The harmony present in any ecological system relies on the inclusion of different types of species, each with a specific role or set of roles. Although to some extent all species contribute in important ways, scientists are now learning about the pivotal parts played by, what are being called foundational and keystone species. Interestingly, the latter represent only a small fraction of the overall animal population within an ecosystem while the former can make up a significant portion of it. In both cases, though, their removal often results in massive changes to, if not the complete destruction of, their ecosystems. Efforts to conserve these species are currently based on the new idea that they prevent the start of a domino effect that would, once started, be unstoppable.
Foundational species are the backbones of the systems they inhabit. These species are referred to as primary producers, which means that they generate a large amount of the abundance other species require to survive. In most cases, they are a type of vegetation or stationary animal. Coral, which tends to grow in large colonies, is often pointed to as an example. Eventually a group of colonies will form a coral reef, which can sustain a complete ecosystem around it. The reef contains the skeletal remains of coral at its bottom and living coral on the top. Numerous animals, including zooplankton and sponges, live in the small crevices located in the rock-like bottom. At the top, the coral interacts with seaweed and other forms of vegetation to regulate the levels of nutrients and gases in the water. There are also more than four thousand types of fish that live on or around most coral reefs. Fish rely on the holes in the reef for protection. The more colorful species of coral provide excellent camouflage for fish, as well. Obviously, without its foundational species, an ecosystem would collapse.
Keystone species play less apparent but equally important roles. The name comes from the keystones used in the building of stone archways. The keystone receives the smallest amount of pressure of all the stones in the arch, but, if it is removed, the arch will collapse. In the same way, research is now showing, certain species within ecosystems, although they are smaller in numbers and biomass than most of the other species present, act like keystones. These species are being categorized as either predators or engineers, depending on their relationships with the various species around them. These categories are not absolute, though, and animals are moved from one to the other as new facts come to light.
In the case of predator keystone species, there are four carnivores that prey on herbivores and other animals. Also, the herbivores usually have no other natural predator in the ecosystem. The sea otter is now considered by many scientists to be a keystone species, because it controls the number of sea urchins, which have few other predators. Both species can be found around kelp forests, which are in the warm parts of the world's oceans. Because kelp is an underwater plant, its roots are not used for the collection of nutrients. Instead, they are there to anchor the plant. As soon as enough sea urchins chewed on the roots, which are fragile, the kelp would be removed from the ecosystem. If the sea otter were to disappear, the urchins would quickly grow in number and destroy all the kelp.
Engineer keystone species maintain the balance in ecosystems in different ways. ■Although bears tend to live in forests, they bring in important sources of nutrients from ocean and sea-based ecosystems. ■ The bears capture large salmon from the water and take them into the forest to eat them. ■ This distributes large amounts of protein in the form of bear waste matter as well as uneaten portions of salmon. The protein eventually supports life in the forest, either as a food source for smaller animals or the vegetation. Another species that plays a part similar to that of bears is the beaver. Through the construction of dams, beavers convert small rivers into ponds and marshes. The new landscape in turn supports a variety of fish, which in turn provides a dependable source of food for the beavers. The removal of beavers from an ecosystem would also remove the landscape it depends on.预测词汇1
in the same way=similarly
Many think that the reason so many animals live with others of their species is that social creatures are higher up the evolutionary scale and so are better adapted and leave more offspring than do animals that live solitary lives. However, in each and every species, generation after generation, relatively social and relatively solitary types compete unconsciously with one another in ways that determine who leaves more offspring on average. In some species, the more social individuals have won out, but in a large majority, it is the solitary types that have consistently left more surviving descendants on average.
But how can living alone ever be superior to living together? Under some conditions, a cost-benefit comparison favors solitary life over a more social existence. For example, among most social species, animals have to expend time and energy competing for social status. Those that do not occupy the top positions regularly have to signal their submissive state to their superiors if they are to be permitted to remain in the group. This can take up a major share of a social subordinate's life. In fact, even in small social groups there are both subtle competition and not-so-subtle competition.
Social groups also offer opportunities for reproductive interference. Breeding males that live in close association with more attractive rivals may lose their mates to these individuals. In addition, sociality has two other potential disadvantages. The first is heightened competition for food, which occurs in animals as different as colonial fieldfares （a kind of songbird） and groups of lions, whose females are often pushed from their food by hungry males. The second is increased vulnerability to parasites and disease, which plague social species of all sorts. While it is true that some social animals have evolved special responses designed to combat parasites and disease, those responses can only reduce, but cannot totally eliminate, the damage caused by those threats, and the responses may even carry their own costs. Thus, honeybees warm their hives in response to an infestation by a fungal pathogen, which apparently helps kill the heat-sensitive fungus, but at the price of time and energy expended by the heat-producing workers.
If social living carries a heightened risk of infection, then the larger the group, the greater the risk. This prediction holds for cliff swallows, which pack their nests side by side in colonies composed of anywhere from a handful of birds to several thousand pairs. The more swallows nesting together, the greater the chance that at least one bird will be infested with swallow bugs, which can then readily spread from one nest to another.
