题目内容：

Directions: Read the following passage. Summarize the main idea and the main point(s) of the passage in no more than 60 words. Use your own words as far as possible.

The Decline of Biodiversity

Biodiversity is the range of living things in an area. During the last 100 years, scientists have seen a great decline in biodiversity. Some studies show that one in eight plant species is threatened with extinction. It is estimated that 140,000 species of plants are lost each year.

Most of the extinctions in the last thousand years are due to humans. The main cause is the chopping down of tropical rain forests. Most of this destruction is done to create pasture (牧场) for beef cattle. Many rainforests in Central and South America have been burnt down to make way for cattle farming, which supplies beef to the rest of the world. It is estimated that for each pound of beef produced, 200 square feet of rainforest are destroyed. The forests are also cut down to make wheat for bread as well as fruit for human consumption.

The introduction of exotic (外来的) species is another threat. The latest research from the University of Southampton has revealed the impact of exotic species upon native wildlife, which could potentially lead to the extinctions of local species. When exotic species are introduced to a place, they try to establish a self-sustaining population. While the local species, which haven’t had a chance to evolve, often lack defenses and thus cannot compete against the exotic species.

The disappearance of certain food animals can be disastrous not only to larger animals but also to the entire ecosystem. It is these small creatures which convert much of the energy in an ecosystem from unusable (i.e. microscopic plants, decaying matter, etc.) into usable forms, namely their bodies. Their absence makes most energy and nutrients trapped in forms unusable to other species. This makes the environment less suitable for healthy living, and less capable of producing resources that humans need.

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