Day 82
Passage 82
Competition among Species
1
Competition is a natural occurrence between organisms occupying the same space at the same time. Organisms may live in harmony with each other, yet if the environment couldn't supply enough resources to meet all organisms’ requirements, they would interfere with each ether; competition then follows.[1] Plants, for example, often compete for access to a limited supply of nutrients, water, sunlight, and space. Two main types of competition exist; intraspecific competition occurs when individuals of the same species vie for access to essential resources, while interspecific competition occurs between different species.
2
Studies show that intraspecific competition can regulate population dynamics because when individuals become crowded as the population grows, resources will become more limited. Some individuals, typically small juveniles, eventually do not acquire enough resources and die before they mature or do not reproduce, thus reducing population size and slowing population growth. Interspecific competition, on the other hand, can not only alter the sizes of many species’ populations at the same time, but even result in the extinction of a local species. Many experiments have suggested that competing species cannot coexist because the best competitor will exclude all other competing species.
3
The concept of competitive exclusion arose from the work of G.F. Gause, a Russian ecologist who raised in test-tubes two different species of Paramecium, a single-celled, ciliated protozoan that feeds on bacteria. Both species thrived when raised alone, reaching some limit set by the volume of the test-tube and the supply of bacteria. Yet when raised together, the growth rate was slowed for both until one species declined and the other then grew to the capacity of the test-tube. Evidently these two ciliates were competing for a limiting resource, sharing the same niche, and competition determined that only one persisted. This study was influential in establishing the fact that competition could occur and could eliminate one species from a shared environment. Gause then formed the competitive exclusion principle, which states that species with ecologically identical life styles and resource needs cannot coexist over the longer term, and the competitively less-fit species will be displaced by the better fit species.
4
While such laboratory studies lead to the formation of the competitive exclusion principle, field observations suggest that niche differences, while sometimes subtle, are invariably found between co-existing species. Joe Connell studied barnacles (shelled marine organisms) that live on rocks along European coastlines in 1961 and found that two barnacle species, Balanus and Chthamalus can coexist because they differ in two traits: growth rate and vulnerability to desiccation. Balanus's growth is rapid, which allows it to smother and crush the slower-growing Chthamalus. However, Balanus is vulnerable to desiccation and mainly lives in the lower inter-tidal zone. In contrast, Chthamalus can tolerate dry conditions and hence can live in the upper inter-tidal zones, even to the point where some individuals are just submerged during wave surges at the highest tide. Consequently, even though Balanus is a better competitor for space, these barnacles coexist because Chthamalus can survive in areas that Balanus cannot survive. This and many other examples actually support the competitive exclusion principle: species can only coexist if they have different niches.
5
Competition is likely to result in evolutionary divergence and specialization among closely related species. This occurs because the individuals of a species with traits similar to competing species always experience strong interspecific competition. These individuals have less reproduction and survival than individuals with traits that differ from their competitors. Consequently, they will not contribute many offspring to future generations. The Galapagos finches appear to be good examples of such divergence. Two finch species, Geospiza fuliginosa and Geospiza fortis can be found alone or together on the Galapagos Islands. Studies show that when G. fortis and G. fuliginosa are present on the same island, G. fuliginosa tends to evolve a small beak and G. fortis tends to evolve a large beak. The observation that competing species’ traits are more different when they live in the same area than in different areas is called character displacement. For the two finch species, beak size was displaced: Beaks became smaller in one species and larger in the other species. Studies of character displacement are important because they provide evidence that competition plays a very important role in determining ecological and evolutionary patterns in nature.
——2011年04月02日北美考试机经
According to Paragraph 4, Chthamalus can coexist with Balanus mainly because____.
A. they are different species
B. they grow at different times
C. they live in different inter-tidal zones
D. they are all vulnerable to desiccation
核心词汇：
词汇练习：
阅读下列句子，用所给单词（或词组）的正确形式填空：
vulnerability trait tolerate submerge smother reproduction regulate
raise occurrence invariably niche interfere identical extinction
evolve eliminate displace desiccation decline
1. The _____ of these eye movements provides evidence that the moving pattern is perceived at some level by the newborn. (TPO-13: Methods of Studying Infant Perception)
2. Large wind farms might also _____ with the flight patterns of migratory birds in certain areas, and they have killed large birds of prey (especially hawks, falcons, and eagles) that prefer to hunt along the same ridge lines that are ideal for wind turbines. (Online Test: Electricity from Wind)
3. It subconsciously adjusts breathing rate and depth in order to _____ the levels of carbon dioxide (CO2) and oxygen (O2), and the acid-base ratio in the blood. (TPO-24: Breathing During Sleep)
4. Similarly, a plant or animal cannot squander all its energy on growing a big body if none would be left over for reproduction, for this is the surest way to _____. (Online Test: Opportunists and Competitors)
5. Newly _____ regiments were normally transferred to another province from whence it was unlikely that individual recruits would ever return. (TPO-19: The Roman Army's Impact on Britain)
6. After the canal went into operation, the cost per mile of transporting a ton of freight from Buffalo to New York City _____ from nearly 20 cents to less than 1 cent. (TPO-20: Westward Migration)
7. Even if the new population is of a different species, it can approximately fill the _____ vacated by the extinct population and keep the food web intact. (TPO-3: The Long-Term Stability of Ecosystems)
8. Researchers are evaluating how serious this problem is and hope to find ways to _____ or sharply reduce this problem. (Online Test: Electricity from Wind)
9. So, in another set of experiments, Kramer put _____ food boxes around the cage, with food in only one of the boxes. (TPO-11: Orientation and Navigation)
10. Opportunists must constantly invade new areas to compensate for being _____ by more competitive species. (Online Test: Opportunists and Competitors)
11. These critics were making a common assumption—that the technological inadequacies of earlier efforts (poor synchronization, weak sound amplification, fragile sound recordings) would _____ occur again. (TPO-12: Transition to Sound in Film)
12. Becoming a reflective practitioner requires extra work (Jaworski, 1993) and has only vaguely defined goals with, perhaps, little initially perceivable reward and the threat of _____. (TPO-9: Reflection in Teaching)
13. Their migration may have been set in motion by an increase in population caused by a movement of peoples fleeing the _____, or drying up, of the Sahara. (TPO-7: Agriculture, Iron, and the Bantu Peoples)
14. Too much snow, for example, can _____ trees, and avalanches and snow creep can damage or destroy them. (TPO-1: Timberline Vegetation on Mountains)
15. In contrast, a complex climax community, such as a temperate forest, will _____ considerable damage from weather to pests. (TPO-3: The Long-Term Stability of Ecosystems)
16. It had been assumed that the ice extended westward from the Alaskan/Canadian mountains to the very edge of the continental shelf, the flat, _____ part of the continent that extends into the ocean. (TPO-9: Colonizing the Americas via the Northwest Coast)
17. Therefore, these social _____ helped to produce the nomadic and daring settlers who kept pushing westward beyond the fringes of settlement. (TPO-20: Westward Migration)
18. All organisms, therefore, allocate energy to growth, _____, maintenance, and storage. (Online Test: Opportunists and Competitors)
19. Sometimes primary group relationships ____ out of secondary group relationships. This happens in many work settings. (TPO-13: Types of Social Groups)
参考答案：
1. occurrence 2. interfere 3. regulate 4. extinction 5. raised
6. declined 7. niche 8. eliminate 9. identical 10. displaced
11. invariably 12. vulnerability 13. desiccation 14. smother 15. tolerate
16. submerged 17. traits 18. reproduction 19. evolve
注释
[1]与今日百句译（Sentence 82）相同的句型。