The parasites and fungi that make life miserable for swallows and other social creatures demonstrate that if sociality is to evolve, the asorted costs of living together must be outweighed by compensatory benefits. Cliff swallows may join others to take advantage of the improved foraging that comes from following companions to good feeding sites, while other animals, such as male imperial penguins, save thermal energy by huddling shoulder to shoulder during the brutal Antarctica winter. Still others, such as lionesses, join forces to fend off enemies of their own species.
The most widespread fitness benefit for social animals, however, probably is improved protection against predators. Many studies have shown that animals in groups gain by reducing the individual risk of being captured, or by spotting danger sooner, or by attacking their enemies in groups. Males in nesting colonies of bluegill sunfish cooperate in driving egg-eating bullhead catfish away from their nests at the bottom of a freshwater lake. While bluegills have adopted social behavior to avoid predation, closely related species that nest alone have evolved means to protect themselves while nesting alone. Thus, the solitary pumpkinseed sunfish, a member of the same genus as the bluegill, has powerful biting jaws and so can repel egg-eating enemies on its own, whereas bluegills have small, delicate mouths good only for inhaling small, soft-bodied insect larvae. Pumpkinseed sunfish are in no way inferior to or less well adapted than bluegills because they are solitary; they simply gain less through social living, which makes solitary nesting the adaptive tactic for them.预测词汇2
Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases （water vapor, ammonia, methane, carbon dioxide, and carbon monoxide） that hold together small pieces of rocky and metallic materials. Many comets travel in very elongated orbits that carry them far beyond Pluto. These long-period comets take hundreds of thousands of years to complete a single orbit around the Sun. However, a few short-period comets （those having an orbital period of less than 200 years）， such as Halley’s Comet, make a regular encounters with the inner solar system.
When a comet first becomes visible from Earth, it appears very small, but as it approaches the Sun, solar energy begins to vaporize the frozen gases, producing a glowing head called the coma. The size of the coma varies greatly from one comet to another. Extremely rare ones exceed the size of the Sun, but most approximate the size of Jupiter. Within the coma, a small glowing nucleus with a diameter of only a few kilometers can sometimes be detected. As comets approach the Sun, some develop a tail that extends for millions of kilometers. Despite the enormous size of their tails and comas, comets are relatively small members of the solar system.
The observation that the tail of a comet points away from the Sun in a slightly curved manner led early astronomers to propose that the Sun has a repulsive force that pushes the particles of the coma away, thereby forming the tail. Today, two solar forces are known to contribute to this formation. One, radiation pressure, pushes dust particles away from the coma. The second, known as solar wind, is responsible for moving the ionized gases, particularly carbon monoxide. Sometimes a single tail composed of both dust and ionized gases is produced, but often two tails—one of dust, the other, a blue streak of ionized gases—are observed.
As a comet moves away from the Sun, the gases forming the coma recondense, the tail disappears, and the comet returns to distant space. Material that was blown from the coma to form the tail is lost from the comet forever. Consequently, it is believed that most comets cannot survive more than a few hundred close orbits of the Sun. Once all the gases are expelled, the remaining material—a swarm of tiny metallic and stony particles—continues the orbit without a coma or a tail.
Comets apparently originate in two regions of the outer solar system. Most short-period comets are thought to orbit beyond Neptune in a region called the Kuiper belt, in honor of the astronomer Gerald Kuiper. During the past decade over a hundred of these icy bodies have been discovered. Most Kuiper belt comets move in nearly circular orbits that lie roughly in the same plane as the planets. A chance collision between two comets, or the gravitational influence of one of the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—may occasionally alter the orbit of a comet in these regions enough to send it to the inner solar system and into our view.
Unlike short-period comets, long-period comets have elliptical orbits that are not confined to the plane of the solar system. These comets appear to be distributed in all directions from the Sun, forming a spherical shell around the solar system, called the Oort cloud, after the Dutch astronomer Jan Oort. Millions of comets are believed to orbit the Sun at distances greater than 10,000 times the Earth-Sun distance. The gravitational effect of a distant passing star is thought to send an occasional Oort cloud comet into a highly eccentric orbit that carries it toward the Sun. However, only a tiny portion of the Oort cloud comets have orbits that bring them into the inner solar system.
The most famous short-period comet is Halley’s Comet, named after English astronomer Edmond Halley. Its orbital period averages 76 years, and every one of its 30 appearances since 240 B.C. has been recorded by Chinese astronomers. When seen in 1910, Halley’s Comet had developed a tail nearly 1.6 million kilometers （1 million miles） long and was visible during daylight hours. Its most recent approach occurred in 1986.预测词汇3
propose=offer the theory
In the northern American colonies, especially New England, tight-knit farming families, organized in communities of several thousand people, dotted the landscape by the mid-eighteenth century. New Englanders staked their future on a mixed economy. They cleared forests for timber used in barrels, ships, houses, and barns. They plumbed the offshore waters for fish to feed local populations. And they cultivated and grazed as much of the thin-soiled, rocky hills and bottomlands as they could recover from the forest.
