Many species in the deer family, particularly white-tailed deer and moose in North America and red deer in Europe, browse(feed on) immature trees such as seedlings and saplings and in so doing curtail the regeneration of trees. In Scotland high numbers of red deer pose a particular problem for the regeneration of Scots pine forests, some of which have not seen substantial regeneration for 300 years. Many smaller mammals, including mice, squirrels, voles (small, mouse-like animals), and rabbits, also regularly eat tree seeds and seedlings, sufficient to alter the course of woodland development.
These animals have a preference for particular tree species and can therefore potentially change woodland composition and future development. Moreover, some tree species can tolerate repeated browsing, but others cannot. Many broad-leaved tree saplings, for example, will readily resprout if the leading stem is browsed, but conifers(trees with needlelike leaves) generally lack this ability. This is significant in northern zones, where voles more than make up for the comparative absence of large animals. In northern Europe, vole outbreaks occur on a three- to four- year cycle (and somewhat more irregularly in North America), during which populations multiply several hundredfold. As voracious feeders, they rapidly consume their preferred plant roots and bulbs and then switch to seedlings of pines, spruces, and other conifers. This can have substantial economic impact for commercial forestry, and in Finland alone the cost of seedling losses to voles has been estimated at €20 million during such outbreaks.
Browsing mammals feed on trees, leaves, and shoots and consequently suppress tree growth. Conversely, grazers feed on grasses and can enhance tree establishment. This is most obviously played out in the African plains. By suppressing trees in African dry forests, elephants, giraffes, and other large browsers facilitate the spread and growth of grasses. This creates a substantial fuel load (amount of burnable material), and the resulting intense fires reduce woody vegetation further and prevent tree regeneration. Grazers such as zebras and wildebeests reverse this process by cropping the grasses, which reduces fire intensity. Trees reestablish, and the resulting forest shade suppresses the grasses. The change of African savannas from woodland to grassland and back again is therefore maintained by different kinds of herbivores(plant eaters).
Yet it is predators that have the key role in triggering the change from woodlands to grasslands and back. Trees provide predators with cover within which they can stalk and ambush herbivores. Herbivores therefore exist within a “landscape of fear”in which they avoid wooded areas in favor of more open grasslands where they can more easily spot predators. The concentration of grazing herbivores in grasslands is therefore brought about by predators, and this serves as the main driver of conversion to woodland. Elephants, by contrast, are more or less immune to predators on account of their bulk and have no aversion to wooded areas. As large browsers they feed on leaves, breaking branches and even knocking over trees to get to them, thereby initiating a return to grassland.
No matter the destruction done by mammalian herbivores, it pales into insignificance compared to that caused by insects. Since 1990 some 30 billion conifers from Alaska to Mexico have been killed by tiny bark beetles. Individually, bark beetles are smaller than a grain of rice, but when conditions are right, their populations explode in epidemics that can last a decade or more. In recent decades a few bark beetle species, such as the mountain pine beetle in North America and the European spruce bark beetle, have been particularly destructive of coniferous forests. The increased incidence of summer droughts in these regions has rendered many trees vulnerable to attack, while milder winters have facilitated the growth of bark beetle populations. Healthy trees defend themselves by drowning the tiny pine beetles in resin, a sticky substance produced by some trees. Female beetles therefore target vulnerable drought-stressed trees, and once located they release a pheromone (chemical signal) to attract other beetles. The tree responds with sticky resin and poisonous gases, but sheer force of numbers eventually overcomes a tree’s defenses. The beetles, which might now number several thousand, lay their eggs under the bark, and the larvae (young) feed on the living tissue, which ultimately kills the trees.
题目
1
These animals have a preference for particular tree species and can therefore potentially change woodland composition and future development. Moreover, some tree species can tolerate repeated browsing, but others cannot. Many broad-leaved tree saplings, for example, will readily resprout if the leading stem is browsed, but conifers(trees with needlelike leaves) generally lack this ability. This is significant in northern zones, where voles more than make up for the comparative absence of large animals. In northern Europe, vole outbreaks occur on a three- to four- year cycle (and somewhat more irregularly in North America), during which populations multiply several hundredfold. As voracious feeders, they rapidly consume their preferred plant roots and bulbs and then switch to seedlings of pines, spruces, and other conifers. This can have substantial economic impact for commercial forestry, and in Finland alone the cost of seedling losses to voles has been estimated at €20 million during such outbreaks.
