habitat loss as human populations spread.
“They have to have water. Well, guess what? Everybody else wants water. And it’s really unfortunate if you’ve tied your fate to water because when you compete with humans for water you often lose. So, people want water from rivers and lakes for building out cities – for helping to cool turbines– or damming rivers,” he said.
It’s estimated hippos kill about 3,000 people a year. The animals may attack, for example, if people get too near a mother and calf.
McCauley said careful thought must be given to the management of water, both for humans and wildlife. He says if managed intelligently, there should be plenty of water to go around, adding that the fates of humans, wildlife and ecosystems are closely tied together.
长难句：
1.we started looking at hippopotamus and realized (that a big part of the story was their poop – was this vast amount of nutrients and energy that are moved across systems via their eating and their defecation).
句子类型：宾语从句+定语从句
句子拆分：
主干：we started looking at hippopotamus and realized…
宾语从句: that a big part of the story was their poop – was this vast amount of nutrients and energy that are moved across systems via their eating and their defecation
定语从句：that are moved across systems via their eating and their defecation
“–”后是对their poop的进一步解释。
翻译：我们研究河马时意识到它们的粪便非常重要，有大量的营养物质和能量通过它们的饮食和排泄来在生态系统内循环。
2.And it turns out (when you start crunching the numbers to be a huge amount of material and energy and nutrients of this sort of natural fertilizer )that is moved across these boundaries
句子类型：宾语从句+时间状语从句
句子拆分：
主干：And it turns out that…
宾语从句: that is moved across these boundaries
时间状语从句: when you start crunching the numbers to be a huge amount of material and energy and nutrients of this sort of natural fertilizer
翻译：所以如果进行计算，就会发现这种天然粪便中有非常大体量的物质、能量和营养物质在不同的系统间运输。
3.So, they have these wonderfully well-developed lips (that they use like a lawnmower to go across these short grass lawns that they actually manicure).
句子类型：定语从句
句子拆分：
主干：they have these wonderfully well-developed lips.
定语从句: that they use like a lawnmower to go across these short grass lawns that they actually manicure，that they actually manicure也是一个定语从句
翻译：所以，它们的嘴唇很发达，在吃短草坪时很像除草机那样修剪杂草，它们让杂草保持啃得很短，因为似乎只有它们自己能吃到这么短的草。
文章大意：
河马对于非洲河流湖泊的生态很重要并分析其中的原因。研究者认为河马数量减少会对生态不利。并介绍了河马生活习性。
加州大学的道格拉斯·麦考利和同事们决定研究河马的粪便，他认为大量的营养物质和能量通过它们的饮食和排泄来在生态系统内循环，河马参与了不同的生态系统。河马在两个不同的领域—陆上和水中进行活动，这意味这些重要的营养物质也在水陆之间传输。因为这种动物很庞大，有4000到8000磅，所以它们胃口很大。所以着眼于整个非洲大陆，现在撒哈拉南部非洲的河马能制造数百万公民的粪便
麦考利说河马甚至使用粪便来作为通讯工具：它们使用粪便作为自己族群中的信号。但河马粪便真正重要的意义在于其在食物链中的地位。
粪便里有营养物质，有大量的氮、碳、甚至还有磷，供给给河中的很多动物。像鱼类这样的捕食者会以食用粪便的昆虫为生。
麦考利称河马粪便是非洲河流湖泊的生命物质，但他说如果河水流动太慢，粪便就会破坏生态系统，就成了污染物质，如果水太多，粪便的价值就会被稀释。
但就在研究者对河马的重要性有更多了解时，河马的数量却在减少。
麦考利说人类要对河马数量减少负很大责任，随着人口的流动，人类的狩猎和占领栖息地导致河马减少。其中因为水的竞争使得其生存受制。麦考利说必须认真管理水，无论是为人类还是为野生。他说如果管理得当，应该有大量的水可供使用，因为人类、野生物和生态系统的命运是密切关联的。
文章架构：
第一部分，1,2段，主旨：河马对于非洲河流湖泊的生态很重要，并对河马的习性简要介绍。
第二部分，3-10段，研究者认为大量的营养物质和能量通过它们的饮食和排泄来在生态系统内循环，11-14段，河马参与了不同的生态系统。并详细说明。
第三部分，15-20段，强调：河马粪便可以作为通讯工具：它们使用粪便作为自己族群中的信号。但河马粪便真正重要但就在研究者对河马的重要性有更多了解时，河马的数量却在减少的意义在于其在食物链中的地位。
第四部分：就在研究者对河马的重要性有更多了解时，河马的数量却在减少，说明了人类影响。
文章题材：
自然科学—河马与生态系统
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