The farmers of the middle colonies-Pennsylvania, Delaware, New Jersey, and New York-set their wooden plows to much richer soils than New Englanders did. They enjoyed the additional advantage of setting an area already partly cleared by Native Americans who had relied more on agriculture than had New England tribes. Thus favored, mid-Atlantic farm families produced modest surpluses of corn, wheat, beef, and pork. By the mid-eighteenth century, ships from New York and Philadelphia were carrying these foodstuffs not only to the West Indies, always a primary market, but also to areas that could no longer feed themselves-England, Spain, Portugal, and even New England.
In the North, the broad ownership of land distinguished farming society from every other agricultural region of the Western world. Although differences in circumstances and ability led gradually toward greater social stratification, in most communities, the truly rich and terribly poor were few and the gap between them small compared with European society. Most men other than indentured servants （servants contracted to work for a specific number of years） lived to purchase or inherit a farm of at least 50 acres. With their family’s labor, they earned a decent existence and provided a small inheritance for each of their children. Settlers valued land highly, for owning land ordinarily guaranteed both economic independence and political rights.
By the eighteenth century, amid widespread property ownership, a rising population pressed against a limited land supply, especially in New England. Family farms could not be divided and subdivided indefinitely, for it took at least fifty acres（of which only a quarter could usually be cropped） to support a single family. In Concurd, Massachusetts, for example, the founders had worked farms averaging about 250 acres. A century later, in the 1730s, the average farm had shrunk by two thirds, as farm owners struggled to provide an inheritance for the three or four sons that the average marriage produced.
The decreasing fertility of the soil compounded the problem of dwindling farm size in New England. When land had been plentiful, farmers planted crops in the same field for three years and then let it lie fallow （unplanted） in pasture seven years or more until it regained its fertility. But on the smaller farms of the eighteenth century, farmers had reduced fallow time to only a year or two. Such intense use of the soil reduced crop yields, forcing farmers to plow marginal land or shift to livestock production.
The diminishing size and productivity of family farms forced many New Englanders to move to the frontier or out of the area altogether in the eighteen century. "Many of our old towns are too full of inhabitants for husbandry, many of them living on small shares of land, " complained one writer. In Concurd, one of every four adult males migrated from town every decade from the 1740s on, and in many towns migration out was even greater. Some drifted south to New York and Pennsylvania. Others sought opportunities as artisans in the coastal towns or took to the sea. More headed for the colonies, western frontier or north into New Hampshire and the eastern frontier of Maine. Several thousand New England families migrated even farther north to the Annapolis Valley of Nova Scotia. Throughout New England after the early eighteenth century, most farmers' sons knew that their destiny lay elsewhere.
Wherever they took up farming, northern cultivators engaged in agricultural work routines that were far less intense than in the south. The growing season was much shorter, and the cultivation of cereal crops required incessant labor only during spring planting and autumn harvesting. This less burdensome work rhythm let many northern cultivators to fill out their calendars with intermittent work as clockmakers, shoemakers, carpenters, and weavers.预测词汇4
Following the Roman Empire conquering the area in the first century A.D., there is a great deal of archaeological evidence for the economic growth of the British Isles. Prior to this event, the economy of the British Isles, which was based on manufacturing, was centered mainly on the household and on craft skills, and where the best quality and greatest range of goods were largely a monopoly of the tribal aristocracies. This was the nature of the economy which lasted in regions of Britain that were unconquered by the Roman Empire, even though some Roman products were utilized in such areas. The majority of these Roman artifacts were glass vessels, pots, as well as small metal objects that were dispersed over a vast region. They perhaps held a symbolic value and were not necessarily used for their originally designed purposes. The spread of Roman objects beyond Roman Britain does not seem to have happened on an enormous scale. In areas where artifacts are more numerous, it is likely due to gift giving during close interactions between the Roman government and the tribes.
In regions that experienced direct economic control under the Romans, however, economic growth is clearly notable. There was an enormous increase in the number and variety of goods in circulation and the range of settlements in which they were found. This is clearly true in the overwhelming majority of excavated sites in Roman Britain, with the only exceptions being some rural regions that continued the pre-Roman, Iron Age pattern. The majority of sites resulted in the discovery of an abundance of iron, glass, and pottery, and good quantities of copper alloys, lead, tin, silver, and occasionally gold. For example, the humble iron nail is found in numbers not repeated until the Industrial Revolution.
The technology levels and range of the manufacturing of these objects also developed alongside the sheer increase in their quantity. During the Iron Age, the typical household objects were usually manufactured using a low technology of craft manufacture. Later, this changed to more specialized and larger-scale production methods. During this time, specialized workers could utilize equipment manufactured through'time and resource investments. In these regions, small-scale workshops used by specialized craftsmen betoken full-time employment in this work. Regardless of the large increase in the scale of manufacturing, there is little evidence of major growth in the size of productive units. We are left with the impression of an economy still based on small-scale craft production.
