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Passage56 Answers
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Although numbers of animals in a given region may
fluctuate from year to year, the fluctuations are often
temporary and, over long periods, trivial. Scientists
have advanced three theories of population control to
(5) account for this relative constancy.
The first theory attributes a relatively constant popu-
lation to periodic climatic catastrophes that decimate
populations with such frequency as to prevent them
from exceeding some particular limit. In the case of
(10) small organisms with short life cycles, climatic changes
need not be catastrophic: normal seasonal changes in
photoperiod (daily amount of sunlight), for example,
can govern population growth. This theory---the
density-independent view---asserts that climatic factors
(15) exert the same regulatory effect on population regard-
less of the number of individuals in a region.
A second theory argues that population growth is
primarily density-dependent---that is, the rate of
growth of a population in a region decreases as the
(20) number of animals increases. The mechanisms that
manage regulation may vary. For example, as numbers
increase, the food supply would probably diminish,
which would increase mortality. In addition, as Lotka
and Volterra have shown, predators can find prey more
(25) easily in high-density populations. Other regulators
include physiological control mechanisms: for example.
Christian and Davis have demonstrated how the
crowding that results from a rise in numbers may bring
about hormonal changes in the pituitary and adrenal
(30) glands that in turn may regulate population by lowering
sexual activity and inhibiting sexual maturation. There
is evidence that these effects may persist for three
generations in the absence of the original provocation.
One challenge for density-dependent theorists is to
(35) develop models that would allow the precise prediction
of the effects of crowding.
A third theory, proposed by Wynne-Edwards and
termed "epideictic," argues that organisms have evolved
a "code"in the form of social or epideictic behavior
(40) displays, such as winter-roosting aggregations or group
vocalizing; such codes provide organisms with infor-
mation on population size in a region so that they can,
if necessary, exercise reproductive restraint. However,
wynne-Edwards' theory, linking animal social behavior
(45) and population control, has been challenged, with some
justification, by several studies.
1. The primary purpose of the passage is to
(A) argue against those scientists who maintain that
animal populations tend to fluctuate
(B) compare and contrast the density-dependent
and epideictic theories of population control
(C) provide example of some of the ways in which
animals exercise reproductive restraint to
control their own numbers
(D) suggests that theories of population control that
concentrate on the social behavior of animals
are more open to debate than are theories that do not
(E) summarize a number of scientific theories that
attempt to explain why animal populations do
not exceed certain limits
2. It can be inferred from the passage that proponents
of the density-dependent theory of population control
have not yet been able to
(A) use their theory to explain the population growth of
organisms with short life cycles
(B) reproduce the results of the study of Christian and
Davis
(C) explain adequately why the numbers of a population
can increase as the population's rate of growth
decreases
(D) make sufficiently accurate predictions about the
effects of crowding
(E) demonstrate how predator populations are
themselves regulated
3. Which of the following, if true, would best support the
density-dependent theory of population control as it is
described in the passage?
(A) As the number of foxes in Minnesota decrease, the
growth rate of this population of foxes begins of
increase.
(B) As the number of woodpeckers in Vermont
decreases, the growth rate of this population of
woodpeckers also begins to decrease.
(C) As the number of prairie dogs in Oklahoma
increases, the growth rate of this population of
prairie dogs also begins to increase.
(D) After the number of beavers in Tennessee decreases,
the number of predators of these beavers begins to
increase.
(E) After the number of eagles in Montana decreases,
the food supply of this population of eagles also
begins to decrease.
4. According to the Wynne-Edwards theory as it is
described in the passage, epideictic behavior displays
serve the function of
(A) determining roosting aggregations
(B) locating food
(C) attracting predators
(D) regulating sexual activity
(E) triggering hormonal changes
5. The challenge posed to the Wynne-Edwards-theory by
several studies is regarded by the author with
(A) complete indifference
(B) qualified acceptance
(C) skeptical amusement
(D) perplexed astonishment
(E) agitated dismay
6. Which of the following statements would provide the
most of logical continuation of the final paragraph of the
passage?
(A) Thus wynne-Edwards' theory raises serious
questions about the constancy of animal population
in a region.
(B) Because Wynne-Edwards' theory is able to explain
more kinds of animal behavior than is the density-
dependent theory, epideictic explanations of
population
regulation are now widely accepted.
(C) The results of one study, for instance, have
suggested that group vocalizing is more often used
to defend territory than to provide information about
population density.
(D) Some of these studies have, in fact, worked out
a systematic and complex code of social behavior
that can regulate population size.
(E) One study, for example, has demonstrated that birds
are more likely to use winter-roosting aggregations
than group vocalizing in order to provide
information
on population size.
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