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澳大利亚有一生态难题,许多牛粪往往将草场上的牧草遮住导致牧草枯死,破坏了草场。从生态系统成分看,牛是____ ,其数量过多,排出的粪便破坏了牧草这一____  ,破坏了原有的生态平衡。从中国引入蜣螂后,清除了粪便,恢复了生态平衡,这里的蜣螂起到了____   的作用。

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     "You see someone just sitting there, looking like they're stupid," explains teenager Manny Logan excitedly. "You just run up to them, slap(掌击) them and run off. It's funny."

    The 16-year-old Logan was describing "happy slapping". It's supposed to be fun but it's a new problem in Britain. Young people will attack passers-by while their friends record the whole thing on camera phones. It was first reported in south London six months ago. But, the practice has spread to other parts of the country and videos of attacks have become popular on the Internet.

    The young people are called "hoodies" because they usually wear a sweatshirt with a hooded top. The top can be used to cover the head or face.

    One study in England published earlier this year suggested that as many as one in four teenagers is an active offender. That includes both serious crimes and small acts like not paying for a bus ticket.

    The fear of youth crime has left some parts of towns empty of ordinary people.

    So, the British government declared war on public disorder last week. The Prime Minister has supported a decision by the country's biggest shopping centre in southeast of England to ban teenagers wearing hooded tops. The clothing helps troublemakers hide from the centre's cameras.

    This week, the government is going further. It plans to force teenage offenders to wear orange jackets while they do public service. The idea is to shame them in front of others.

    But some teenagers and experts have complained that, in their rush to stop the hoodies, politicians risk tarring all teenagers with the same brush.