Where we do see an important change is in the removal of any exclusive association between the best traditional craftsmen and the governing elite. The powerful could show off their status in new ways, particularly by using Roman architecture and domestic decoration, but the traditional classes of decorative metalwork manufacture no longer seem to have been under the control of the tribal leaders. Rich objects from a wide range of archaeological sites imply the deterioration of this monopoly. There are a number of contributing factors. The control of precious metals moved to the imperial government immediately after the conquest, and gold and silver were also removed from circulation when captured as booty during the invasion. Similarly, changes in taste and the fashions of wealth and status display were stimulated by the arrival of new things like Roman dress, architecture, and sculpture.
These changes in manufacture were accompanied by increased distances over which many goods were transported to their consumers. ■ The bulk of pottery and other items originated locally, during the Iron Age; but after the Roman invasion, these objects had been produced over a far greater range of distances. ■ In this way, vast regions of the Roman province were incorporated into a society where there was wide access to material wealth. _ New changes in manufacturing production were coupled with huge increase in the importation of goods from elsewhere in the empire. ■ These commodities, which included Mediterranean foodstuffs such as olive oil as well as comparatively low-value objects such as decorated pottery, also achieved a wide distribution and are found in many different types of site.预测词汇5
dispersed = spread
Industrial output increased smartly across nearly all of Europe between 1450 and 1575. Although trade with the Americas had something to do with this, the main determinants of this industrial advance lay within Europe itself.
Population grew from 61 million in 1500 to 78 million a century later, and the proportion of Europeans living in cities of 10,000 or more—and thus dependent on the market for what they consumed—expanded from less than 6 percent to nearly 8 percent during the same period. More important than sheer numbers, many Europeans’ incomes rose. This was especially true among more fully employed urban groups, farmers who benefited from higher prices and the intensifying commercialization and specialization in agriculture （which also led them to shed much non-agricultural production in favor of purchased goods）， and landlords and other property owners who collected mounting rents. Government activities to build and strengthen the state were a stimulus to numerous industries, notably shipbuilding, textiles, and metallurgy. To cite just one example, France hastened to develop its own iron industry when the Hapsburgs—the family that governed much of Europe, and whom France fought repeatedly in the sixteenth century—came to dominate the manufacture of weapons in Germany and the cities of Liege and Milan, which boasted Europe’s most advanced technology.
The supply of goods was also significantly modified. Migration had long been critical for the diffusion of knowledge that spawned new trades or revived others. Now thousands of workers, and sizeable amounts of capital, moved from one region to another. At the same time, new commodities appeared on the market, often broadening and deepening demand. Most were inexpensive items destined for individual consumers. Knitted stockings, ribbon and lace, buttons, starch, soap, vinegar brewed from beer, knives and tools, pots and ovens, and many more goods, formerly made only for local sale, now entered into channels of national or international trade. The best-known and most widely adopted new industry was printing with movable type, which spread swiftly throughout Europe after Johannes Gutenberg perfected his innovation in 1453. Despite isolated cases of resistance—the scribes’ guild （an association of book copiers） delayed printing’s introduction into Paris for twenty years, for example—more than 380 working presses had sprung up by 1480, and 1,000 （in nearly 250 towns） by 1500. Between 1453 and 1500, all the presses of Europe together turned out some 40,000 editions （known as incunabula）， but from 1501 to 1600, that same quantity was produced in Lyon and Paris alone.
In metals and mining, technical improvements were available that saved substantially on raw materials and fuel, causing prices to drop. The construction of ever-larger furnaces capable of higher temperatures culminated in the blast furnace, which used cheaper ores and economized on scarce and expensive wood, cutting costs per ton by 20 percent while boosting output substantially. A new technique for separating silver from copper allowed formerly worthless ores to be exploited. Better drainage channels, pumps, and other devices made it possible to tunnel more deeply into the earth as surface deposits began to be exhausted. In most established industries, however, technological change played little role, as in the past, new customers were sought by developing novel products based on existing technologies, such as a new type of woolen cloth with the texture of silk.
Sharply declining transaction costs （the direct and indirect expenses associated with transporting, distributing, and marketing goods and services） were more influential. On a general level, the decrease was due to greater security thanks to the lessening of wartime disruptions and to the economies of scale achieved when selling to large, concentrated urban populations. More specifically, it can be traced to transport innovations such as the carrack, a large ship that reduced rates for oceanborne freight by up to 25 percent, and big four-wheeled Hesse carts for overland routes. The spread of efficient organizational forms further contributed to declining costs, as did falling interest rates, which dropped from 20 percent or 25 percent in the mid-fifteenth century to 10 percent 100 years later.预测词汇6
diffusion = dispersal
perfected = completed
A rapid population of Japan occurred during its medieval times. Japan’s population was around 7 million but it rose keenly to 12 million from year 1200 to 1600. In this period, numerous hamlets formed throughout the country. They were mostly formed in the lands listed as ”unsettled” or as a ”wasteland” before 1300. There were many facets in increase in number of new hamlets，but by far the most significant characteristic of newly formed hamlets were that they were much bigger in terms of size compared to that of the hamlets built before 1300. There are many factors for forming of such large towns that contributed to increase in population mass. Some factors that can be considered are people’s demand for local authority, voluntarily, to defend themselves against outside threats or to form religious communities. Whatever the impetus, such formation of large villages was due to improvements in the agricultural technologies. Some improvements in technologies involved turning over of fields, irrigation methods, and usage of waterwheels, iron tools and diversification of crop output.
