完整版练起来!托福阅读tpo真题的重要性大家都是有目共睹的，很多同学能高分完胜，很大部分都是因为做了托福阅读tpo真题，所以但凡考
Architecture
Architecture is the art and science of designing structures that organize and enclose space for practical and symbolic purposes. Because architecture grows out of human needs and aspirations, it clearly communicates cultural values. Of all the visual arts, architecture affects our lives most directly for it determines the character of the human environment in major ways.
Architecture is a three-dimensional form. It utilizes space, mass, texture, line, light, and color. To be architecture, a building must achieve a working harmony with a variety of elements. Humans instinctively seek structures that will shelter and enhance their way of life. It is the work of architects to create buildings that are not simply constructions but also offer inspiration and delight. Buildings contribute to human life when they provide shelter, enrich space, complement their site, suit the climate, and are economically feasible. The client who pays for the building and defines its function is an important member of the architectural team. The mediocre design of many contemporary buildings can be traced to both clients and architects.
In order for the structure to achieve the size and strength necessary to meet its purpose, architecture employs methods of support that, because they are based on physical laws, have changed little since people first discovered them—even while building materials have changed dramatically. The world’s architectural structures have also been devised in relation to the objective limitations of materials. Structures can be analyzed in terms of how they deal with downward forces created by gravity. They are designed to withstand the forces of compression (pushing together), tension (pulling apart), bending, or a combination of these in different parts of the structure.
Even development in architecture has been the result of major technological changes. Materials and methods of construction are integral parts of the design of architecture structures. In earlier times it was necessary to design structural systems suitable for the materials that were available, such as wood, stone, brick. Today technology has progressed to the point where it is possible to invent new building materials to suit the type of structure desired. Enormous changes in materials and techniques of construction within the last few generations have made it possible to enclose space with much greater ease and speed and with a minimum of material. Progress in this area can be measured by the difference in weight between buildings built now and those of comparable size built one hundred years ago.
Modern architectural forms generally have three separate components comparable to elements of the human body: a supporting skeleton or frame, an outer skin enclosing the interior spaces, and equipment, similar to the body’s vital organs and systems. The equipment includes plumbing, electrical wiring, hot water, and air-conditioning. Of course in early architecture—such as igloos and adobe structures—there was no such equipment, and the skeleton and skin were often one.
Much of the world’s great architecture has been constructed of stone because of its beauty, permanence, and availability. In the past, whole cities grew from the arduous task of cutting and piling stone upon. Some of the world’s finest stone architecture can be seen in the ruins of the ancient Inca city of Machu Picchu high in the eastern Andes Mountains of Peru. The doorways and windows are made possible by placing over the open spaces thick stone beams that support the weight from above. A structural invention had to be made before the physical limitations of stone could be overcome and new architectural forms could be created. That invention was the arch, a curved structure originally made of separate stone or brick segments. The arch was used by the early cultures of the Mediterranean area chiefly for underground drains, but it was the Romans who first developed and used the arch extensively in aboveground structures. Roman builders perfected the semicircular arch made of separate blocks of stone. As a method of spanning space, the arch can support greater weight than a horizontal beam. It works in compression to divert the weight above it out to the sides, where the weight is borne by the vertical elements on either side of the arch. The arch is among the many important structural breakthroughs that have characterized architecture throughout the centuries.
Paragraph 1: Architecture is the art and science of designing structures that organize and enclose space for practical and symbolic purposes. Because architecture grows out of human needs and aspirations, it clearly communicates cultural values. Of all the visual arts, architecture affects our lives most directly for it determines the character of the human environment in major ways.
1. According to paragraph 1, all of the following statements about architecture are true EXCEPT:
○Architecture is visual art.
○Architecture reflects the cultural values of its creators.
○Architecture has both artistic and scientific dimensions.
○Architecture has an indirect effect on life.
Paragraph 2: Architecture is a three-dimensional form. It utilizes space, mass, texture, line, light, and color. To be architecture, a building must achieve a working harmony with a variety of elements. Humans instinctively seek structures that will shelter and enhance their way of life. It is the work of architects to create buildings that are not simply constructions but also offer inspiration and delight. Buildings contribute to human life when they provide shelter, enrich space, complement their site, suit the climate, and are economically feasible. The client who pays for the building and defines its function is an important member of the architectural team. The mediocre design of many contemporary buildings can be traced to both clients and architects.
