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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 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 (45) 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.
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[阅读&逻辑 ]新版冲分救命题 -11894
Select the sentence that presents a conventional requirement that is rejected by the author of the passage.
[阅读&逻辑 ]精选题 -11861
Which of the following, if true of the cave, most weakens the argument?
[阅读&逻辑 ]精选题 -11860
It can be inferred that the “sediments
[阅读&逻辑 ]精选题 -11859
Which of the following best describes the issue raised by the question in the highlighted portion of the passage?
[阅读&逻辑 ]精选题 -11858
It can be inferred that which of the following is true of the “geologic evidence”?
[阅读&逻辑 ]精选题 -11857
Which of the following elements in the debate over the origin of Varanus is NOT provided in the passage?
[阅读&逻辑 ]精选题 -11856
It can be inferred from the passage that the geographical distribution of the Komodo dragon is
[阅读&逻辑 ]精选题 -11855
Which of the following, if discovered to be true, would most tend to suggest that the “correlation” mentioned in the passage held true for the people of the West Branch site?
[阅读&逻辑 ]精选题 -11854
The final sentence of the passage primarily serves to do which of the following?
[阅读&逻辑 ]精选题 -11853
The passage implies which of the following about beavers?
[阅读&逻辑 ]精选题 -11852
The passage provides support for which of the following statements about beaver dams?
[阅读&逻辑 ]精选题 -11851
According to the passage, which of the following best accounts for the apparent bias in antelope-migration direction?
[阅读&逻辑 ]精选题 -11850
The author of the passage implies that during the
[阅读&逻辑 ]精选题 -11849
According to the passage, which of the following is true of an Eurasia-Africa land bridge during ice ages?
[填空 ]精选题 -11843
The political upheaval caught most people by surprise: despite the ______ warning of some commentators, it had never seemed that imminent.
[填空 ]精选题 -11842
Even if the story now seems a surprisingly innocuous overture to the author’s later, more fully developed narrations, it ____ some of the key traits of those bleaker tales.
[填空 ]精选题 -11841
William Perkins, his ______ speaking style notwithstanding, has long been seen as the moderate face of his political party.
[填空 ]精选题 -11840
Just because, as a photographer, Friedlander (i)____ places that most people consider ugly does not mean that he is out to prove they are beautiful. Instead, his work suggests that the photographer simply cannot ignore so much of the built American landscape but is obligated to (ii)_____ what we pass through day in and day out, regardless of (iii)_____.
[填空 ]精选题 -11839
In the search for truth, the knowledge gained by scientists consists of approximations with varying degrees of certainty. Such (i)_____ truth can be highly (ii)_____, as the rapid and relatively steady progress of medical knowledge well exemplifies.
[填空 ]精选题 -11838
History teaches us that science is not _________ enterprise, indeed, it is quite the opposite, a motley assortment of tools designed to safeguard researchers against their own biases.
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