Among many improvements in agriculture, field leveling was the most basic practice used to optimize the land for farming. The farmers would create flat land for farming by leveling a field. They then would use the surfeit soil from the field to level the slightly slanted field. As a result, two fields of difference in altitude would be formed. Such difference in elevation allowed farmers to use the lower for rice paddy and the higher for dry crops. Practice of field leveling allowed a paddy culture to settle and allowed vast variety in dry crops to be produced due to the formation of drop crop field. Though the labor involved in formation of fields was enormous, the field preparation enabled marshlands alongside the rivers to be used for husbandry even if the rivers were uncontrolled.
Rice crops require ample water for growth and it takes much time until they are ready to be harvested. So farmers naturally worked by places where they had access to ample supply of water, such as riverbanks, streams and ponds. However, natural water supply was inefficient for the growth of rice, especially in sweltering summers. This led to the usage and development of ditches and dikes. Development in drawing the water from the distant locations led number of dams to increase and directed them to wherever they needed them. This was most evident in Yamato Basin where famers built permanent dams. The water detained in the pools was kept for times when they needed water for farming in droughty seasons. Such development led to keen proliferation of crop output as the heated water metabolized the germination of crops and caused crops to mature even faster.
By mid 1500s，one quarter of all paddy land were used to double crop. The farmers not only used fields to grow two crops in a year, but they even grew three in a single annual cycle. ■ An envoy from Korea stated that Japanese farmers from Hyogo region would grow barley and sow in winter and harvest them in summer. ■ Followed by rice cropping in summer and fall, buckwheat was harvested in winter. ■ As time went on agriculture advanced, such technique progressed from generation to generation. ■ Farming became more consistent and the crop output became even greater. A greater sense of discipline in land tilling and wide range of crops being planted in the same piece of land broadened the understanding of agriculture in farmers.
Crops harvested were used for farmers themselves. In many cases, one hectare of decent land was enough to sustain their entire family. They would only plow enough land for food to cater families for several reasons. Much of the medieval Japanese reclamation of land was due to the search for enough arable land to meet the food needed for just a single household. In case a farmer having enough fields for crop output required for his family, he would expand his fields no that one had to put into farming for his crops to grow. This was further compounded by the scarcity of land for fanning as well as limiting capacity for water and fertilizer supplies, not to mention the likelihood of antagonizing neighbors. Taking these variable factors into consideration, farmers of this period persisted with single hectare or less of arable land, just enough to sustain their families.预测词汇7
optimize= make the best use of
As the first cities formed in Mesopotamia in the Middle East, probably around 3000 B.C., it became necessarily to provide food for larger populations, and thus to find ways of increasing agricultural production. This, in turn, led to the problem of obtaining sufficient water.
Irrigation must have started on a small scale with rather simple constructions, but as its value became apparent, more effort was invested in new construction to divert more water into the canals and to extend the canal system to reach greater areas of potential farmland. Because of changing water levels and clogging by waterborne particles, canals and their intakes required additional labor to maintain, besides the normal labor required to guide water from field to field. Beyond this,some personnel had to be devoted to making decisions about the allocation of available water among the users and ensuring that these directions were carried out. With irrigation water also came potential problems, the most obvious being the susceptibility of low-lying farmlands to disastrous flooding and the longer-term problem of salinization （elevated levels of salt in the soil）。 To combat flooding from rivers, people from early historic times until today have constructed protective levees （raised barriers of earth） between the river and the settlement or fields to be protected. This, of course, is effective up to a certain level of flooding but changes the basic water patterns of the area and can multiply the damage when the flood level exceeds the height of the levee.
Salinization is caused by an accumulation of salt in the soil near its surface. This salt is carried by river water from the sedimentary rocks in the mountains and deposited on the Mesopotamian fields during natural flooding or purposeful irrigation. Evaporation of water sitting on the surface in hot climates is rapid, concentrating the salts in the remaining water that then descends through the soil to the underlying water table. In southern Mesopotamia, for example, the natural water table comes to within roughly six feet of the surface. Conditions of excessive irrigation bring the water table to eighteen inches, and water can rise further to the root zone, where the high concentration of salts would kill most plants.