2.The word “feasible” in the passage is closet in meaning to
○in existence
○without question
○achievable
○most likely
3. The word “enhance” in the passage is closest in meaning to
○protect
○improve
○organize
○match
Paragraph 3: In order for the structure to achieve the size and strength necessary to meet its purpose, architecture employs methods of support that, because they are based on physical laws, have changed little since people first discovered them—even while building materials have changed dramatically. The world’s architectural structures have also been devised in relation to the objective limitations of materials. Structures can be analyzed in terms of how they deal with downward forces created by gravity. They are designed to withstand the forces of compression (pushing together), tension (pulling apart), bending, or a combination of these in different parts of the structure.
4. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
○Unchanging physical laws have limited the size and strength of buildings that can be made with materials discovered long ago.
○Building materials have changed in order to increase architectural size and strength, but physical laws of structure have not changed.
○When people first started to build, the structural methods used to provide strength and size were inadequate because they were not based on physical laws.
○Unlike building materials, the methods of support used in architecture have not changed over time because they are based on physical laws.
5. The word “devised” in the passage is closest in meaning to
○combined
○created
○introduced
○suggested
Paragraph 4: Even development in architecture has been the result of major technological changes. Materials and methods of construction are integral parts of the design of architecture structures. In earlier times it was necessary to design structural systems suitable for the materials that were available, such as wood, stone, brick. Today technology has progressed to the point where it is possible to invent new building materials to suit the type of structure desired. Enormous changes in materials and techniques of construction within the last few generations have made it possible to enclose space with much greater ease and speed and with a minimum of material. Progress in this area can be measured by the difference in weight between buildings built now and those of comparable size built one hundred years ago.
6. The word “integral” is closet in meaning to
○essential
○variable
○practical
○independent
7. According to paragraph 4, which of the following is true about materials used in the construction of buildings?
○Because new building materials are hard to find, construction techniques have changed very little from past generations.来源：北京新航道托福培训
○The availability of suitable building materials no longer limits the types of structures that may be built.
○The primary building materials that are available today are wood, stone, and brick.
○Architects in earlier times did not have enough building materials to enclose large spaces.
8. In paragraph 4, what does the author imply about modern buildings?
○They occupy much less space than buildings constructed one hundred years ago.
○They are not very different from the building of a few generations ago.
○They weigh less in relation to their size than buildings constructed one hundred years ago.
○They take a long time to build as a result of their complex construction methods.
Paragraph 5: Modern architectural forms generally have three separate components comparable to elements of the human body: a supporting skeleton or frame, an outer skin enclosing the interior spaces, and equipment, similar to the body’s vital organs and systems. The equipment includes plumbing, electrical wiring, hot water, and air-conditioning. Of course in early architecture—such as igloos and adobe structures—there was no such equipment, and the skeleton and skin were often one.
9. Which of the following correctly characterizes the relationship between the human body and architecture that is described in paragraph 5?
○Complex equipment inside buildings is the one element in modern architecture that resembles a component of the human body.
○The components in early buildings were similar to three particular elements of the human body.
○Modern buildings have components that are as likely to change as the human body is.
○In general, modern buildings more closely resemble the human body than earlier buildings do.
Paragraph 6: Much of the world’s great architecture has been constructed of stone because of its beauty, permanence, and availability. In the past, whole cities grew from the arduous task of cutting and piling stone upon. Some of the world’s finest stone architecture can be seen in the ruins of the ancient Inca city of Machu Picchu high in the eastern Andes Mountains of Peru. The doorways and windows are made possible by placing over the open spaces thick stone beams that support the weight from above. A structural invention had to be made before the physical limitations of stone could be overcome and new architectural forms could be created. That invention was the arch, a curved structure originally made of separate stone or brick segments. The arch was used by the early cultures of the Mediterranean area chiefly for underground drains, but it was the Romans who first developed and used the arch extensively in aboveground structures. Roman builders perfected the semicircular arch made of separate blocks of stone. As a method of spanning space, the arch can support greater weight than a horizontal beam. It works in compression to divert the weight above it out to the sides, where the weight is borne by the vertical elements on either side of the arch. The arch is among the many important structural breakthroughs that have characterized architecture throughout the centuries.
10. The word “arduous” in the passage is closest in meaning to
○difficult
○necessary
○skilled
○shared
11. Why does the author include a description of how the “doorways and windows” of Machu Picchu were constructed?
○To indicate that the combined skeletons and skins of the stone buildings of Machu Picchu were similar to igloos and adobe structures
○To indicate the different kinds of stones that had to be cut to build Machu Picchu
○To provide an illustration of the kind of construction that was required before arches were invented
○To explain how ancient builders reduced the amount of time necessary to construct buildings from stone
12.According to paragraph 6, which of the following statements is true of the arch?
