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A basic principle of ecology is that population size is [#permalink]
26 Jun 2011, 00:36
A basic principle of ecology is that population size is partly a reflection of available food resources. Recent experiments suggest that the relationship is more complex than formerly thought. Specifically, the browsing of certain rodents appears to trigger chemical reactions in food plants which, in turn, affect the size of the rodent populations. Two examples of such regulation have been reported.
Berger has demonstrated the power of a naturally occurring chemical called 6-MBOA to stimulate reproductive behavior in the mountain vole, a small mouse-like rodent. 6-MBOA forms in young grass in response to browsing by voles. Berger experimented by feeding oats coated with 6-MBOA to non-breeding winter populations of voles. After three weeks, she found a high incidence of pregnancy among females. Since the timing of reproduction is crucial to the short-lived vole in an environment in which the onset of vegetative growth may be considerably delayed, the phytochemical triggering of reproductive behavior represents a significant biological adaptation.
In an example reported by Bryant, plants appear to have developed a phytochemical defense against the depredations of snowshoe hares in Canada. Every ten years, for reasons that are unclear, the hare population swells. The result is overbrowsing of certain deciduous trees and shrubs. Bryant found that trees favored by the hare produce young shoots high in terpene and phenolic resins, which discourage hare browsing. After treating non-resinous willow twigs with resinous extracts and placing treated and untreated samples at hare feeding stations, Bryant found that samples containing at least half of the resin concentration of natural twigs were untouched. The avoidance of resinous shoots, he concludes, may play a role in the decline of the hare population to normal levels.
Both of these reports suggest areas for further research. For example, data should be reviewed to determine if periodic population explosions among lemmings (another small rodent living in a northern environment) occur during years in which there is an early onset of vegetative growth; if so, a triggering mechanism similar to that prompted by the vole may be involved.
1) The author provides specific information to answer which of the following questions? (A) What factors other than food supply affect the population size of rodents? (B) Why is the timing of the voles’ reproductive effort important? (C) Are phytochemical reactions found only in northern environments? (D) How does 6-MBOA trigger reproductive activity in the mountain vole? (E) What are the causes of the periodic increase in the snowshoe hare population?
2) The passage describes the effect of 6-MBOA on voles as a “significant biological adaptation” (line 23) because it (A) limits reproductive behavior in times of food scarcity (B) prompts the vole population to seek new food sources (C) supports species survival during periods of fluctuating food supply (D) maximizes the number of offspring in individual litters (E) minimizes territorial competition
3) Which of the following statements can be inferred about plant shoots containing large amounts of terpene and phenolic resins? I. They serve as a form of natural defense. II. Their growth is stimulated by increases in the hare population. III. They are unappetizing to hares. (A) I only (B) II only (C) III only (D) I and III only (E) I, II, and III
4) It can be inferred that the study of lemmings proposed by the author would probably (A) strengthen the conclusions of Bryant (B) cast doubt on the conclusions of Bryant (C) support the specific findings of Berger (D) provide evidence as to whether Berger’s conclusions can be generalized (E) disprove common beliefs about the relationship between population size and food supply
5) The author of the passage is primarily concerned with (A) reviewing findings about phytochemical regulation of rodent populations (B) outlining the role of 6-MBOA in regulating population size (C) summarizing knowledge on population size of rodents (D) explaining why earlier studies of population size were wrong (E) describing mechanisms used by plants to protect themselves
6) Bryant’s interpretation of the results of his experiment (lines 36–38) depends on which of the following assumptions? (A) The response of hares to resinous substances may be different in nature than under experimental conditions. (B) The periodic rise in the hare population is triggered by an unknown phytochemical response. (C) Many hares will starve to death rather than eat resinous shoots. (D) Hares alter their breeding behavior in response to the declining availability of food. (E) Significant numbers of hares die from ingesting the resins in shoots
7) The experiments described in the passage did each of the following EXCEPT (A) measure changes in the behavior of test animals (B) measure changes in the populations of experimental animals (C) simulate a hypothesized phytochemical effect in nature (D) measure the consumption of foods by test animals (E) analyze the effects of food on breeding behavior
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