Solutions for salinization were not as straightforward as for flooding, but even in ancient times it was understood that the deleterious effects of salinization could be minimized by removing harmful elements through leaching the fields with additional fresh water, digging deep wells to lower the water table, or instituting a system of leaving fields uncultivated. The first two cures would have required considerable labor, and the third solution would have led to diminished productivity, not often viewed as a likely decision in periods of growing population. An effective irrigation system laid the foundation for many of the world’s early civilizations, but it also required a great deal of labor input.
Growing agrarian societies often tried to meet their food-producing needs by farming less-desirable hill slopes surrounding the favored low-lying valley bottoms. Since bringing irrigation water to a hill slope is usually impractical, the key is effective utilization of rainfall. Rainfall either soaks into the soil or runs off of it due to gravity. A soil that is deep, well-structured, and covered by protective vegetation and much will normally absorb almost all of the rain that falls on it, provided that the slope is not too steep. However, soils that have lost their vegetative cover and surface mulch will absorb much less, with almost half the water being carried away by runoff in more extreme conditions. This runoff carries with it topsoil particles, nutrients, and humus （decayed vegetable matter） that are concentrated in the topsoil. The loss of this material reduces the thickness of the rooting zone and its capacity to absorb moisture for crop needs.
The most direct solution to this problem of slope runoff was to lay lines of stones along the contours of the slope and hence, perpendicular to the probable flow of water and sediment. These stones could then act as small dams, slowing the downhill flow of water and allowing more water to infiltrate and soil particles to collect behind the dam. This provided a buildup of sediments for plants and improved the landscape’s water-retention properties.预测词汇8
There are both great similarities and considerable diversity in the ecosystems that evolved on the islands of Oceania in and around the Pacific Ocean. The islands, such as New Zealand, that were originally parts of continents still carry some small plant and animal remnants of their earlier biota （animal and plant life）， and they also have been extensively modified by evolution, adaptation, and the arrival of new species. By contrast, the other islands, which emerged via geological processes such as volcanism, possessed no terrestrial life, but over long periods, winds, ocean currents, and the feet, feathers, and digestive tracts of birds brought the seeds of plants and a few species of animals. Only those species with ways of spreading to these islands were able to undertake the long journeys, and the various factors at play resulted in diverse combinations of new colonists on the islands. One estimate is that the distribution of plants was 75 percent by birds, 23 percent by floating, and 2 percent by wind.
The migration of Oceanic biota was generally from west to east, with four major factors influencing their distribution and establishment. The first was the size and fertility of the islands on which they landed, with larger islands able to provide hospitality for a wider range of species. Second, the further east the islands, generally the less the species diversity, largely because of the distance that had to be crossed and because the eastern islands tended to be smaller, more scattered, and remote. This easterly decline in species diversity is well demonstrated by birds and coral fish. It is estimated that there were over 550 species of birds in New Guinea, 127 in the Solomon Islands, 54 in Fiji, and 17 in the Society Islands. From the west across the Pacific, the Bismarck Archipelago and the Solomon Islands have more than 90 families of shore fish （with many species within the families）， Fiji has 50 families, and the Society Islands have 30. Third, the latitude of the islands also influenced the biotic mix, as those islands in relatively cooler latitudes, notably New Zealand, were unsuited to supporting some of the tropical plants with which Pacific islands are generally associated.
Finally, a fourth major factor in species distribution, and indeed in the shaping of Pacific ecosystems, was wind. It takes little experience on Pacific islands to be aware that there are prevailing winds. To the north of the equator these are called north-easterlies, while to the south they are called south-easterlies. Further south, from about 30°south, the winds are generally from the west. As a result on nearly every island of significant size there is an ecological difference between its windward and leeward （away from the wind） sides. Apart from the wind action itself on plants and soils, wind has a major effect on rain distribution. The Big Island of Hawaii offers a prime example; one can leave Kona on the leeward side in brilliant sunshine and drive across to the windward side where the city of Hilo is blanketed in mist and rain.
While such localized plant life and climatic conditions are very noticeable, over Oceania as a whole there is relatively little biodiversity, and the smaller the island and the further east it lies, the less there is likely to be. When humans moved beyond the islands of Near Oceania （Australia, New Guinea, and the Solomon Islands）， they encountered no indigenous mammals except for flying foxes, fruit bats, and seals on some islands. Other vertebrate species were restricted to flying animals and a few small reptiles. However, local adaptations and evolution over long periods of isolation promoted fascinating species adaptations to local conditions. Perhaps most notable, in the absence of mammals and other predators, are the many species of flightless and ground-nesting birds. Another consequence of evolution was that many small environments boasted their own endemic （native） species, often small in number, unused to serious predation, limited in range, and therefore vulnerable to disruption. In Hawaii, for example, the highly adapted 39 species and subspecies of honeycreepers, several hundred species of fruit flies, and more than 750 species of tree snails are often cited to epitomize the extent of localized Oceanic endemism （species being native to the area）。预测词汇9
The planets of our solar system all revolve around the Sun in the same direction and in orbits that lie in nearly the same plane. This is strong evidence that the planets formed simultaneously from a single disk of material that rotated in the same direction as the modern planets.
