骈文GRE阅读——暗示推理题

2021年1月16日发(作者：瞿元镜)
GRE阅读——暗示推理题

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 populations 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