○The Romans were the first people to use the stone arch.
○The invention of the arch allowed new architectural forms to be developed.
○The arch worked by distributing the structural load of a building toward the center of the arch.
○The Romans followed earlier practices in their use of arches.
Paragraph 5: Modern architectural forms generally have three separate components comparable to elements of the human body; a supporting skeleton or frame, an outer skin enclosing the interior spaces, and equipment, similar to the body’s vital organs and systems. The equipment includes plumbing, electrical wiring, hot water, and air-conditioning. Of course in early architecture—such as igloos and adobe structures—there was no such equipment, and the skeleton and skin were often one.
13.Look at the four squares [] that indicate where the following sentence could be added to the passage.
However, some modern architectural designs, such as those using folded plates of concreter or air-inflated structures, are again unifying skeleton and skin.
Where would the sentence best fit?
14.Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Architecture uses forms and space to express cultural values.
●
●
●
Answer choices
○Architects seek to create buildings that are both visually appealing and well suited for human use.
○Over the course of the history of building, innovations in material and methods of construction have given architects ever greater freedom to express themselves.
○Throughout history buildings have been constructed like human bodies, needing distinct “organ” systems in order to function.
○Both clients and architects are responsible for the mediocre designs of some modern buildings.
○Modern buildings tend to lack the beauty of ancient stone buildings such as those of Machu Picchu.
○The discovery and use of the arch typifies the way in which architecture advances by developing more efficient types of structures.
参考答案：
1. ○ 4
2. ○ 3
3. ○2
4. ○ 4
5. ○ 2
6. ○ 1
7. ○ 2
8. ○ 3
9. ○ 4
10. ○ 1
11. ○ 3
12. ○ 2
13. ○ 4
14. Architects seek to create…
Over the course of the…
The discovery and use of…
Depletion of the Ogallala Aquifer
The vast grasslands of the High Plains in the central United States were settled by farmers and ranchers in the 1880s. This region has a semiarid climate, and for 50 years after its settlement, it supported a low-intensity agricultural economy of cattle ranching and wheat farming. In the early twentieth century, however, it was discovered that much of the High Plains was underlain by a huge aquifer (a rock layer containing large quantities of groundwater). This aquifer was named the Ogallala aquifer after the Ogallala Sioux Indians, who once inhabited the region.
The Ogallala aquifer is a sandstone formation that underlies some 583,000 square kilometers of land extending from northwestern Texas to southern South Dakota. Water from rains and melting snows has been accumulating in the Ogallala for the past 30,000 years. Estimates indicate that the aquifer contains enough water to fill Lake Huron, but unfortunately, under the semiarid climatic conditions that presently exist in the region, rates of addition to the aquifer are minimal, amounting to about half a centimeter a year.
The first wells were drilled into the Ogallala during the drought years of the early 1930s. The ensuing rapid expansion of irrigation agriculture, especially from the 1950s onward, transformed the economy of the region. More than 100,000 wells now tap the Ogallala. Modern irrigation devices, each capable of spraying 4.5 million liters of water a day, have produced a landscape dominated by geometric patterns of circular green islands of crops. Ogallala water has enabled the High Plains region to supply significant amounts of the cotton, sorghum, wheat, and corn grown in the United States. In addition, 40 percent of American grain-fed beef cattle are fattened here.
This unprecedented development of a finite groundwater resource with an almost negligible natural recharge rate—that is, virtually no natural water source to replenish the water supply—has caused water tables in the region to fall drastically. In the 1930s, wells encountered plentiful water at a depth of about 15 meters; currently, they must be dug to depths of 45 to 60 meters or more. In places, the water table is declining at a rate of a meter a year, necessitating the periodic deepening of wells and the use of ever-more-powerful pumps. It is estimated that at current withdrawal rates, much of the aquifer will run dry within 40 years. The situation is most critical in Texas, where the climate is driest, the greatest amount of water is being pumped, and the aquifer contains the least water. It is projected that the remaining Ogallala water will, by the year 2030, support only 35 to 40 percent of the irrigated acreage in Texas that is supported in 1980.
The reaction of farmers to the inevitable depletion of the Ogallala varies. Many have been attempting to conserve water by irrigating less frequently or by switching to crops that require less water. Others, however, have adopted the philosophy that it is best to use the water while it is still economically profitable to do so and to concentrate on high-value crops such as cotton. The incentive of the farmers who wish to conserve water is reduced by their knowledge that many of their neighbors are profiting by using great amounts of water, and in the process are drawing down the entire region’s water supplies.
