Directions: In the following text, some sentences have been removed. For Questions 41-45, choose the most suitable one from the list A-F to fit into each of the numbered blanks. There is one extra choice, which does not fit in any of the blanks. Mark your answers on the ANSWER SHEET. (10 points)
Population Changing
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. (41) 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.
(42) 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. These rates fluctuate in a way that is wholly independent of population density.
The dichotomy has its uses, but it can cause problems if taken too literally. (43) 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. 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 important. (44)
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 density-dependent effects act to produce “nose” 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 effect described, even though the causative biological mechanism may remain unknown. (45) But it now seems clear that all populations are regulated by a mixture of density-dependent and density-independent effects in varying proportions.
A. It is sometimes possible to infer the existence of a density-independent factor controlling population growth without understanding its causative mechanism.
B. 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.
C. However, whether recognized or not, they will usually determine the long-term average population density.
D. To impose some order on this kaleidoscope of patterns, one school of thought proposes dividing populations into two groups.
E. For irregularly fluctuating populations, we are likely to have too few observations to have any hope of extracting the signal from the overwhelming noise.
F. For one thing, no population can be driven entirely by density-independent factors all the time.
