题干

    Nick is a 14-year-old school boy. His life is full of exams and studies on weekdays. He has little free time. He thinks playing computer games is the best way to make him relax. When he has free time, he sits in front of the computer. Just like that way, he neither eats nor drinks for several hours.
    Last weekend, he played games on the computer again. He was too excited and didn't want to move. He didn’t have anything for six hours. When he had to go to the bathroom, he found he could not move. He was taken to the hospital. The doctor told him he should do some more different kinds of activities. In other words, he needs more exercise and outdoor activities to make him have a healthy body.
After coming back from the hospital, Nick follows the doctor’s advice. He often plays soccer with his friends. Sometimes he still plays computer games on weekends, but he never does it for long. Now, he lives a happy and healthy life.

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回答下列关于草原生态系统的问题.

(1)草原上不同地段上分布着种类和密度不同的牧草,在水平方向上它们通常呈____ 分布.草原上的植被能防风固沙、保持水土,对于缓解相邻地区的沙尘天气有一定的作用,这体现了生物多样性的____ 价值.

(2)蝗灾已成为草原农牧业生产的重大危害之一.图甲示研究者对草原上四种主要蝗虫密度对牧草产量(牧草产量=牧草补偿量一蝗虫取食量)影响的实验结果;图乙为农业部门对某次“亚洲小车蝗”的爆发和防治过程中种群密度的调查结果.

①由图甲可知,该实验的因变量是____ ,甲图所示结果为研究者依据实验结果建立的____模型.当狭翅雏蝗密度为5头/m2时,牧草产量增加,原因是____,说明蝗虫只有达到一定量才会导致危害,因为生态系统具有____ .

②若图乙是对蝗虫的幼虫跳蝻进行调查所获得的结果,则该调查采用的方法是____ .为探究治理蝗灾的最好措施,农业工作者在蝗灾期间,分别采用了农药防治和生物防治的方法.由图乙可知,图中____ 点是进行了农药防治,随农药使用次数的增多防治效果变差;图中____ 点是引入害虫的天敌进行生物防治,利用生物间的____ 等种间关系,使防治效果持久而稳定.

(3)严重的草原火灾会破坏生态系统,但适当的“地面火”能烧掉枯枝落叶,加速生态系统的____ ,还可使部分位置获得更多光线,促进植物的光合作用,利于植被生长.火灾后的草原演替的过程中,生物种类、数量及有机物含量的变化趋势是____ (填“增加”、“减少”或“不变”).