Precisely when the planets came into being has been a difficult issue to resolve. While Earth’s water is necessary for life, its abundance near the planet’s surface makes rapid erosion inevitable. Continuous alteration of the crust by erosion and also by igneous （volcanic） and metamorphic （pressure and heat within Earth） processes makes unlikely any discovery of rocks nearly as old as Earth. Thus geologists have had to look beyond this planet in their efforts to date Earth’s origin. Fortunately, we do have samples of rock that appear to represent the primitive material of the solar system. These samples are meteorites, which originate as extraterrestrial objects, called meteors, that have been captured in Earth’s gravitational field and have then crashed into our planet.
Some meteorites consist of rocky material and, accordingly, are called stony meteorites. Others are metallic and have been designated iron meteorites even though they contain lesser amounts of elements other than iron. Still others consist of mixtures of rocky and metallic material and thus are called stony-iron meteorites. Meteors come in all sizes, from small particles to the small planets known as asteroids; no asteroid, however, has struck Earth during recorded human history. Many meteorites appear to be fragments of larger bodies that have undergone collisions and broken into pieces. Iron meteorites are fragments of the interiors of these bodies, comparable to Earth’s core, and stony meteorites are from outer portions of these bodies, comparable to Earth’s mantle （the layer between the core and outer crust）。
Meteorites have been radiometrically dated by means of several decay systems, including rubidium-strontium, potassium-argon, and uranium-thorium. The dates thus derived tend to cluster around 4.6 billion years, which suggests that this is the approximate age of the solar system. After many meteorites had been dated, it was gratifying to find that the oldest ages obtained for rocks gathered on the surface of the Moon also were approximately 4.6 billion years. This must, indeed, be the age of the solar system. Ancient rocks can be found on the Moon because the lunar surface, unlike that of Earth, has no water to weather and erode rocks and is characterized by only weak movements of its crust.
Determining the age of the universe has been more complicated. Most stars in the universe are clustered into enormous disk-like galaxies. The distance between our galaxy, known as the Milky Way, and all others is increasing. In fact, all galaxies are moving away from one another, evidence that the universe is expanding. It is not the galaxies themselves that are expanding but the space between them. What is happening is analogous to inflating a balloon with small coins attached to its surface. The coins behave like galaxies: although they do not expand, the space between them does. Before the galaxies formed, matter that they contain was concentrated with infinite density at a single point from which it exploded in an event called the big bang. Even after it assembled into galaxies, matter continued to spread in all directions from the site of the big bang.
The evidence that the universe is expanding makes it possible to estimate its age. This evidence, called the redshift, is an increase in the wavelengths of light waves traveling through space—a shift toward the red end of the visible spectrum of wavelengths. Expansion of the space between galaxies causes this shift by stretching light waves as they pass through it. The farther these light waves have traveled through space, the greater the redshift they have undergone. For this reason, light waves that reach Earth from distant galaxies have larger redshifts than those from nearby galaxies. Calculations based on these redshifts indicate that about 13.7 billion years ago all of the galaxies would have been at one spot, the site of the big bang. This, then, is the approximate date of the big bang and the age of the universe.预测词汇10
In order to move from one home base to another, animals must expend calories not only while moving but even before the dispersal when they invest in the development of the muscles needed to move. For example, if a cricket is to leave a deteriorating environment and move to a new and better place, it will need large flight muscles to fly away. Presumably, the calories and materials that go into flight muscle development and maintenance have to come out of the general energy budget of the animal. This means that other organ systems cannot develop as rapidly as they could otherwise, which may mean that the flight-capable individual is, in some other respects, less fit to survive.
Dispersing individuals not only have to pay energetic, developmental, and travel costs but are also more often exposed to predators—all of which raises the question, why are animals so often willing to leave home even when this means leaving a familiar, resource-rich location? This question is particularly pertinent for species in which some individuals disperse while others do not or do not disperse as far. One species in which some individuals travel farther than others is Belding’s ground squirrel. Young male squirrels travel about 150 meters from the burrow in which they were born, whereas young females usually settle down only 50 meters or so from where they were born. Why should young Belding’s ground squirrels disperse at all, and why should the males disperse farther than their sisters?
According to one argument, dispersal by juvenile animals of many species may be an adaptation against problems associated with inbreeding. When two closely related individuals mate, their offspring are more likely to manifest genetic diseases than are the offspring of genetically unrelated individuals, and as a result, inbreeding tends to produce animals that are less likely to survive to adulthood and reproduce. Dispersal of juveniles makes inbreeding less likely. If avoidance of inbreeding is the point of dispersing, then one might expect as many female ground squirrels as males to travel 150 meters from their natal burrow. In fact females do not disperse as far as males, perhaps because the costs and benefits of dispersal differ for the two sexes. It has been suggested that the reproductive success of female Belding’s ground squirrels depends on their possession of a territory in which to rear their young. Female ground squirrels that remain near their birthplace enjoy assistance from their mothers in the defense of their burrows against rival females. Thus, the benefits of remaining on familiar ground are greater for females than for males.
