【正确答案】Opportunist species colonize an area when an environment undergoes a change unfavorable to the dominant plant species. Their dominance is often fast and widespread, but temporary, because opportunist plants can be easily warded off by the original species when the conditions are favorable to the original equilibrium species. For example, in the Great Plain of the Western U.S., the blue-bunch is a common and dominant species in the area, but under the threat of cattle. Once the plants are, by large, consumed by cattle, opportunist species, the leafy spurge comes to predominate the place. But if the cattle are removed or reduced to the degree that the blue-bunch is able to re-establish themselves, the opportunist species will be warded off by the dominant species.

【答案解析】 讲座主要讲了机会主义植物是怎样在一个地区达到全胜再走向灭绝的。“Here is a common example of such plant species in the Western U．S．”讲座用美国西部的一种植物来解释这一过程。