The Evolution of Grass and Herbivores
When blades of grass are cut, it is their oldest parts that are removed. The critical basal parts of grasses, where leaf growth takes place and new leaves emerge, remain intact. Grasses are thus highly adapted to a regime of frequent, intense cutting, because they sacrifice the least functional parts of their food-producing machinery while hiding the portions of the plant responsible for making it grow back. The key to understanding the modem success of grasses is thus to identify the situations in which the attributes of grasses as well as their defense mechanisms are superior to those of other plants, and to document how plant eaters and the environments they live in have changed through the ages.
One long-held view is that grasses acquired their unique features and then became important components of the world’s vegetation in response to climatic change. The great grass-dominated prairies, steppes, and savannas of the world thrive in regions where annual precipitation is less than about five hundred millimeters. Closed-canopy forests, where grasses constitute a small fraction of plant cover, grow under conditions of higher rainfall. As climatic conditions became increasingly favorable to grasses during the past twenty-five million years, so the argument goes, at least ten lineages of mammals gave rise to highly mobile grazers (animals that clip vegetation at or near ground level), which roamed the new dry-adapted grass ecosystems in herds beginning ten million years ago.
The argument against this explanation rests on two intriguing discoveries. Fossil deposits containing the silica bodies of grasses indicate that these plants originated in the understory of forested habitats, not in dry, open areas. The oldest occurrences so far known are from the latest Cretaceous period, about seventy million years ago. in India; but related plant families may date back to one hundred million years ago or more. In a late stage of their history near the end of the Cretaceous, large herbivorous (plant-eating) dinosaurs were living alongside small mammals. However, it was not until much later—thirty million years ago in South America, twenty-five million years ago in North America, and perhaps fifteen million years ago in Africa—that grasses became the signature architects of landscapes, which they have remained ever since. Soils characteristic of tail-grass and short-grass prairie vegetations became common in North America in the Great Plains about twenty million years ago, at a time when hoofed mammals were evolving rapidly. According to detailed studies of teeth and skulls of herbivorous animals carried out by Christine Janis, a brilliant vertebrate paleontologist at Brown University, these early mammals were either browsers, specialized on a diet of tree and shrub vegetation, or mixed feeders on a wide variety of plants, including grass. The high-crowned teeth of grass eaters appeared in
numerous lineages of mammals whose diet included large amounts of fibrous plant material as well as gritty dust.
The idea that herbivores were responsible for the evolution of the peculiarities of the grass habitat, as well as for the subsequent development of grasslands, is supported by the second discovery. Although grasslands and the grazers inhabiting them occur on all the major continents, they are largely absent from the native ecosystems of Madagascar and New Zealand, places where the climate is conducive to grasslands and where such grasslands have become widespread since humans introduced domesticated grazers to these islands. Extinct moas—giant flightless birds, some weighing up to two hundred kilograms—and living flightless kaka parrots evolved as incidental grazers in New Zealand, but only in parts of the South Island above treeline did grasses form the dominant component of the vegetation. No other herbivores on these or other smaller land masses developed the appetites associated with grass feeding in continental mammals. In other words, a suitable climate is not sufficient to explain the success of grass.
This observation implies that grasslands thrive only in places where herbivores are large enough and common enough to restrict trees and shrubs, which might dominate the same environment in the absence of grazers. In other words, the intensity of plant consumption in grasslands is so high that the defenses of other kinds of plants are no longer adequate. Only plants whose vulnerable growth zones are hidden from the depredations of plant eaters can flourish as dominant members of the vegetation in the face of intense grazing.
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P1: When blades of grass are cut, it is their oldest parts that are removed. The critical basal parts of grasses, where leaf growth takes place and new leaves emerge, remain intact. Grasses are thus highly adapted to a regime of frequent, intense cutting, because they sacrifice the least functional parts of their food-producing machinery while hiding the portions of the plant responsible for making it grow back. The key to understanding the modem success of grasses is thus to identify the situations in which the attributes of grasses as well as their defense mechanisms are superior to those of other plants, and to document how plant eaters and the environments they live in have changed through the ages.
1.The word “intact” in the passage is closest in meaning to
A.whole
B.covered
C.fertile
D.healthy
2.According to paragraph 1, grasses adapted to the frequent habit of intense cutting by
A.producing an increased number of leaves
B.generating strong leaves that are difficult to cut
C.hiding their basal parts
D.evolving numerous alternative food-producing parts
3.According to paragraph 1, which of the following is an important part of the success of modern grasses?
A.The frequent, intense cutting they experience
B.The superiority of their defense mechanisms
C.The removal of their oldest parts
D.Their ability to grow back their basal parts
P2: One long-held view is that grasses acquired their unique features and then became important components of the world’s vegetation in response to climatic change. The great grass-dominated prairies, steppes, and savannas of the world thrive in regions where annual precipitation is less than about five hundred millimeters. Closed-canopy forests, where grasses constitute a small fraction of plant cover, grow under conditions of higher rainfall. As climatic conditions became increasingly favorable to grasses during the past twenty-five million years, so the argument goes, at least ten lineages of mammals gave rise to highly mobile grazers (animals that clip vegetation at or near ground level), which roamed the new dry-adapted grass ecosystems in herds beginning ten million years ago.
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.
The change of climate led to the existence of new types of grasses that grew in many environments, not just dry ecosystems.
