托福阅读真题+题目+答案：Foundational and Keystone Species
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.
1. The word extent in the passage is closest in meaning to
2. According to paragraph 1, foundational species are usually
(A) responsible for environmental damage
(B) present only in certain ecosystems
(C) close to complete extinction
(D) found in the largest number
3. The word sustain in the passage could best be replaced by
4. The word it in the passage refers to
(B) coral reef
5. 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.
(A) The foundational species become obvious once the system has collapsed.
(B) Ecosystems will not collapse as the foundational species do not exist.
(C) It is clear that the system would fall apart if it did not have its foundational species.
(D) Without the collapse, the foundational species are not obvious in the ecosystem.
6. The author mentions arches in paragraph 3 in order to
(A) help understand what keystone species means
(B) provide an example of an ecological imbalance
(C) show the problems with a foundational species
(D) explain the role of a type of animal species
7. Which of the following can be inferred from paragraph 3 about keystone species?
(A) Their roles have been understood for a long time.
(B) They are related to a large number of ecosystems.
(C) They are now seen as less important in nature.
(D) They are the subjects of ongoing research.
8. The phrase In the same way in the passage could best be replaced by
9. It is NOT true according to paragraph 4 that sea otters
(A) hunt sea urchins
(B) damage kelp roots
(C) eat keystone species
(D) live near water
10. According to paragraph 5, which of the following is true about beavers?
(A) They are predator keystone species.
(B) They live in the same areas as bears.
(C) They usually eat fish from ponds.
(D) They can create large, strong dams.
11. Look at the four squares [■] that indicate where the following sentence can be added to the passage.
The bears that live on the West Coast of North America are often cited as an engineer species.
Where would the sentence best fit?
12. Directions: Ecological systems require the contributions of various species types. Complete the table by matching the appropriate species with their characteristics. This question is worth 3 points.
Foundational Species Keystone Species
A. They are large in number.
B. Ecosystems form around them.
C. They are more beautiful.
D. They are smaller in number.
E. Four carnivorous animals belong to them.
F. They are more dangerous than anything else.
G. Without them, ecosystems would collapse.