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As Gilbert White, Darwin, and others observed long ago, all species appear
to have the innate capacity to increase their numbers from generation to
generation. The task for ecologists is to untangle the environmental and
biological factors that hold this intrinsic capacity for population growth in
check over the long run. The great variety of dynamic behaviors exhibited by
different population makes this task more difficult: some populations remain
roughly constant from year to year; others exhibit regular cycles of abundance
and scarcity; still others vary wildly, with outbreaks and crashes that are in
some cases plainly correlated with the weather, and in other cases not.
To impose some order on this kaleidoscope of patterns, one school of
thought proposes dividing populations into two groups. These ecologists posit
that the relatively steady populations have “density-dependent” growth
parameters; that is, rates of birth, death, and migration which depend strongly
on population density. The highly varying populations have “density-independent”
growth parameters, with vital rates buffeted by environmental events; these
rates fluctuate in a way that is wholly independent of population density.
This dichotomy has its uses, but it can cause problems if taken too
literally. For one thing, no population can be driven entirely by
density-independent factors all the time. No matter how severely or
unpredictably birth, death, and migration rates may be fluctuating around their
long-term averages, if there were no density-dependent effects, the population
would, in the long run, either increase or decrease without bound (barring a
miracle by which gains and losses canceled exactly). Put another way, it may be
that on average 99 percent of all deaths in a population arise from
density-independent causes, and only one percent from factors varying with
density. The factors making up the one percent may seem unimportant, and their
cause may be correspondingly hard to determine. Yet, whether recognized or not,
they will usually determine the long-term average population density.
In order to understand the nature of the ecologist’s investigation, we may
think of the density-dependent effects on growth parameters as the “signal”
ecologists are trying to isolate and interpret, one that tends to make the
population increase from relatively low values or decrease from relatively high
ones, while the density-independent effects act to produce “noise” in the
population dynamics. For populations that remain relatively constant, or that
oscillate around repeated cycles, the signal can be fairly easily characterized
and its effects described, even though the causative biological mechanism may
remain unknown. For irregularly fluctuating populations, we are likely to have
too few observations to have any hope of extracting the signal from the
overwhelming noise. But it now seems clear that all populations are regulated by
a mixture of density-dependent and density-independent effects in varying
proportions.
1. The author of the text is primarily concerned with
[A] discussing two categories of factors that control population growth and
assessing their relative importance.
[B] describing how growth rates in natural populations fluctuate over time
and explaining why these changes occur.
[C] proposing a hypothesis concerning population size and suggesting ways
to test it.
[D] posing a fundamental question about environmental factors in population
growth and presenting some currently accepted answer.
2. It can be inferred from the text that the author considers the dichotomy
discussed to be
[A] applicable only to erratically fluctuating populations.
[B] instrumental, but only if its limitations are recognized.
[C] dangerously misleading in most circumstances.
[D] a complete and sufficient way to account for observed phenomena.
3. According to the text, all of the following behaviors have been
exhibited by different populations EXCEPT
[A] roughly constant population levels from year to year.
[B] regular cycles of increases and decreases in numbers.
[C] erratic increases in numbers correlated with the weather.
[D] unchecked increases in numbers over many generations.
4. The discussion concerning population in the third paragraph serves
primarily to
[A] demonstrate the difficulties ecologists face in studying
density-dependent factors limiting population growth.
[B] advocate more rigorous study of density-dependent factors in population
growth.
[C] prove that the death rates of any population are never entirely
density-independent.
[D] underline the importance of even small density-dependent factors in
regulating long-term population densities.
5. In the text, the author does all of the following EXCEPT
[A] cite the views of other biologists.
[B] define a basic problem that the text addresses.
[C] present conceptual categories used by other biologists.
[D] describe the results of a particular study. |
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发表于 2017-8-6 14:08:12