In the face of the upcoming water supply crisis, a number of grandiose schemes have been developed to transport vast quantities of water by canal or pipeline from the Mississippi, the Missouri, or the Arkansas rivers. Unfortunately, the cost of water obtained through any of these schemes would increase pumping costs at least tenfold, making the cost of irrigated agricultural products from the region uncompetitive on the national and international markets. Somewhat more promising have been recent experiments for releasing capillary water (water in the soil) above the water table by injecting compressed air into the ground. Even if this process proves successful, however, it would almost triple water costs. Genetic engineering also may provide a partial solution, as new strains of drought-resistant crops continue to be developed. Whatever the final answer to the water crisis may be, it is evident that within the High Plains, irrigation water will never again be the abundant, inexpensive resource it was during the agricultural boom years of the mid-twentieth century.
Paragraph 1: The vast grasslands of the High Plains in the central United States were settled by farmers and ranchers in the 1880s. This region has a semiarid climate, and for 50 years after its settlement, it supported a low-intensity agricultural economy of cattle ranching and wheat farming. In the early twentieth century, however, it was discovered that much of the High Plains was underlain by a huge aquifer (a rock layer containing large quantities of groundwater). This aquifer was named the Ogallala aquifer after the Ogallala Sioux Indians, who once inhabited the region.
1.According to paragraph 1, which of the following statements about the High Plains is true?
○Until farmers and ranchers settled there in the 1880s, the High Plains had never been inhabited.
○The climate of the High Plains is characterized by higher-than-average temperatures.
○The large aquifer that lies underneath the High Plains was discovered by the Ogallala Sioux Indians.
○Before the early 1900s there was only a small amount of farming and ranching in the High Plains.
Paragraph 2: The Ogallala aquifer is a sandstone formation that underlies some 583,000 square kilometers of land extending from northwestern Texas to southern South Dakota. Water from rains and melting snows has been accumulating in the Ogallala for the past 30,000 years. Estimates indicate that the aquifer contains enough water to fill Lake Huron, but unfortunately, under the semiarid climatic conditions that presently exist in the region, rates of addition to the aquifer are minimal, amounting to about half a centimeter a year.
2. According to paragraph 2, all of the following statements about the Ogallala aquifer are true EXCEPT:
○The aquifer stretches from South Dakota to Texas.
○The aquifer’s water comes from underground springs.
○Water has been gathering in the aquifer for 30,000 years.
○The aquifer’s water is stored in a layer of sandstone.
3. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
○Despite the current impressive size of the Ogallala aquifer, the region’s climate keeps the rates of water addition very small.
○Although the aquifer has been adding water at the rate of only half a centimeter a year, it will eventually accumulate enough water of fill Lake Huron.
○Because of the region’s present climatic conditions, water is being added each year to the aquifer.
○Even when the region experiences unfortunate climatic conditions, the rates of addition of water continue to increase.
Paragraph 3: The first wells were drilled into the Ogallala during the drought years of the early 1930s. The ensuing rapid expansion of irrigation agriculture, especially from the 1950s onward, transformed the economy of the region. More than 100,000 wells now tap the Ogallala. Modern irrigation devices, each capable of spraying 4.5 million liters of water a day, have produced a landscape dominated by geometric patterns of circular green islands of crops. Ogallala water has enabled the High Plains region to supply significant amounts of the cotton, sorghum, wheat, and corn grown in the United States. In addition, 40 percent of American grain-fed beef cattle are fattened here.
4. The word “ensuing” in the passage is closest in meaning to
○continuing
○surprising
○initial
○subsequent
5. In paragraph 3, why does the author provide the information that 40 percent of American cattle are fattened in the High Plains?
○To suggest that crop cultivation is not the most important part of the economy of the High Plains
○To indicate that not all economic activity in the High Plains is dependent on irrigation
○To provide another example of how water from the Ogallala has transformed the economy of the High Plains
○To contrast cattle-fattening practices in the High Plains with those used in other region of the United States
Paragraph 4: This unprecedented development of a finite groundwater resource with an almost negligible natural recharge rate—that is, virtually no natural water source to replenish the water supply—has caused water tables in the region to fall drastically. In the 1930s, wells encountered plentiful water at a depth of about 15 meters; currently, they must be dug to depths of 45 to 60 meters or more. In places, the water table is declining at a rate of a meter a year, necessitating the periodic deepening of wells and the use of ever-more-powerful pumps. It is estimated that at current withdrawal rates, much of the aquifer will run dry within 40 years. The situation is most critical in Texas, where the climate is driest, the greatest amount of water is being pumped, and the aquifer contains the least water. It is projected that the remaining Ogallala water will, by the year 2030, support only 35 to 40 percent of the irrigated acreage in Texas that is supported in 1980.
