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.
extent=amount
sustain=support
in the same way=similarly
Oxygen and nitrogen are major components of our current atmosphere. But the kinds of hydrogen reactions with other gases that are required to transform simple organic molecules into complex ones are interrupted by oxygen, which combines with hydrogen atoms from other compounds. Therefore, life on Earth must have originated when there was very little oxygen in Earth’s atmosphere. The modern scientific theory of life’s origin was first formulated in the 1920s by Russian scientist Aleksandr Oparin and independently by British scientist J. B. S. Haldane. The assumption that life sprang up from chemical reactions that were initiated in the early atmosphere（oxygen-poor/hydrogen-rich） and came to completion in the early oceans was posited by the Oparin-Haldane hypothesis, as it came to be called. Oparin and Haldane suggested that the hydrogen-containing gases caused to react with each other to form organic compounds by energy sources such as sunlight and lightning.
With regard to the view that these complex organic compounds could have begun to shape in Earth’s oceans, some researchers remain skeptical. The probability that the fundamental building blocks of life, formaldehyde （H2CO） and hydrogen cyanide（HCN），even though they were probably available, would have been concentrated sufficiently to allow further reactions to occur was likely small. And the more complex organic compounds that might have formed in this way would not have lasted long in the surface-ocean environment, because photochemical and thermal reactions would have destroyed them. Therefore, researchers have sought alternative explanations for how complex organic compounds formed.
There is one possibility that the relevant organic compounds were created in space, and asteroids or comets brought them to Earth, probably as tiny dust particles. Recovered from the stratosphere （an upper region of Earth’s atmosphere）， interplanetary dust particles （IDPs） is tiny particles that are known to be extraterrestrial origin. From various researches, we know that organic compounds, including amino acids, exist in IDPs as well as in some meteorite. Now, we actually have identified the fact that amino acids and many other complex organic compounds in interstellar dust clouds. It is believed that they form from reactions between charged particles and neutral molecules. Those atoms appear in interstellar dust clouds at very low temperatures-on the order of 200 or more degrees below zero on the Celsius scale. It may seem surprising that organic chemistry could occur in the interstellar environment, but it is precisely the extremely low temperatures involved that allow complex organic molecules to exist because temperatures are too cold to allow them to decompose.
■It is thought that some of the molecules created in the interstellar environment have survived the collapse of the gas and dust cloud that formed our solar nebula and Sun.
■ They would have been incorporated into solid materials that condensed out of the nebula and formed asteroids and comets.
■ Such materials might have been delivered to Earth in great quantities during the heavy bombardment period of solar system history, between 4.5 and 3.8 billion years ago.
■The hypothesis that life took place in or around hydrothermal vents （hot springs）， where new seafloor is being created along mid-ocean ridges （underwater mountain chains） at the ocean’s bottom is the third theory of life’s origin. By seawater that flows a kilometer or more down through crevices in the rock, is heated, and then rises rapidly back to the surface, the ridges are cooled. During the process, the water gathers substances such as hydrogen, hydrogen sulfide, and dissolved ferrous iron. When it meets the cold water, the extremely hot （350°C） vent water generates a dark plume comprised mostly of iron sulfide, a compound produced by the reaction between ferrous iron and hydrogen sulfide.
Still, the explanation that submarine hydrothermal vents a likely place for life to have originated is controversial. In vent systems, there are various types of materials from which organic molecules can be synthesized. However, complex organic molecules are not stable at the high temperatures observed in vents positioned directly on the axis of a ridge. If life did originate at the mid-ocean ridges, it probably did so in cooler, off-axis vents. Some researchers claim that perfect place for life to have begun would be in some near-freezing surface environment because even the off-axis vents are too warm. The dispute as to whether life originated in a hot or cold environment is unlikely to stop.
预测词汇2skeptical = doubtful
sufficiently = adequately
sought = looked for
been incorporated into = become part of
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3exceed=go beyond
detected=noticed
propose=offer the theory
roughly=approximately