There may, however, be another reason why male mammals disperse greater distances than females. The usual rule is that males, not females, fight with one another for access to mates, and, therefore, males that lose such conflicts may find it advantageous to move away from same-sex rivals that they cannot subdue. Although this hypothesis probably does not apply to Belding’s ground squirrels, since young males have not been seen fighting with older ones around the time of dispersal, the idea is more plausible with respect to some other species, such as lions.
Lions live in large groups, or prides, from which young males disperse. In contrast, the daughters of the resident lionesses usually spend their entire lives close to where they were born. The sedentary females benefit from their familiarity with good hunting grounds and safe breeding dens in their natal territory, among other things. The departure of many young male lions coincides with the arrival of new mature males that violently displace the previous masters of the pride and chase off the males that are not yet adults in the pride as well. These observations support the mate-competition hypothesis for male dispersal. However, if young males are not evicted after a pride takeover, they often leave anyway without any coercion from adult males and without ever having attempted to mate with their female relatives. Moreover, mature males that have claimed a pride sometimes disperse again, expanding their range to add a second pride of females, at a time when their daughters in the first pride are becoming sexually mature. Inhibitions against inbreeding apparently exist in lions and cause males to leave home.预测词汇11
presumably= It is reasonable to assume
coincide with= occurs at the same time as
A species of small mongooses in Africa called meerkats share sentinel （guard） duties to warn other group members by repeating alarm calls if a predator is seen. This is an important job, because when meerkats are foraging, their heads are in the ground seeking prey, and they cannot see a predator coming.
The question is, why do group members take turns acting as sentinels? Kin selection, that is, being able to save the lives of family members can be one hypothesis for this type of sentinel behavior. Family members share copies of a meerkat's genes. Kin selection is achieved by helping a meerkat’s own offspring as well as non descendant kin, including sibling, nieces, nephews, aunts, and uncles.
Therefore, if members of a certain group are closely related, a sentinel ensures that copies of its genes can be passed on to future generations by saving the majority of family members by alerting others, even at the expense of its own life. Assuming this hypothesis is true, we can predict that group members have close genetic ties. Otherwise, kin selection would not work. But this prediction does hold true. A dominant, breeding female is mother to 75 percent of all the litters in a group, and one dominant male fathers 75 percent of all the pups born. Even though a typical meerkat group includes a few immigrants, most subordinate adults are siblings or half siblings. Therefore, it is likely that subordinate adults share 25 or 50 percent of their genes.
On account of most meerkat group members being family, it is possible that kin selection has favored sentinel behavior. Nonetheless, by itself, a close inherent relationship is not enough evidence to conclude that kin selection has played a role. Thus, we need further evidence, and must improve the prediction. Based on the same hypothesis, a more specific prediction is that each mongoose should increase the frequency of sentinel behavior when they are guarding family members. This new prediction needed testing, so the group was observed to determine which members stand guard and when. The immigrants without any kin relations to other group members acted as sentinels just as much as the individuals with many relatives nearby. Therefore, the result of this test does not support the kin-selection hypothesis.
Another hypothesis that is often suggested to explain such cooperative behavior is that it results from reciprocal altruism~each individual takes turns standing guard to benefit the rest of the group, rather than itself. The reciprocal altruism theory can work only when those who cheat by avoiding guard duty can be identified and punished by the rest of the group. This hypothesis produces the prediction that there should be a regular rotation of sentry duty within the group and that the ones who neglect this duty should be chastised. However, this is not observed. In fact, the group members do take turns on sentry duty, but there is no predetermined order for this. In addition, when some members shorten their shift, other group members increase their contributions to compensate. The predictions and observations of the reciprocal-altruism hypothesis do not coincide with each other.
Yet another hypothesis for the evolution of meerkat sentinel behavior is that it results from selfish antipredator behavior. This idea stems from the fact that the meerkat watching for predators increases its personal safety, and warning others does not harbor any disadvantage. So, when a meerkat has had enough to eat, it should watch for predators. The sentinel on duty can then return to foraging. This hypothesis produces a prediction that sentinel duty is not dangerous or risky in any way. This does seem to be true. Over the course of 2,000 hours of observation, no sentinels were attacked or killed by predators. They may actually be safer because they are the first to sense the predator. Moreover, they generally stand guard within 5 meters of a burrow, and are the first underground when a predator comes close. ■ If a meerkat’s personal safety is increased with serving as a sentinel, it would be possible to predict that an individual would spend a proportion of its time guarding, whether it was solitary or part of a group. ■ As predicted, individual meerkats spend about the same time on guard duty as members of large groups. ■ Groups with more members suffer less predation because there is a sentinel for a longer portion of foraging time than in small groups. ■预测词汇12