VOA标准英语2011--Species Loss Compromises Earth’s Vital

（单词翻译）

Species Loss Compromises Earth’s Vital Systems

Scientists have long thought that diverse ecosystems² like forests, lakes and streams are especially good at removing pollutants⁴ that human activities put into the environment.
A new study in the journal Nature, confirms that theory.
University of Michigan ecologist Bradley Cardinale demonstrates how water quality improves in a more diverse ecosystem¹.
In his laboratory Cardinale built 150 meter-long cases that enclosed mock streams. He cultured between one and eight common algae⁵ species in each waterway. The tiny micro-organisms are important actors in removing pollution from the water. He then exposed the streams to nitrate, a common pollutant³ from agricultural run-off and auto⁶ emissions⁷.
Bradley Cardinale
University of Michigan graduate students monitor a water quality experiment.
It took Cardinale three years to set up the experiment, run it and process the data. On average streams with eight species were cleaned four-and-one-half times faster than water with just one species. Cardinale says specific algae species adapt to specific stream habitat.
"As you added more and more species to the stream what happened is that all of these different habitats got filled up and the stream as a whole became a much better bio-filter for this particular nutrient⁸ pollutant."
Cardinale says scientists as far back as Charles Darwin in the 1860s proposed that every species plays a specific role in the ecosystem and compliments each other.
"Basically a diverse world always comes down to having unique niches¹⁰ that allow species somehow to exist with each other. So any system where niche⁹ partitioning is a key biological phenomenon, the results of the study I've shown here and it's implication for water quality should probably apply."
Bradley Cardinale
This sample mock stream contains algae growth after six months and represents between 12 to 15 generations and millions of algae cells.
While Cardinale's study showed that water quality improved in the streams with greater biodiversity, the ecologist notes it did not address how many species would be needed to completely remove the pollutant from soil and water.
"We previously¹¹ expected that somewhere between three and five species was enough to clean nitrate out of soil and water. And my study extends those results to at least to eight species where we didn't even begin to see a plateau."
Cardinale says he expects the number is more than eight, but far less than the several hundred found in a stream. The University of Michigan ecologist hopes future studies will determine that tipping point.