2021年7月10日托福阅读回忆及解析：生物学

　　2021年7月10日托福阅读回忆及解析，本文内容由环球教育北京学校张泽老师提供整理，以供大家学习参考。更多托福资料以及备考托福的干货内容，请关注环球教育，环球教育托福暑假班正在火热报名中，感兴趣的同学欢迎随时咨询我们的在线客服老师 。

　　托福阅读

　　Passage two 生物学

　　题目：鸟类和他的食物

　　内容回忆：有一些insects会摄入一些植物产生的毒素进入体内，这对于这些insects是有很大帮助的。这些吃了并且在身体内储存的毒素的insects，本身的身体会呈现出各种各样奇怪的颜色。这个颜色本身并不能防止自己被自己的predator吃掉，但是当鸟类吃掉这些虫子之后，鸟类自己的身体就会生病，但是不会死掉，鸟类会很不舒服，甚至会掉毛。而这些insect本身并不是经常吃这种有毒素的植物，他们其实是在年幼的时候，吃了这种带毒素的植物的，但是他们吃了之后，就会一直把毒素存在自己的体内。鸟类吃了这种有毒的insects之后，身体会不舒服，因此他们就会记住，这种虫子会让他们不舒服，然后他们以后就不会再吃这种虫子了。之所以这种带毒素的虫子不会让鸟类死亡，是因为鸟类一旦死亡了，那么鸟类的首领就会换为其他的鸟类。而且他的鸟类因为没吃这种虫子，因此就不会告诉其他的鸟类不要吃这种虫子，因此带毒的虫子就还会继续被吃。因为这些鸟类不会死掉，因此这些鸟类就继续做首领，他们就会告诉其他的鸟类不要去吃这种带毒素的虫子，这样同类的insects存活率就会大增，因为鸟类会长期在这个固定的区域移动，而虫子也会在这个固定的区域移动，鸟类认识了这种虫子，他们就不会再吃这种虫子了，因此这种虫子的存活率就增高了。

　　参考阅读：Predator-Prey Cycles

　　How do predators affect populations of the prey animals? The answer is not as simple as might be thought. Moose reached Isle Royale in Lake Superior by crossing over winter ice and multiplied freely there in isolation without predators. When wolves later reached the island, naturalists widely assumed that the wolves would play a key role in controlling the moose population. Careful studies have demonstrated, however, that this is not the case. The wolves eat mostly old or diseased animals that would not survive long anyway. In general, the moose population is controlled by food availability, disease, and other factors rather than by the wolves.

　　When experimental populations are set up under simple laboratory conditions, the predator often exterminates its prey and then becomes extinct itself, having nothing left to eat. However, if safe areas like those prey animals have in the wild are provided, the prey population drops to low levels but not to extinction. Low prey population levels then provide inadequate food for the predators, causing the predator population to decrease. When this occurs, the prey population can rebound. In this situation the predator and prey populations may continue in this cyclical pattern for some time.

　　Population cycles are characteristic of some species of small mammals, and they sometimes appear to be brought about by predators. Ecologists studying hare populations have found that the North American snowshoe hare follows a roughly ten-year cycle. Its numbers fall tenfold to thirtyfold in a typical cycle, and a hundredfold change can occur. Two factors appear to be generating the cycle: food plants and predators.

　　The preferred foods of snowshoe hares are willow and birch twigs. As hare density increases, the quantity of these twigs decreases, forcing the hares to feed on low-quality, high-fiber food. Lower birth rates, low juvenile survivorship, and low growth rates follow, so there is a corresponding decline in hare abundance. Once the hare population has declined, it takes two to three years for the quantity of twigs to recover.

　　A key predator of the snowshoe hare is the Canada lynx. The Canada lynx shows a ten-year cycle of abundance that parallels the abundance cycle of hares. As hare numbers increase, lynx numbers do too, rising in response to the increased availability of lynx food.

　　What causes the predator-prey oscillations? Do increasing numbers of hares lead to overharvesting of plants, which in turn results in reduced hare populations, or do increasing numbers of lynx lead to overharvesting of hares? Field experiments carried out by Charles Krebs and coworkers in 1992 provide an answer. Krebs investigated experimental plots in Canada`s Yukon territory that contained hare populations. When food was added to these plots (no food effect) and predators were excluded (no predator effect) from an experimental area, hare numbers increased tenfold and stayed there-the cycle was lost. However, the cycle was retained if either of the factors was allowed to operate alone: if predators were excluded but food was not added (food effect alone), or if food was added in the presence of predators (predator effect alone). Thus, both factors can affect the cycle, which, in practice, seems to be generated by the conjunction of the two factors.

　　Predators are an essential factor in maintaining communities that are rich and diverse in species. Without predators, the species that is the best competitor for food, shelter, nesting sites, and other environmental resources tends to dominate and exclude the species with which it competes. This phenomenon is known as "competitor exclusion." However, if the community contains a predator of the strongest competitor species, then the population of that competitor is controlled. Thus even the less competitive species are able to survive. For example, sea stars prey on a variety of bivalve mollusks and prevent these bivalves from monopolizing habitats on the sea floor. This opens up space for many other organisms. When sea stars are removed, species diversity falls sharply. Therefore, from the standpoint of diversity, it is usually a mistake to eliminate a major predator from a community.

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