The word”tolerate”in the passage is closest in meaning to
Aprevent
Bstay free of
Cdiscourage
Dsurvive
2
These animals have a preference for particular tree species and can therefore potentially change woodland composition and future development. Moreover, some tree species can tolerate repeated browsing, but others cannot. Many broad-leaved tree saplings, for example, will readily resprout if the leading stem is browsed, but conifers(trees with needlelike leaves) generally lack this ability. This is significant in northern zones, where voles more than make up for the comparative absence of large animals. In northern Europe, vole outbreaks occur on a three- to four- year cycle (and somewhat more irregularly in North America), during which populations multiply several hundredfold. As voracious feeders, they rapidly consume their preferred plant roots and bulbs and then switch to seedlings of pines, spruces, and other conifers. This can have substantial economic impact for commercial forestry, and in Finland alone the cost of seedling losses to voles has been estimated at €20 million during such outbreaks.
Paragraph 2 supports all of the following statements about vole outbreaks EXCEPT:
AThey occur every few years in Northern Europe.
BThey occur more often in North America than in Europe.
CThey cause the mass destruction of conifer seedlings.
DThey cause millions of euros’ worth of damage to commercial forests.
3
Browsing mammals feed on trees, leaves, and shoots and consequently suppress tree growth. Conversely, grazers feed on grasses and can enhance tree establishment. This is most obviously played out in the African plains. By suppressing trees in African dry forests, elephants, giraffes, and other large browsers facilitate the spread and growth of grasses. This creates a substantial fuel load (amount of burnable material), and the resulting intense fires reduce woody vegetation further and prevent tree regeneration. Grazers such as zebras and wildebeests reverse this process by cropping the grasses, which reduces fire intensity. Trees reestablish, and the resulting forest shade suppresses the grasses. The change of African savannas from woodland to grassland and back again is therefore maintained by different kinds of herbivores(plant eaters).
According to paragraph 3,the eating habits of elephants and giraffes have all the following effects on African plains EXCEPT:
AThey discourage trees from growing.
BThey encourage grasses to cover more area.
CThey discourage grazers from cropping grasses.
DThey increase the amount of material available that can burn.
4
Yet it is predators that have the key role in triggering the change from woodlands to grasslands and back. Trees provide predators with cover within which they can stalk and ambush herbivores. Herbivores therefore exist within a “landscape of fear”in which they avoid wooded areas in favor of more open grasslands where they can more easily spot predators. The concentration of grazing herbivores in grasslands is therefore brought about by predators, and this serves as the main driver of conversion to woodland. Elephants, by contrast, are more or less immune to predators on account of their bulk and have no aversion to wooded areas. As large browsers they feed on leaves, breaking branches and even knocking over trees to get to them, thereby initiating a return to grassland.
According to paragraph 4, elephants affect woodlands more than other herbivores do because
Apredators generally do not attack animals as large as elephants
Bother herbivores avoid the “landscape of fear”that elephants create
Celephants can spot predators more easily than other kinds of herbivores can
Delephants graze in especially concentrated areas
5
Yet it is predators that have the key role in triggering the change from woodlands to grasslands and back. Trees provide predators with cover within which they can stalk and ambush herbivores. Herbivores therefore exist within a “landscape of fear”in which they avoid wooded areas in favor of more open grasslands where they can more easily spot predators. The concentration of grazing herbivores in grasslands is therefore brought about by predators, and this serves as the main driver of conversion to woodland. Elephants, by contrast, are more or less immune to predators on account of their bulk and have no aversion to wooded areas. As large browsers they feed on leaves, breaking branches and even knocking over trees to get to them, thereby initiating a return to grassland.
