A basic principle of ecology is that population size is

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