【正文】 their debts and contracts and received a league guarantee for them. This device proved a decisive factor in the later development of credit and merce in northern Europe.These developments added up to what one modern scholar has called a mercial revolution. In the long run, the mercial revolution of the High Middle Ages (A D 10001300) brought about radical change in European society. One remarkable aspect of this change was that the mercial classes constituted a small part of the total population—never more than 10 percent. They exercised an influence far in excess of their numbers. The mercial revolution created a great deal of new wealth, which meant a higher standard of living. The existence of wealth did not escape the attention of kings and other rulers. Wealth could be taxed, and through taxation, kings could create strong and centralized states. In the years to e, alliances with themiddle classes were to enable kings to weaken aristocratic interests and build the states that came to be called modern.The mercial revolution also provided the opportunity for thousands of agricultural workers to improve their social position. The slow but steady transformation of European society from almost pletely rural and isolated to relatively more urban constituted the greatest effect of the mercial revolution that began in the eleventh century. Even so, merchants and business people did not run medieval munities, except in central and northern Italy and in the county of Flanders. Most towns remained small. The nobility and churchmen determined the predominant social attitudes, values, and patterns of thought and behavior. The mercial changes of the eleventh through fourteenth centuries did however, lay the economic foundation for the development of urban life and culture.1. According to paragraph 1, one effect of the increased use of cash was thatO an individual merchant no longer performed all aspects of trading operationsO a pany39。3. Ecosystem Diversity and StabilityConservation biologists have long been concerned that species extinction could have significant consequences for the stability of entire ecosystems—groups of interacting organisms and the physical environment that they inhabit. An ecosystem could survive the loss of some species, but if enough species were lost, the ecosystem would be severely degraded. In fact, it is possible that the loss of a single important species could start a cascade of extinctions that might dramatically change an entire ecosystem. A good illustration of this occurred after sea otters were eliminated from some Pacific kelp (seaweed) bed ecosystems: the kelp beds were practically obliterated too because in the absence of sea otter predation, sea urchin populations exploded and consumed most of the kelp and other macroalgae.It is usually claimed that speciesrich ecosystems tend to be more stable than speciespoor ecosystems. Three mechanisms by which higher diversity increases ecosystem stability have been proposed. First, if there are more species in an ecosystem, then its food web will be more plex, with greater redundancy among species in terms of their nutritional roles. In other words, in a rich system if a species is lost, there is a good chance that other species will take over its function as prey, predator, producer, deposer, or whatever role it played. Second, diverse ecosystems may be less likely to be invaded by new species, notably exotics (foreign species living outside their native range), that would disrupt the ecosystem’s structure and function. Third, in a speciesrich ecosystem, diseases may spread more slowly because most species will be relatively less abundant, thus increasing the average distance between individuals of the same species and hampering disease transmission among individuals.Scientific evidence to illuminate these ideas has been slow in ing, and many shadows remain. ■ One of the first studies to provide data supporting a relationship between diversity and stability examined how grassland plants responded to a drought. ■ Researchers D. Tilman and J A. Downing used the ratio of aboveground biomass in 1988 (after two years of drought) to that in 1986 (predrought) in 207 plots in a grassland field in the Cedar Creek Natural History Area in Minnesota as an index of ecosystem response to disruption by drought. ■ In an experiment that began in 1982, they pared these values with the number of plant species in each plot and discovered that the plots with a greater number of plant species experienced a less dramatic reduction in biomass. ■ Plots with more than ten species had about half as much biomass in 1988 as in 1986, whereas those with fewer than five species only produced roughly oneeighth as much biomass after the twoyear drought. Apparently, speciesrich plots were likely to contain some droughtresistant plant species that grew better in drought years, pensating for the poor growth of lesstolerant species.To put this result in more general terms, a speciesrich ecosystem may be more stable because it is more likely to have species with a wide array of responses to variable conditions such as droughts. Furthermore, a speciesrich ecosystem is more likely to have species with similar ecological functions, so that if a species is lost from an ecosystem, another species, probably a petitor, is likely to flourish and occupy its functional role. Both of these, variability in responses and functional redundancy, could be thought of as insurance against disturbances.The Minnesota grassland research has been widely accepted as strong evidence for the diversity stability theory。托福TPO46閱讀文本+題目+答案下載閱讀一: 閱讀二: 閱讀三: 薃肀莂蒃袂肀肂蠆袈聿芄薂螄肈莇螇蝕肇葿薀罿肆腿莃裊肅芁薈螁膄莃莁蚇膄肅薇薃膃芅荿羈膂莈蚅袇膁蒀蒈螃膀膀蚃蠆腿節(jié)蒆羈羋莄蟻襖羋蒆蒄螀芇膆蝕蚆袃莈蒃螞袂蒁螈羀袁膀薁袆袁芃螆螂袀蒞蕿蚈衿蕆莂羇羈膇薇袃羇艿莀蝿羆蒂薆螅羅膁蒈蟻羅芄蚄罿羄莆蕆裊羃蒈螞螁羂膈蒅蚇肁芀蟻薃肀莂蒃袂肀肂蠆袈聿芄薂螄肈莇螇蝕肇葿薀罿肆腿莃裊肅芁薈螁膄莃莁蚇膄肅薇薃膃芅荿羈膂莈蚅袇膁蒀蒈螃膀膀蚃蠆腿節(jié)蒆羈羋莄蟻襖羋蒆蒄螀芇膆蝕蚆袃莈蒃螞袂蒁螈羀袁膀薁袆袁芃螆螂袀蒞蕿蚈衿蕆莂羇羈膇薇袃羇艿莀蝿羆蒂薆螅羅膁蒈蟻羅芄蚄罿羄莆蕆裊羃蒈螞螁羂膈蒅蚇肁芀蟻薃肀莂蒃袂肀肂蠆袈聿芄薂螄肈莇螇