6. The word “unprecedented” in the passage is closest in meaning to
○difficult to control
○without any restriction
○unlike anything in the past
○rapidly expanding
7. The word “virtually” in the passage is closest in meaning to
○clearly
○perhaps
○frequently
○almost
8. According to paragraph 4, all of following are consequences of the heavy use of the Ogallala aquifer for irrigation EXCEPT:
○The recharge rate of the aquifer is decreasing.
○Water tables in the region are becoming increasingly lower.
○Wells now have to be dug to much greater depths than before.
○Increasingly powerful pumps are needed to draw water from the aquifer.
9. According to paragraph 4, compared with all other states that use Ogallala water for irrigation, Texas
○has the greatest amount of farmland being irrigated with Ogallala water
○contains the largest amount of Ogallala water underneath the soil
○is expected to face the worst water supply crisis as the Ogallala runs dry
○uses the least amount of Ogallala water for its irrigation needs
Paragraph 5: The reaction of farmers to the inevitable depletion of the Ogallala varies. Many have been attempting to conserve water by irrigating less frequently or by switching to crops that require less water. Others, however, have adopted the philosophy that it is best to use the water while it is still economically profitable to do so and to concentrate on high-value crops such as cotton. The incentive of the farmers who wish to conserve water is reduced by their knowledge that many of their neighbors are profiting by using great amounts of water, and in the process are drawing down the entire region’s water supplies.
10. The word “inevitable” in the passage is closest in meaning to
○unfortunate
○predictable
○unavoidable
○final
11. Paragraph 5 mentions which of the following as a source of difficulty for some farmers who try to conserve water?
○Crops that do not need much water are difficult to grow in the High Plains.
○Farmers who grow crops that need a lot of water make higher profits.
○Irrigating less frequently often leads to crop failure.
○Few farmers are convinced that the aquifer will eventually run dry.
Paragraph 6: In the face of the upcoming water supply crisis, a number of grandiose schemes have been developed to transport vast quantities of water by canal or pipeline from the Mississippi, the Missouri, or the Arkansas rivers. Unfortunately, the cost of water obtained through any of these schemes would increase pumping
costs at least tenfold, making the cost of irrigated agricultural products from the region uncompetitive on the national and international markets. Somewhat more promising have been recent experiments for releasing capillary water (water in the soil) above the water table by injecting compressed air into the ground. Even if this process proves successful, however, it would almost triple water costs. Genetic engineering also may provide a partial solution, as new strains of drought-resistant crops continue to be developed. Whatever the final answer to the water crisis may be, it is evident that within the High Plains, irrigation water will never again be the abundant, inexpensive resource it was during the agricultural boom years of the mid-twentieth century.
12.According to paragraph 6, what is the main disadvantage of the proposed plans to transport river water to the High Plains?
○The rivers cannot supply sufficient water for the farmer’s needs.
○Increased irrigation costs would make the products too expensive.
○The costs of using capillary water for irrigation will increase.
○Farmers will be forced to switch to genetically engineered crops.
Paragraph 5: The reaction of farmers to the inevitable depletion of the Ogallala varies. Many have been attempting to conserve water by irrigating less frequently or by switching to crops that require less water. Others, however, have adopted the philosophy that it is best to use the water while it is still economically profitable to do so and to concentrate on high-value crops such as cotton. The incentive of the farmers who wish to conserve water is reduced by their knowledge that many of their neighbors are profiting by using great amounts of water, and in the process are drawing down the entire region’s water supplies.
Paragraph 6: In the face of the upcoming water supply crisis, a number of grandiose schemes have been developed to transport vast quantities of water by canal or pipeline from the Mississippi, the Missouri, or the Arkansas rivers. Unfortunately, the cost of water obtained through any of these schemes would increase pumping costs at least tenfold, making the cost of irrigated agricultural products from the region uncompetitive on the national and international markets. Somewhat more promising have been recent experiments for releasing capillary water (water in the soil) above the water table by injecting compressed air into the ground. Even if this process proves successful, however, it would almost triple water costs. Genetic engineering also may provide a partial solution, as new strains of drought-resistant crops continue to be developed. Whatever the final answer to the water crisis may be, it is evident that within the High Plains, irrigation water will never again be the abundant, inexpensive resource it was during the agricultural boom years of the mid-twentieth century.