According to paragraph 4, most herbivores of the African plains prefer open grasslands because these areas allow them to
Agraze efficiently
Bbetter detect attackers
Cfind cover from predators
Davoid competition with elephants
6
No matter the destruction done by mammalian herbivores, it pales into insignificance compared to that caused by insects. Since 1990 some 30 billion conifers from Alaska to Mexico have been killed by tiny bark beetles. Individually, bark beetles are smaller than a grain of rice, but when conditions are right, their populations explode in epidemics that can last a decade or more. In recent decades a few bark beetle species, such as the mountain pine beetle in North America and the European spruce bark beetle, have been particularly destructive of coniferous forests. The increased incidence of summer droughts in these regions has rendered many trees vulnerable to attack, while milder winters have facilitated the growth of bark beetle populations. Healthy trees defend themselves by drowning the tiny pine beetles in resin, a sticky substance produced by some trees. Female beetles therefore target vulnerable drought-stressed trees, and once located they release a pheromone (chemical signal) to attract other beetles. The tree responds with sticky resin and poisonous gases, but sheer force of numbers eventually overcomes a tree’s defenses. The beetles, which might now number several thousand, lay their eggs under the bark, and the larvae (young) feed on the living tissue, which ultimately kills the trees.
According to paragraph 5, which of the following developments has contributed to an increase in bark beetle epidemics in North America and Europe?
AThe increased vulnerability of trees because of poisonous chemicals released by beetles
BThe spread of disease that affects resin production in trees
CThe spread of a few bark beetle species between Europe and North America in recent decades
DThe presence of less severe weather conditions during the coldest time of year
7
No matter the destruction done by mammalian herbivores, it pales into insignificance compared to that caused by insects. Since 1990 some 30 billion conifers from Alaska to Mexico have been killed by tiny bark beetles. Individually, bark beetles are smaller than a grain of rice, but when conditions are right, their populations explode in epidemics that can last a decade or more. In recent decades a few bark beetle species, such as the mountain pine beetle in North America and the European spruce bark beetle, have been particularly destructive of coniferous forests. The increased incidence of summer droughts in these regions has rendered many trees vulnerable to attack, while milder winters have facilitated the growth of bark beetle populations. Healthy trees defend themselves by drowning the tiny pine beetles in resin, a sticky substance produced by some trees. Female beetles therefore target vulnerable drought-stressed trees, and once located they release a pheromone (chemical signal) to attract other beetles. The tree responds with sticky resin and poisonous gases, but sheer force of numbers eventually overcomes a tree’s defenses. The beetles, which might now number several thousand, lay their eggs under the bark, and the larvae (young) feed on the living tissue, which ultimately kills the trees.
In paragraph 5, why does the author discuss the number of female bark beetles that may be involved in attacking a tree?
ATo show that bark beetles change their attack strategy when trees are healthy
BTo help explain why a tree’s defense system becomes overwhelmed during an attack
CTo show that bark beetles are attracted to the gases released by trees
DTo explain why it is necessary for bark beetles to lay their eggs under a tree’s bark
8
No matter the destruction done by mammalian herbivores, it pales into insignificance compared to that caused by insects. Since 1990 some 30 billion conifers from Alaska to Mexico have been killed by tiny bark beetles. Individually, bark beetles are smaller than a grain of rice, but when conditions are right, their populations explode in epidemics that can last a decade or more. In recent decades a few bark beetle species, such as the mountain pine beetle in North America and the European spruce bark beetle, have been particularly destructive of coniferous forests. The increased incidence of summer droughts in these regions has rendered many trees vulnerable to attack, while milder winters have facilitated the growth of bark beetle populations. Healthy trees defend themselves by drowning the tiny pine beetles in resin, a sticky substance produced by some trees. Female beetles therefore target vulnerable drought-stressed trees, and once located they release a pheromone (chemical signal) to attract other beetles. The tree responds with sticky resin and poisonous gases, but sheer force of numbers eventually overcomes a tree’s defenses. The beetles, which might now number several thousand, lay their eggs under the bark, and the larvae (young) feed on the living tissue, which ultimately kills the trees.
Paragraph 5 suggests that female bark beetles target drought-stressed trees for which of the following reasons?
ADrought-stressed trees provide more protection from winter weather.
BDrought-stressed trees release chemical signals that are attractive to beetles.
CDrought-stressed trees produce less resin than well-watered trees do.
DDrought-stressed trees have thinner bark, which is easier for the beetles to get through.
