question 1

Passage 56

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