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