托福考试 复习
TPO 27—3 Predator-Prey Cycles
原文：
【1】How do predators affect populations of the prey animals? The answer is not as simple as might be thought. Moo 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 ass
umed that the wolves would play a key role in controlling the moo population. Careful studies have demonstrated, however, that this is not the ca. The wolves eat mostly old or diad animals that would not survive long anyway. In general, the moo population is controlled by food availability, dia and other factors rather than by wolves.
【2】When experimental populations are t up under simple laboratory conditions, the predator often exterminates its pre and then becomes extinct itlf, having nothing left to eat. However, if safe areas like tho prey animals have in the wild are provided, the prey population drops to low level but not extinction. Low prey population levels then provide inadequate food for the predators, causing the predator population to decrea. When this occurs, the prey population can rebound. In this situation the predator and prey population may continue in this cyclical pattern for some time.
【3】Population cycles are characteristic of small mammals, and they sometimes appear to be brought about by predators. Ecologists studying hare populations have found that the North American snow shoe hare follows a roughlyten-year cycle.
Its numbers fall tenfold to thirty in a typical cycle, and a hundredfold change can occur. Two factors appear to be generating the cycle: food plants and predators. 【4】The preferred foods of snowshoe
hares are willow and birch twigs. As hare density increas, the quantity of the twigs decreas, 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 year for the quantity of twigs to recover.
【5】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 fall, so do lynx numbers, as their food supply depleted.
【6】What caus the predator-prey oscillations? Do increasing number of hares lead to overharvesting of plants, which in turn results in reduced hare populations, or do increasing numbers of lynx lead to overharvesting 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 tho plots (no food effect) and predators were excluded (no predator effect) from an experimental area, hare numbers incread 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 prence of predators (predator effect alone). Thus both factors can affect the
cycle,裤子粘毛怎么解决
which, in practice, ems to be generated by conjunction of the two factors. 【7】Predators are an esntial factor in maintaining communities that are rich and diver 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, a stars prey on a variety of bivalve mollusks and prevent the bivalves from monopolizing habitats on the a floor. This opens up space for many other organisms. When a stars are removed, species diversity falls sharply. Therefore, from the stand point of diversity, it is usually a mistake to eliminate a major predator from a community
题目：
1.In paragraph 1, why does the author discuss the moo and wolves on Isle Royale?邓超哪里人
实干
A.T o provide an example of predators moving to new habitats by following migrating prey.
五朵玫瑰花代表什么意思B.To show that the interactions between predator populations and prey populations are not always might be expected.
C.To suggest that prey populations are more influenced by predation than food availability and dia.
描写田野的句子D.T o argue that studies of geographically isolated populations tend not to be uful
to naturalists.
2.The word “rebound” in the passage(paragra ph 2)is clost inmeaning to
A.escape.
少年书架
C.survive.
关于坚持的文章
3.Paragraph 2 implies which of the following about experimental environments in which predators be
come extinct?
A.They may yield results that do not accurate predict changes of populations in the wild.
B.In the environments, the prey species is better adapted than the predator species.
C.The environments are appropriate only for studying small populations of predators and prey.
D.They are unrealistic becau some predators are also the prey of other predators.
4.Which of the following can be inferred from paragraphs 2 and 3 about the small mammals that experience population cycles?
A.Their population cycles are not affected by predators.
商业保险一年交多少钱B.Their predators’ populations periodically dis appear.
C.They typically undergo ten-year cycles.
D.They have access to places safe from predators.
5.The word “roughly” in the passage is clost in meaning to