13.Look at the four squares [] that indicate where the following sentence could be added to the passage.
But even if uncooperative farmers were to join in the conservation efforts, this would only delay the depletion of the aquifer.
Where would the sentence best fit? Click on a square to add the sentence to the passage.
14.Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
The Ogallala aquifer is a large underground source of water in the High Plains region of the United States.
●
●
●
Answer choices
○The use of the Ogallala for irrigation has allowed the High Plains to become one of the most productive agricultural regions in the United States.
○Given the aquifer’s low recharge rate, its use for irrigation is causing water tables to drop and will eventually lead to its depletion.
○Releasing capillary water and introducing drought-resistant crops are less-promising solutions to the water supply crisis than bringing in river water
○The periodic deepening of wells and the use of more-powerful pumps would help increase the natural recharge rate of the Ogallala.
○In Texas, a great deal of attention is being paid to genetic engineering because it is there that the most critical situation exists.
○Several solutions to the upcoming water supply crisis have been proposed, but none of them promises to keep the costs of irrigation low.
参考答案：
1. ○4
2. ○2
3. ○1
4. ○4
5. ○3
6. ○3
7. ○4
8. ○1
9. ○3
10. ○3
11. ○2
12. ○2
13. ○3
14. The use of the Ogallala for…
Given the aquifer’s low recharge…
Several solutions to the upcoming…
The Long-Term Stability of Ecosystems
Plant communities assemble themselves flexibly, and their particular structure depends on the specific history of the area. Ecologists use the term “succession” to refer to the changes that happen in plant communities and ecosystems over time. The first community in a succession is called a pioneer community, while the long-lived community at the end of succession is called a climax community. Pioneer and successional plant communities are said to change over periods from 1 to 500 years. These changes—in plant numbers and the mix of species—are cumulative. Climax communities themselves change but over periods of time greater than about 500 years.
An ecologist who studies a pond today may well find it relatively unchanged in a year’s time. Individual fish may be replaced, but the number of fish will tend to be the same from one year to the next. We can say that the properties of an ecosystem are more stable than the individual organisms that compose the ecosystem.
At one time, ecologists believed that species diversity made ecosystems stable. They believed that the greater the diversity the more stable the ecosystem. Support for this idea came from the observation that long-lasting climax communities usually have more complex food webs and more species diversity than pioneer communities. Ecologists concluded that the apparent stability of climax ecosystems depended on their complexity. To take an extreme example, farmlands dominated by a single crop are so unstable that one year of bad weather or the invasion of a single pest can destroy the entire crop. In contrast, a complex climax community, such as a temperate forest, will tolerate considerable damage from weather to pests.
The question of ecosystem stability is complicated, however. The first problem is that ecologists do not all agree what “stability” means. Stability can be defined as simply lack of change. In that case, the climax community would be considered the most stable, since, by definition, it changes the least over time. Alternatively, stability can be defined as the speed with which an ecosystem returns to a particular form following a major disturbance, such as a fire. This kind of stability is also called resilience. In that case, climax communities would be the most fragile and the least stable, since they can require hundreds of years to return to the climax state.
Even the kind of stability defined as simple lack of change is not always associated with maximum diversity. At least in temperate zones, maximum diversity is often found in mid-successional stages, not in the climax community. Once a redwood forest matures, for example, the kinds of species and the number of individuals growing on the forest floor are reduced. In general, diversity, by itself, does not ensure stability. Mathematical models of ecosystems likewise suggest that diversity does not guarantee ecosystem stability—just the opposite, in fact. A more complicated system is, in general, more likely than a simple system to break down. A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle.
Ecologists are especially interested to know what factors contribute to the resilience of communities because climax communities all over the world are being severely damaged or destroyed by human activities. The destruction caused by the volcanic explosion of Mount St. Helens, in the northwestern United States, for example, pales in comparison to the destruction caused by humans. We need to know what aspects of a community are most important to the community’s resistance to destruction, as well as its recovery.
Many ecologists now think that the relative long-term stability of climax communities comes not from diversity but from the “patchiness” of the environment, an environment that varies from place to place supports more kinds of organisms than an environment that is uniform. A local population that goes extinct is quickly replaced by immigrants from an adjacent community. Even if the new population is of a different species, it can approximately fill the niche vacated by the extinct population and keep the food web intact.
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