It has been argued that climate conditions of the past twenty five million years led to the spread of grasses, which in turn gave rise to grazers.
It is believed that the climate conditions that were present ten million years ago resulted in the development of at least ten species of grazers.
Mammals that existed twenty-five million years ago formed herds of highly mobile grazers that preferred dry-adapted grasses.
5.In paragraph 2, why does the author provide the information that grasses are only a small part of the plant cover in closed-canopy forests?
A.To compare the rainfall differences between prairies and areas with less rainfall
B.To indicate that grasses grow better in areas with lower rainfall, such as savannas
C.To explain how grasses acquired unique features that helped their success
D.To provide evidence that grass is able to adapt to multiple climates and environments
P3: The argument against this explanation rests on two intriguing discoveries. Fossil deposits containing the silica bodies of grasses indicate that these plants originated in the understory of forested habitats, not in dry, open areas. The oldest occurrences so far known are from the latest Cretaceous period, about seventy million years ago. in India; but related plant families may date back to one hundred million years ago or more. In a late stage of their history near the end of the Cretaceous, large herbivorous (plant-eating) dinosaurs were living alongside small mammals. However, it was not until much later—thirty million years ago in South America, twenty-five million years ago in North America, and perhaps fifteen million years ago in Africa—that grasses became the signature architects of landscapes, which they have remained ever since. Soils characteristic of tail-grass and short-grass prairie vegetations became common in North America in the Great Plains about twenty million years ago, at a time when hoofed mammals were evolving rapidly. According to detailed studies of teeth and skulls of herbivorous animals carried out by Christine Janis, a brilliant vertebrate paleontologist at Brown University, these early mammals were either browsers, specialized on a diet of tree and shrub vegetation, or mixed feeders on a wide variety of plants, including grass. The high-crowned teeth of grass eaters appeared in numerous lineages of mammals whose diet included large amounts of fibrous plant material as well as gritty dust.
6.According to “detailed studies of teeth and skulls of herbivorous animals carried out by Christine Janis,” all of the following are true of the early herbivorous mammals she studied EXCEPT:
A.The diets of some of them consisted of trees and shrubs.
B.The diets of some of them included dust.
C.The diets of some of them included a variety of plants.
D.The diets of all of them included grass.
7.Which of the following conclusions is suggested by the grass fossil deposits mentioned in paragraph 3 ?
A.Grasses evolved more recently than previously thought.
B.Grasses first appeared as a result of climate change.
C.Grasses did not originally evolve in dry climates.
D.Grasses did not exist at the same time as dinosaurs.
P4: The idea that herbivores were responsible for the evolution of the peculiarities of the grass habitat, as well as for the subsequent development of grasslands, is supported by the second discovery. Although grasslands and the grazers inhabiting them occur on all the major continents, they are largely absent from the native ecosystems of Madagascar and New Zealand, places where the climate is conducive to grasslands and where such grasslands have become widespread since humans introduced domesticated grazers to these islands. Extinct moas—giant flightless birds, some weighing up to two hundred kilograms—and living flightless kaka parrots evolved as incidental grazers in New Zealand, but only in parts of the South Island above treeline did grasses form the dominant component of the vegetation. No other herbivores on these or other smaller land masses developed the appetites associated with grass feeding in continental mammals. In other words, a suitable climate is not sufficient to explain the success of grass.
8.Why does the author point out that Madagascar and New Zealand are “places where the climate is conducive to grasslands and where such grasslands have become widespread since humans introduced domesticated grazers to these islands”?
A.To explain why moas and kaka parrots evolved as grazers
B.To argue that grasslands evolved first on the major continents and later on islands
C.To suggest that for grasslands and grazers to spread, humans needed to encourage the process
D.To support the idea that herbivores were responsible for the development of grasslands
P1: When blades of grass are cut, it is their oldest parts that are removed. ■The critical basal parts of grasses, where leaf growth takes place and new leaves emerge, remain intact. ■ Grasses are thus highly adapted to a regime of frequent, intense cutting, because they sacrifice the least functional parts of their food-producing machinery while hiding the portions of the plant responsible for making it grow back. ■ The key to understanding the modem success of grasses is thus to identify the situations in which the attributes of grasses as well as their defense mechanisms are superior to those of other plants, and to document how plant eaters and the environments they live in have changed through the ages. ■
9.Look at the four squares [■] that indicate where the following sentence could be added to the passage.
These parts are at the top, positioned to be sacrificed.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
10.
A.Scientists have recently discovered that the most important part of grasses is the exposed part of the blade because it is the regenerative part of the plant responsible for the growth of leaves.
B.In contrast to previously held views on the evolution of grasses, it has been discovered that grasses did not first appear in dry habitats and that their evolution was influenced by herbivores.
C.Scientists suggest that grasses that grow in closed-canopy forests require climatic conditions, such as increased moisture, that are different from the climate requirements of grasses in other habitats.
D.For a long time, it was believed that the evolution of grasses was mainly caused by changes in the weather that led to a more suitable habitat for grasses.
E.Some evidence suggests that grasses probably flourished before the Cretaceous period together with other related plant families when herbivorous dinosaurs were present.
F.The absence of grasses in environments suitable for their growth is evidence that grasses evolved not only because of the effect of climate but also because of large plant-eating animals.
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