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OG A small number of the forest species

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OG A small number of the forest species

Post Thu Jul 27, 2017 8:18 pm
A small number of the forest species of lepidoptera (moths and butterflies, which exist as caterpillars Line during most of their life cycle) exhibit regularly recurring patterns of population growth and decline-such fluctuations in population are known as population cycles. Although many different variables influence population levels, a regular pattern such as a population cycle seems to imply a dominant, driving force. Identification of that driving force, however, has proved surprisingly elusive despite considerable research. The common approach of studying causes of population cycles by measuring the mortality caused by different agents, such as predatory birds or parasites, has been unproductive in the case of lepidoptera. Moreover, population ecologists’ attempts to alter cycles by changing the caterpillars’ habitat and by reducing caterpillar populations have not succeeded. In short, the evidence implies that these insect populations, if not self-regulating, may at least be regulated by an agent more intimately connected with the insect than are predatory birds or parasites.
Recent work suggests that this agent may be a virus. For many years, viral disease had been reported in declining populations of caterpillars, but population ecologists had usually considered viral disease to have contributed to the decline once it was underway rather than to have initiated it. The recent work has been made possible by new techniques of molecular biology that allow viral DNA to be detected at low concentrations in the environment. Nuclear polyhedrosis viruses are hypothesized to be the driving force behind population cycles in lepidoptera in part because the viruses themselves follow an infectious cycle in which, if protected from direct sun light, they may remain virulent for many years in the environment, embedded in durable crystals of polyhedrin protein. Once ingested by a caterpillar, the crystals dissolve, releasing the virus to infect the insect’s cells.Late in the course of the infection, millions of new virus particles are formed and enclosed in polyhedrin crystals. These crystals reenter the environment after the insect dies and decomposes, thus becoming available to infect other caterpillars.
One of the attractions of this hypothesis is its broad applicability. Remarkably, despite significant differences in habitat and behavior, many species of lepidoptera have population cycles of similar length, between eight and eleven years. Nuclear polyhedrosis viral infection is one factor these disparate species share.

454) The primary purpose of the passage is to
A. describe the development of new techniques that may help to determine the driving force behind population cycles in lepidoptera
B. present evidence that refutes a particular theory about the driving force behind population cycles in lepidoptera
C. present a hypothesis about the driving force behind population cycles in lepidoptera
D. describe the fluctuating patterns of population cycles in Lepidoptera
E. question the idea that a single driving force is behind population cycles in Lepidoptera

455) It can be inferred from the passage that the mortality caused by agents such as predatory birds or parasites was measured in an attempt to
A. develop an explanation for the existence of lepidoptera population cycles
B. identify behavioral factors in lepidoptera that affect survival rates
C. identify possible methods for controlling Lepidoptera population growth
D. provide evidence that lepidoptera populations are self-regulating
E. determine the life stages of lepidoptera at which mortality rates are highest

456) Which of the following, if true, would most weaken the author’s conclusion in lines 25- 30 (bold lines)?
A. New research reveals that the number of species of birds and parasites that prey on lepidoptera has dropped significantly in recent years.
B. New experiments in which the habitats of lepidoptera are altered in previously untried ways result in the shortening of lepidoptera population cycles.
C. Recent experiments have revealed that the nuclear polyhedrosis virus is present in a number of predators and parasites of lepidoptera.
D. Differences among the habitats of lepidoptera species make it difficult to assess the effects of weather on lepidoptera population cycles.
E. Viral disease is typically observed in a large proportion of the lepidoptera population.

457) According to the passage, before the discovery of new techniques for detecting viral DNA, population ecologists believed that viral diseases
A. were not widely prevalent among insect populations generally
B. affected only the caterpillar life stage of lepidoptera
C. were the driving force behind Lepidoptera population cycles
D. attacked already declining caterpillar populations
E. infected birds and parasites that prey on various species of lepidoptera

458) According to the passage, nuclear polyhedrosis viruses can remain virulent in the environment only when
A. the polyhedrin protein crystals dissolve
B. caterpillar population are in decline
C. they are present in large numbers
D. their concentration in a particular area remains low
E. they are sheltered from direct sunlight

459) It can be inferred from the passage that while inside its polyhedrin protein crystals, the nuclear polyhedrosis virus
A. is exposed to direct sunlight
B. is attractive to predators
C. cannot infect caterpillars' cells
D. cannot be ingested by caterpillars
E. cannot be detected by new techniques of molecular biology

Q454: C
Q455: A
Q456: B
Q457: D
Q458: E
Q459: C

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